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Edit File: pcre2.txt

----------------------------------------------------------------------------- This file contains a concatenation of the PCRE2 man pages, converted to plain text format for ease of searching with a text editor, or for use on systems that do not have a man page processor. The small individual files that give synopses of each function in the library have not been included. Neither has the pcre2demo program. There are separate text files for the pcre2grep and pcre2test commands. ----------------------------------------------------------------------------- PCRE2(3) Library Functions Manual PCRE2(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) INTRODUCTION PCRE2 is the name used for a revised API for the PCRE library, which is a set of functions, written in C, that implement regular expression pattern matching using the same syntax and semantics as Perl, with just a few differences. Some features that appeared in Python and the origi- nal PCRE before they appeared in Perl are also available using the Python syntax. There is also some support for one or two .NET and Onig- uruma syntax items, and there are options for requesting some minor changes that give better ECMAScript (aka JavaScript) compatibility. The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or 32-bit code units, which means that up to three separate libraries may be installed. The original work to extend PCRE to 16-bit and 32-bit code units was done by Zoltan Herczeg and Christian Persch, respec- tively. In all three cases, strings can be interpreted either as one character per code unit, or as UTF-encoded Unicode, with support for Unicode general category properties. Unicode support is optional at build time (but is the default). However, processing strings as UTF code units must be enabled explicitly at run time. The version of Uni- code in use can be discovered by running pcre2test -C The three libraries contain identical sets of functions, with names ending in _8, _16, or _32, respectively (for example, pcre2_com- pile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or 32, a program that uses just one code unit width can be written using generic names such as pcre2_compile(), and the documentation is written assuming that this is the case. In addition to the Perl-compatible matching function, PCRE2 contains an alternative function that matches the same compiled patterns in a dif- ferent way. In certain circumstances, the alternative function has some advantages. For a discussion of the two matching algorithms, see the pcre2matching page. Details of exactly which Perl regular expression features are and are not supported by PCRE2 are given in separate documents. See the pcre2pattern and pcre2compat pages. There is a syntax summary in the pcre2syntax page. Some features of PCRE2 can be included, excluded, or changed when the library is built. The pcre2_config() function makes it possible for a client to discover which features are available. The features them- selves are described in the pcre2build page. Documentation about build- ing PCRE2 for various operating systems can be found in the README and NON-AUTOTOOLS_BUILD files in the source distribution. The libraries contains a number of undocumented internal functions and data tables that are used by more than one of the exported external functions, but which are not intended for use by external callers. Their names all begin with "_pcre2", which hopefully will not provoke any name clashes. In some environments, it is possible to control which external symbols are exported when a shared library is built, and in these cases the undocumented symbols are not exported. SECURITY CONSIDERATIONS If you are using PCRE2 in a non-UTF application that permits users to supply arbitrary patterns for compilation, you should be aware of a feature that allows users to turn on UTF support from within a pattern. For example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8 mode, which interprets patterns and subjects as strings of UTF-8 code units instead of individual 8-bit characters. This causes both the pat- tern and any data against which it is matched to be checked for UTF-8 validity. If the data string is very long, such a check might use suf- ficiently many resources as to cause your application to lose perfor- mance. One way of guarding against this possibility is to use the pcre2_pat- tern_info() function to check the compiled pattern's options for PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when calling pcre2_compile(). This causes an compile time error if a pattern contains a UTF-setting sequence. The use of Unicode properties for character types such as \d can also be enabled from within the pattern, by specifying "(*UCP)". This fea- ture can be disallowed by setting the PCRE2_NEVER_UCP option. If your application is one that supports UTF, be aware that validity checking can take time. If the same data string is to be matched many times, you can use the PCRE2_NO_UTF_CHECK option for the second and subsequent matches to avoid running redundant checks. The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead to problems, because it may leave the current matching point in the middle of a multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C option can be used by an application to lock out the use of \C, causing a compile-time error if it is encountered. It is also possible to build PCRE2 with the use of \C permanently disabled. Another way that performance can be hit is by running a pattern that has a very large search tree against a string that will never match. Nested unlimited repeats in a pattern are a common example. PCRE2 pro- vides some protection against this: see the pcre2_set_match_limit() function in the pcre2api page. USER DOCUMENTATION The user documentation for PCRE2 comprises a number of different sec- tions. In the "man" format, each of these is a separate "man page". In the HTML format, each is a separate page, linked from the index page. In the plain text format, the descriptions of the pcre2grep and pcre2test programs are in files called pcre2grep.txt and pcre2test.txt, respectively. The remaining sections, except for the pcre2demo section (which is a program listing), and the short pages for individual func- tions, are concatenated in pcre2.txt, for ease of searching. The sec- tions are as follows: pcre2 this document pcre2-config show PCRE2 installation configuration information pcre2api details of PCRE2's native C API pcre2build building PCRE2 pcre2callout details of the callout feature pcre2compat discussion of Perl compatibility pcre2demo a demonstration C program that uses PCRE2 pcre2grep description of the pcre2grep command (8-bit only) pcre2jit discussion of just-in-time optimization support pcre2limits details of size and other limits pcre2matching discussion of the two matching algorithms pcre2partial details of the partial matching facility pcre2pattern syntax and semantics of supported regular expression patterns pcre2perform discussion of performance issues pcre2posix the POSIX-compatible C API for the 8-bit library pcre2sample discussion of the pcre2demo program pcre2stack discussion of stack usage pcre2syntax quick syntax reference pcre2test description of the pcre2test command pcre2unicode discussion of Unicode and UTF support In the "man" and HTML formats, there is also a short page for each C library function, listing its arguments and results. AUTHOR Philip Hazel University Computing Service Cambridge, England. Putting an actual email address here is a spam magnet. If you want to email me, use my two initials, followed by the two digits 10, at the domain cam.ac.uk. REVISION Last updated: 16 October 2015 Copyright (c) 1997-2015 University of Cambridge. ------------------------------------------------------------------------------ PCRE2API(3) Library Functions Manual PCRE2API(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) #include <pcre2.h> PCRE2 is a new API for PCRE. This document contains a description of all its functions. See the pcre2 document for an overview of all the PCRE2 documentation. PCRE2 NATIVE API BASIC FUNCTIONS pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length, uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset, pcre2_compile_context *ccontext); void pcre2_code_free(pcre2_code *code); pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize, pcre2_general_context *gcontext); pcre2_match_data *pcre2_match_data_create_from_pattern( const pcre2_code *code, pcre2_general_context *gcontext); int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext); int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext, int *workspace, PCRE2_SIZE wscount); void pcre2_match_data_free(pcre2_match_data *match_data); PCRE2 NATIVE API AUXILIARY MATCH FUNCTIONS PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data); uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data); PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data); PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data); PCRE2 NATIVE API GENERAL CONTEXT FUNCTIONS pcre2_general_context *pcre2_general_context_create( void *(*private_malloc)(PCRE2_SIZE, void *), void (*private_free)(void *, void *), void *memory_data); pcre2_general_context *pcre2_general_context_copy( pcre2_general_context *gcontext); void pcre2_general_context_free(pcre2_general_context *gcontext); PCRE2 NATIVE API COMPILE CONTEXT FUNCTIONS pcre2_compile_context *pcre2_compile_context_create( pcre2_general_context *gcontext); pcre2_compile_context *pcre2_compile_context_copy( pcre2_compile_context *ccontext); void pcre2_compile_context_free(pcre2_compile_context *ccontext); int pcre2_set_bsr(pcre2_compile_context *ccontext, uint32_t value); int pcre2_set_character_tables(pcre2_compile_context *ccontext, const unsigned char *tables); int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext, PCRE2_SIZE value); int pcre2_set_newline(pcre2_compile_context *ccontext, uint32_t value); int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext, uint32_t value); int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext, int (*guard_function)(uint32_t, void *), void *user_data); PCRE2 NATIVE API MATCH CONTEXT FUNCTIONS pcre2_match_context *pcre2_match_context_create( pcre2_general_context *gcontext); pcre2_match_context *pcre2_match_context_copy( pcre2_match_context *mcontext); void pcre2_match_context_free(pcre2_match_context *mcontext); int pcre2_set_callout(pcre2_match_context *mcontext, int (*callout_function)(pcre2_callout_block *, void *), void *callout_data); int pcre2_set_match_limit(pcre2_match_context *mcontext, uint32_t value); int pcre2_set_offset_limit(pcre2_match_context *mcontext, PCRE2_SIZE value); int pcre2_set_recursion_limit(pcre2_match_context *mcontext, uint32_t value); int pcre2_set_recursion_memory_management( pcre2_match_context *mcontext, void *(*private_malloc)(PCRE2_SIZE, void *), void (*private_free)(void *, void *), void *memory_data); PCRE2 NATIVE API STRING EXTRACTION FUNCTIONS int pcre2_substring_copy_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen); int pcre2_substring_copy_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen); void pcre2_substring_free(PCRE2_UCHAR *buffer); int pcre2_substring_get_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen); int pcre2_substring_get_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen); int pcre2_substring_length_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_SIZE *length); int pcre2_substring_length_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_SIZE *length); int pcre2_substring_nametable_scan(const pcre2_code *code, PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last); int pcre2_substring_number_from_name(const pcre2_code *code, PCRE2_SPTR name); void pcre2_substring_list_free(PCRE2_SPTR *list); int pcre2_substring_list_get(pcre2_match_data *match_data, PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr); PCRE2 NATIVE API STRING SUBSTITUTION FUNCTION int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext, PCRE2_SPTR replacementzfP, PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer, PCRE2_SIZE *outlengthptr); PCRE2 NATIVE API JIT FUNCTIONS int pcre2_jit_compile(pcre2_code *code, uint32_t options); int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext); void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext); pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize, PCRE2_SIZE maxsize, pcre2_general_context *gcontext); void pcre2_jit_stack_assign(pcre2_match_context *mcontext, pcre2_jit_callback callback_function, void *callback_data); void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack); PCRE2 NATIVE API SERIALIZATION FUNCTIONS int32_t pcre2_serialize_decode(pcre2_code **codes, int32_t number_of_codes, const uint8_t *bytes, pcre2_general_context *gcontext); int32_t pcre2_serialize_encode(const pcre2_code **codes, int32_t number_of_codes, uint8_t **serialized_bytes, PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext); void pcre2_serialize_free(uint8_t *bytes); int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes); PCRE2 NATIVE API AUXILIARY FUNCTIONS pcre2_code *pcre2_code_copy(const pcre2_code *code); pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code); int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer, PCRE2_SIZE bufflen); const unsigned char *pcre2_maketables(pcre2_general_context *gcontext); int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where); int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data); int pcre2_config(uint32_t what, void *where); PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES There are three PCRE2 libraries, supporting 8-bit, 16-bit, and 32-bit code units, respectively. However, there is just one header file, pcre2.h. This contains the function prototypes and other definitions for all three libraries. One, two, or all three can be installed simul- taneously. On Unix-like systems the libraries are called libpcre2-8, libpcre2-16, and libpcre2-32, and they can also co-exist with the orig- inal PCRE libraries. Character strings are passed to and from a PCRE2 library as a sequence of unsigned integers in code units of the appropriate width. Every PCRE2 function comes in three different forms, one for each library, for example: pcre2_compile_8() pcre2_compile_16() pcre2_compile_32() There are also three different sets of data types: PCRE2_UCHAR8, PCRE2_UCHAR16, PCRE2_UCHAR32 PCRE2_SPTR8, PCRE2_SPTR16, PCRE2_SPTR32 The UCHAR types define unsigned code units of the appropriate widths. For example, PCRE2_UCHAR16 is usually defined as `uint16_t'. The SPTR types are constant pointers to the equivalent UCHAR types, that is, they are pointers to vectors of unsigned code units. Many applications use only one code unit width. For their convenience, macros are defined whose names are the generic forms such as pcre2_com- pile() and PCRE2_SPTR. These macros use the value of the macro PCRE2_CODE_UNIT_WIDTH to generate the appropriate width-specific func- tion and macro names. PCRE2_CODE_UNIT_WIDTH is not defined by default. An application must define it to be 8, 16, or 32 before including pcre2.h in order to make use of the generic names. Applications that use more than one code unit width can be linked with more than one PCRE2 library, but must define PCRE2_CODE_UNIT_WIDTH to be 0 before including pcre2.h, and then use the real function names. Any code that is to be included in an environment where the value of PCRE2_CODE_UNIT_WIDTH is unknown should also use the real function names. (Unfortunately, it is not possible in C code to save and restore the value of a macro.) If PCRE2_CODE_UNIT_WIDTH is not defined before including pcre2.h, a compiler error occurs. When using multiple libraries in an application, you must take care when processing any particular pattern to use only functions from a single library. For example, if you want to run a match using a pat- tern that was compiled with pcre2_compile_16(), you must do so with pcre2_match_16(), not pcre2_match_8(). In the function summaries above, and in the rest of this document and other PCRE2 documents, functions and data types are described using their generic names, without the 8, 16, or 32 suffix. PCRE2 API OVERVIEW PCRE2 has its own native API, which is described in this document. There are also some wrapper functions for the 8-bit library that corre- spond to the POSIX regular expression API, but they do not give access to all the functionality. They are described in the pcre2posix documen- tation. Both these APIs define a set of C function calls. The native API C data types, function prototypes, option values, and error codes are defined in the header file pcre2.h, which contains def- initions of PCRE2_MAJOR and PCRE2_MINOR, the major and minor release numbers for the library. Applications can use these to include support for different releases of PCRE2. In a Windows environment, if you want to statically link an application program against a non-dll PCRE2 library, you must define PCRE2_STATIC before including pcre2.h. The functions pcre2_compile(), and pcre2_match() are used for compiling and matching regular expressions in a Perl-compatible manner. A sample program that demonstrates the simplest way of using them is provided in the file called pcre2demo.c in the PCRE2 source distribution. A listing of this program is given in the pcre2demo documentation, and the pcre2sample documentation describes how to compile and run it. Just-in-time compiler support is an optional feature of PCRE2 that can be built in appropriate hardware environments. It greatly speeds up the matching performance of many patterns. Programs can request that it be used if available, by calling pcre2_jit_compile() after a pattern has been successfully compiled by pcre2_compile(). This does nothing if JIT support is not available. More complicated programs might need to make use of the specialist functions pcre2_jit_stack_create(), pcre2_jit_stack_free(), and pcre2_jit_stack_assign() in order to control the JIT code's memory usage. JIT matching is automatically used by pcre2_match() if it is available, unless the PCRE2_NO_JIT option is set. There is also a direct interface for JIT matching, which gives improved performance. The JIT-specific functions are discussed in the pcre2jit documentation. A second matching function, pcre2_dfa_match(), which is not Perl-com- patible, is also provided. This uses a different algorithm for the matching. The alternative algorithm finds all possible matches (at a given point in the subject), and scans the subject just once (unless there are lookbehind assertions). However, this algorithm does not return captured substrings. A description of the two matching algo- rithms and their advantages and disadvantages is given in the pcre2matching documentation. There is no JIT support for pcre2_dfa_match(). In addition to the main compiling and matching functions, there are convenience functions for extracting captured substrings from a subject string that has been matched by pcre2_match(). They are: pcre2_substring_copy_byname() pcre2_substring_copy_bynumber() pcre2_substring_get_byname() pcre2_substring_get_bynumber() pcre2_substring_list_get() pcre2_substring_length_byname() pcre2_substring_length_bynumber() pcre2_substring_nametable_scan() pcre2_substring_number_from_name() pcre2_substring_free() and pcre2_substring_list_free() are also pro- vided, to free the memory used for extracted strings. The function pcre2_substitute() can be called to match a pattern and return a copy of the subject string with substitutions for parts that were matched. Functions whose names begin with pcre2_serialize_ are used for saving compiled patterns on disc or elsewhere, and reloading them later. Finally, there are functions for finding out information about a com- piled pattern (pcre2_pattern_info()) and about the configuration with which PCRE2 was built (pcre2_config()). Functions with names ending with _free() are used for freeing memory blocks of various sorts. In all cases, if one of these functions is called with a NULL argument, it does nothing. STRING LENGTHS AND OFFSETS The PCRE2 API uses string lengths and offsets into strings of code units in several places. These values are always of type PCRE2_SIZE, which is an unsigned integer type, currently always defined as size_t. The largest value that can be stored in such a type (that is ~(PCRE2_SIZE)0) is reserved as a special indicator for zero-terminated strings and unset offsets. Therefore, the longest string that can be handled is one less than this maximum. NEWLINES PCRE2 supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (line- feed) character, the two-character sequence CRLF, any of the three pre- ceding, or any Unicode newline sequence. The Unicode newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029). Each of the first three conventions is used by at least one operating system as its standard newline sequence. When PCRE2 is built, a default can be specified. The default default is LF, which is the Unix stan- dard. However, the newline convention can be changed by an application when calling pcre2_compile(), or it can be specified by special text at the start of the pattern itself; this overrides any other settings. See the pcre2pattern page for details of the special character sequences. In the PCRE2 documentation the word "newline" is used to mean "the character or pair of characters that indicate a line break". The choice of newline convention affects the handling of the dot, circumflex, and dollar metacharacters, the handling of #-comments in /x mode, and, when CRLF is a recognized line ending sequence, the match position advance- ment for a non-anchored pattern. There is more detail about this in the section on pcre2_match() options below. The choice of newline convention does not affect the interpretation of the \n or \r escape sequences, nor does it affect what \R matches; this has its own separate convention. MULTITHREADING In a multithreaded application it is important to keep thread-specific data separate from data that can be shared between threads. The PCRE2 library code itself is thread-safe: it contains no static or global variables. The API is designed to be fairly simple for non-threaded applications while at the same time ensuring that multithreaded appli- cations can use it. There are several different blocks of data that are used to pass infor- mation between the application and the PCRE2 libraries. The compiled pattern A pointer to the compiled form of a pattern is returned to the user when pcre2_compile() is successful. The data in the compiled pattern is fixed, and does not change when the pattern is matched. Therefore, it is thread-safe, that is, the same compiled pattern can be used by more than one thread simultaneously. For example, an application can compile all its patterns at the start, before forking off multiple threads that use them. However, if the just-in-time optimization feature is being used, it needs separate memory stack areas for each thread. See the pcre2jit documentation for more details. In a more complicated situation, where patterns are compiled only when they are first needed, but are still shared between threads, pointers to compiled patterns must be protected from simultaneous writing by multiple threads, at least until a pattern has been compiled. The logic can be something like this: Get a read-only (shared) lock (mutex) for pointer if (pointer == NULL) { Get a write (unique) lock for pointer pointer = pcre2_compile(... } Release the lock Use pointer in pcre2_match() Of course, testing for compilation errors should also be included in the code. If JIT is being used, but the JIT compilation is not being done immedi- ately, (perhaps waiting to see if the pattern is used often enough) similar logic is required. JIT compilation updates a pointer within the compiled code block, so a thread must gain unique write access to the pointer before calling pcre2_jit_compile(). Alternatively, pcre2_code_copy() or pcre2_code_copy_with_tables() can be used to obtain a private copy of the compiled code. Context blocks The next main section below introduces the idea of "contexts" in which PCRE2 functions are called. A context is nothing more than a collection of parameters that control the way PCRE2 operates. Grouping a number of parameters together in a context is a convenient way of passing them to a PCRE2 function without using lots of arguments. The parameters that are stored in contexts are in some sense "advanced features" of the API. Many straightforward applications will not need to use contexts. In a multithreaded application, if the parameters in a context are val- ues that are never changed, the same context can be used by all the threads. However, if any thread needs to change any value in a context, it must make its own thread-specific copy. Match blocks The matching functions need a block of memory for working space and for storing the results of a match. This includes details of what was matched, as well as additional information such as the name of a (*MARK) setting. Each thread must provide its own copy of this memory. PCRE2 CONTEXTS Some PCRE2 functions have a lot of parameters, many of which are used only by specialist applications, for example, those that use custom memory management or non-standard character tables. To keep function argument lists at a reasonable size, and at the same time to keep the API extensible, "uncommon" parameters are passed to certain functions in a context instead of directly. A context is just a block of memory that holds the parameter values. Applications that do not need to adjust any of the context parameters can pass NULL when a context pointer is required. There are three different types of context: a general context that is relevant for several PCRE2 operations, a compile-time context, and a match-time context. The general context At present, this context just contains pointers to (and data for) external memory management functions that are called from several places in the PCRE2 library. The context is named `general' rather than specifically `memory' because in future other fields may be added. If you do not want to supply your own custom memory management functions, you do not need to bother with a general context. A general context is created by: pcre2_general_context *pcre2_general_context_create( void *(*private_malloc)(PCRE2_SIZE, void *), void (*private_free)(void *, void *), void *memory_data); The two function pointers specify custom memory management functions, whose prototypes are: void *private_malloc(PCRE2_SIZE, void *); void private_free(void *, void *); Whenever code in PCRE2 calls these functions, the final argument is the value of memory_data. Either of the first two arguments of the creation function may be NULL, in which case the system memory management func- tions malloc() and free() are used. (This is not currently useful, as there are no other fields in a general context, but in future there might be.) The private_malloc() function is used (if supplied) to obtain memory for storing the context, and all three values are saved as part of the context. Whenever PCRE2 creates a data block of any kind, the block contains a pointer to the free() function that matches the malloc() function that was used. When the time comes to free the block, this function is called. A general context can be copied by calling: pcre2_general_context *pcre2_general_context_copy( pcre2_general_context *gcontext); The memory used for a general context should be freed by calling: void pcre2_general_context_free(pcre2_general_context *gcontext); The compile context A compile context is required if you want to change the default values of any of the following compile-time parameters: What \R matches (Unicode newlines or CR, LF, CRLF only) PCRE2's character tables The newline character sequence The compile time nested parentheses limit The maximum length of the pattern string An external function for stack checking A compile context is also required if you are using custom memory man- agement. If none of these apply, just pass NULL as the context argu- ment of pcre2_compile(). A compile context is created, copied, and freed by the following func- tions: pcre2_compile_context *pcre2_compile_context_create( pcre2_general_context *gcontext); pcre2_compile_context *pcre2_compile_context_copy( pcre2_compile_context *ccontext); void pcre2_compile_context_free(pcre2_compile_context *ccontext); A compile context is created with default values for its parameters. These can be changed by calling the following functions, which return 0 on success, or PCRE2_ERROR_BADDATA if invalid data is detected. int pcre2_set_bsr(pcre2_compile_context *ccontext, uint32_t value); The value must be PCRE2_BSR_ANYCRLF, to specify that \R matches only CR, LF, or CRLF, or PCRE2_BSR_UNICODE, to specify that \R matches any Unicode line ending sequence. The value is used by the JIT compiler and by the two interpreted matching functions, pcre2_match() and pcre2_dfa_match(). int pcre2_set_character_tables(pcre2_compile_context *ccontext, const unsigned char *tables); The value must be the result of a call to pcre2_maketables(), whose only argument is a general context. This function builds a set of char- acter tables in the current locale. int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext, PCRE2_SIZE value); This sets a maximum length, in code units, for the pattern string that is to be compiled. If the pattern is longer, an error is generated. This facility is provided so that applications that accept patterns from external sources can limit their size. The default is the largest number that a PCRE2_SIZE variable can hold, which is effectively unlim- ited. int pcre2_set_newline(pcre2_compile_context *ccontext, uint32_t value); This specifies which characters or character sequences are to be recog- nized as newlines. The value must be one of PCRE2_NEWLINE_CR (carriage return only), PCRE2_NEWLINE_LF (linefeed only), PCRE2_NEWLINE_CRLF (the two-character sequence CR followed by LF), PCRE2_NEWLINE_ANYCRLF (any of the above), or PCRE2_NEWLINE_ANY (any Unicode newline sequence). When a pattern is compiled with the PCRE2_EXTENDED option, the value of this parameter affects the recognition of white space and the end of internal comments starting with #. The value is saved with the compiled pattern for subsequent use by the JIT compiler and by the two inter- preted matching functions, pcre2_match() and pcre2_dfa_match(). int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext, uint32_t value); This parameter ajusts the limit, set when PCRE2 is built (default 250), on the depth of parenthesis nesting in a pattern. This limit stops rogue patterns using up too much system stack when being compiled. The limit applies to parentheses of all kinds, not just capturing parenthe- ses. int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext, int (*guard_function)(uint32_t, void *), void *user_data); There is at least one application that runs PCRE2 in threads with very limited system stack, where running out of stack is to be avoided at all costs. The parenthesis limit above cannot take account of how much stack is actually available. For a finer control, you can supply a function that is called whenever pcre2_compile() starts to compile a parenthesized part of a pattern. This function can check the actual stack size (or anything else that it wants to, of course). The first argument to the callout function gives the current depth of nesting, and the second is user data that is set up by the last argu- ment of pcre2_set_compile_recursion_guard(). The callout function should return zero if all is well, or non-zero to force an error. The match context A match context is required if you want to change the default values of any of the following match-time parameters: A callout function The offset limit for matching an unanchored pattern The limit for calling match() (see below) The limit for calling match() recursively A match context is also required if you are using custom memory manage- ment. If none of these apply, just pass NULL as the context argument of pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match(). A match context is created, copied, and freed by the following func- tions: pcre2_match_context *pcre2_match_context_create( pcre2_general_context *gcontext); pcre2_match_context *pcre2_match_context_copy( pcre2_match_context *mcontext); void pcre2_match_context_free(pcre2_match_context *mcontext); A match context is created with default values for its parameters. These can be changed by calling the following functions, which return 0 on success, or PCRE2_ERROR_BADDATA if invalid data is detected. int pcre2_set_callout(pcre2_match_context *mcontext, int (*callout_function)(pcre2_callout_block *, void *), void *callout_data); This sets up a "callout" function, which PCRE2 will call at specified points during a matching operation. Details are given in the pcre2call- out documentation. int pcre2_set_offset_limit(pcre2_match_context *mcontext, PCRE2_SIZE value); The offset_limit parameter limits how far an unanchored search can advance in the subject string. The default value is PCRE2_UNSET. The pcre2_match() and pcre2_dfa_match() functions return PCRE2_ERROR_NOMATCH if a match with a starting point before or at the given offset is not found. For example, if the pattern /abc/ is matched against "123abc" with an offset limit less than 3, the result is PCRE2_ERROR_NO_MATCH. A match can never be found if the startoffset argument of pcre2_match() or pcre2_dfa_match() is greater than the off- set limit. When using this facility, you must set PCRE2_USE_OFFSET_LIMIT when calling pcre2_compile() so that when JIT is in use, different code can be compiled. If a match is started with a non-default match limit when PCRE2_USE_OFFSET_LIMIT is not set, an error is generated. The offset limit facility can be used to track progress when searching large subject strings. See also the PCRE2_FIRSTLINE option, which requires a match to start within the first line of the subject. If this is set with an offset limit, a match must occur in the first line and also within the offset limit. In other words, whichever limit comes first is used. int pcre2_set_match_limit(pcre2_match_context *mcontext, uint32_t value); The match_limit parameter provides a means of preventing PCRE2 from using up too many resources when processing patterns that are not going to match, but which have a very large number of possibilities in their search trees. The classic example is a pattern that uses nested unlim- ited repeats. Internally, pcre2_match() uses a function called match(), which it calls repeatedly (sometimes recursively). The limit set by match_limit is imposed on the number of times this function is called during a match, which has the effect of limiting the amount of backtracking that can take place. For patterns that are not anchored, the count restarts from zero for each position in the subject string. This limit is not relevant to pcre2_dfa_match(), which ignores it. When pcre2_match() is called with a pattern that was successfully pro- cessed by pcre2_jit_compile(), the way in which matching is executed is entirely different. However, there is still the possibility of runaway matching that goes on for a very long time, and so the match_limit value is also used in this case (but in a different way) to limit how long the matching can continue. The default value for the limit can be set when PCRE2 is built; the default default is 10 million, which handles all but the most extreme cases. If the limit is exceeded, pcre2_match() returns PCRE2_ERROR_MATCHLIMIT. A value for the match limit may also be sup- plied by an item at the start of a pattern of the form (*LIMIT_MATCH=ddd) where ddd is a decimal number. However, such a setting is ignored unless ddd is less than the limit set by the caller of pcre2_match() or, if no such limit is set, less than the default. int pcre2_set_recursion_limit(pcre2_match_context *mcontext, uint32_t value); The recursion_limit parameter is similar to match_limit, but instead of limiting the total number of times that match() is called, it limits the depth of recursion. The recursion depth is a smaller number than the total number of calls, because not all calls to match() are recur- sive. This limit is of use only if it is set smaller than match_limit. Limiting the recursion depth limits the amount of system stack that can be used, or, when PCRE2 has been compiled to use memory on the heap instead of the stack, the amount of heap memory that can be used. This limit is not relevant, and is ignored, when matching is done using JIT compiled code. However, it is supported by pcre2_dfa_match(), which uses recursive function calls less frequently than pcre2_match(), but which can be caused to use a lot of stack by a recursive pattern such as /(.)(?1)/ matched to a very long string. The default value for recursion_limit can be set when PCRE2 is built; the default default is the same value as the default for match_limit. If the limit is exceeded, pcre2_match() and pcre2_dfa_match() return PCRE2_ERROR_RECURSIONLIMIT. A value for the recursion limit may also be supplied by an item at the start of a pattern of the form (*LIMIT_RECURSION=ddd) where ddd is a decimal number. However, such a setting is ignored unless ddd is less than the limit set by the caller of pcre2_match() or pcre2_dfa_match() or, if no such limit is set, less than the default. int pcre2_set_recursion_memory_management( pcre2_match_context *mcontext, void *(*private_malloc)(PCRE2_SIZE, void *), void (*private_free)(void *, void *), void *memory_data); This function sets up two additional custom memory management functions for use by pcre2_match() when PCRE2 is compiled to use the heap for remembering backtracking data, instead of recursive function calls that use the system stack. There is a discussion about PCRE2's stack usage in the pcre2stack documentation. See the pcre2build documentation for details of how to build PCRE2. Using the heap for recursion is a non-standard way of building PCRE2, for use in environments that have limited stacks. Because of the greater use of memory management, pcre2_match() runs more slowly. Func- tions that are different to the general custom memory functions are provided so that special-purpose external code can be used for this case, because the memory blocks are all the same size. The blocks are retained by pcre2_match() until it is about to exit so that they can be re-used when possible during the match. In the absence of these func- tions, the normal custom memory management functions are used, if sup- plied, otherwise the system functions. CHECKING BUILD-TIME OPTIONS int pcre2_config(uint32_t what, void *where); The function pcre2_config() makes it possible for a PCRE2 client to discover which optional features have been compiled into the PCRE2 library. The pcre2build documentation has more details about these optional features. The first argument for pcre2_config() specifies which information is required. The second argument is a pointer to memory into which the information is placed. If NULL is passed, the function returns the amount of memory that is needed for the requested information. For calls that return numerical values, the value is in bytes; when requesting these values, where should point to appropriately aligned memory. For calls that return strings, the required length is given in code units, not counting the terminating zero. When requesting information, the returned value from pcre2_config() is non-negative on success, or the negative error code PCRE2_ERROR_BADOP- TION if the value in the first argument is not recognized. The follow- ing information is available: PCRE2_CONFIG_BSR The output is a uint32_t integer whose value indicates what character sequences the \R escape sequence matches by default. A value of PCRE2_BSR_UNICODE means that \R matches any Unicode line ending sequence; a value of PCRE2_BSR_ANYCRLF means that \R matches only CR, LF, or CRLF. The default can be overridden when a pattern is compiled. PCRE2_CONFIG_JIT The output is a uint32_t integer that is set to one if support for just-in-time compiling is available; otherwise it is set to zero. PCRE2_CONFIG_JITTARGET The where argument should point to a buffer that is at least 48 code units long. (The exact length required can be found by calling pcre2_config() with where set to NULL.) The buffer is filled with a string that contains the name of the architecture for which the JIT compiler is configured, for example "x86 32bit (little endian + unaligned)". If JIT support is not available, PCRE2_ERROR_BADOPTION is returned, otherwise the number of code units used is returned. This is the length of the string, plus one unit for the terminating zero. PCRE2_CONFIG_LINKSIZE The output is a uint32_t integer that contains the number of bytes used for internal linkage in compiled regular expressions. When PCRE2 is configured, the value can be set to 2, 3, or 4, with the default being 2. This is the value that is returned by pcre2_config(). However, when the 16-bit library is compiled, a value of 3 is rounded up to 4, and when the 32-bit library is compiled, internal linkages always use 4 bytes, so the configured value is not relevant. The default value of 2 for the 8-bit and 16-bit libraries is sufficient for all but the most massive patterns, since it allows the size of the compiled pattern to be up to 64K code units. Larger values allow larger regular expressions to be compiled by those two libraries, but at the expense of slower matching. PCRE2_CONFIG_MATCHLIMIT The output is a uint32_t integer that gives the default limit for the number of internal matching function calls in a pcre2_match() execu- tion. Further details are given with pcre2_match() below. PCRE2_CONFIG_NEWLINE The output is a uint32_t integer whose value specifies the default character sequence that is recognized as meaning "newline". The values are: PCRE2_NEWLINE_CR Carriage return (CR) PCRE2_NEWLINE_LF Linefeed (LF) PCRE2_NEWLINE_CRLF Carriage return, linefeed (CRLF) PCRE2_NEWLINE_ANY Any Unicode line ending PCRE2_NEWLINE_ANYCRLF Any of CR, LF, or CRLF The default should normally correspond to the standard sequence for your operating system. PCRE2_CONFIG_PARENSLIMIT The output is a uint32_t integer that gives the maximum depth of nest- ing of parentheses (of any kind) in a pattern. This limit is imposed to cap the amount of system stack used when a pattern is compiled. It is specified when PCRE2 is built; the default is 250. This limit does not take into account the stack that may already be used by the calling application. For finer control over compilation stack usage, see pcre2_set_compile_recursion_guard(). PCRE2_CONFIG_RECURSIONLIMIT The output is a uint32_t integer that gives the default limit for the depth of recursion when calling the internal matching function in a pcre2_match() execution. Further details are given with pcre2_match() below. PCRE2_CONFIG_STACKRECURSE The output is a uint32_t integer that is set to one if internal recur- sion when running pcre2_match() is implemented by recursive function calls that use the system stack to remember their state. This is the usual way that PCRE2 is compiled. The output is zero if PCRE2 was com- piled to use blocks of data on the heap instead of recursive function calls. PCRE2_CONFIG_UNICODE_VERSION The where argument should point to a buffer that is at least 24 code units long. (The exact length required can be found by calling pcre2_config() with where set to NULL.) If PCRE2 has been compiled without Unicode support, the buffer is filled with the text "Unicode not supported". Otherwise, the Unicode version string (for example, "8.0.0") is inserted. The number of code units used is returned. This is the length of the string plus one unit for the terminating zero. PCRE2_CONFIG_UNICODE The output is a uint32_t integer that is set to one if Unicode support is available; otherwise it is set to zero. Unicode support implies UTF support. PCRE2_CONFIG_VERSION The where argument should point to a buffer that is at least 12 code units long. (The exact length required can be found by calling pcre2_config() with where set to NULL.) The buffer is filled with the PCRE2 version string, zero-terminated. The number of code units used is returned. This is the length of the string plus one unit for the termi- nating zero. COMPILING A PATTERN pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length, uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset, pcre2_compile_context *ccontext); void pcre2_code_free(pcre2_code *code); pcre2_code *pcre2_code_copy(const pcre2_code *code); pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code); The pcre2_compile() function compiles a pattern into an internal form. The pattern is defined by a pointer to a string of code units and a length. If the pattern is zero-terminated, the length can be specified as PCRE2_ZERO_TERMINATED. The function returns a pointer to a block of memory that contains the compiled pattern and related data, or NULL if an error occurred. If the compile context argument ccontext is NULL, memory for the com- piled pattern is obtained by calling malloc(). Otherwise, it is obtained from the same memory function that was used for the compile context. The caller must free the memory by calling pcre2_code_free() when it is no longer needed. The function pcre2_code_copy() makes a copy of the compiled code in new memory, using the same memory allocator as was used for the original. However, if the code has been processed by the JIT compiler (see below), the JIT information cannot be copied (because it is position- dependent). The new copy can initially be used only for non-JIT match- ing, though it can be passed to pcre2_jit_compile() if required. The pcre2_code_copy() function provides a way for individual threads in a multithreaded application to acquire a private copy of shared com- piled code. However, it does not make a copy of the character tables used by the compiled pattern; the new pattern code points to the same tables as the original code. (See "Locale Support" below for details of these character tables.) In many applications the same tables are used throughout, so this behaviour is appropriate. Nevertheless, there are occasions when a copy of a compiled pattern and the relevant tables are needed. The pcre2_code_copy_with_tables() provides this facility. Copies of both the code and the tables are made, with the new code pointing to the new tables. The memory for the new tables is automati- cally freed when pcre2_code_free() is called for the new copy of the compiled code. NOTE: When one of the matching functions is called, pointers to the compiled pattern and the subject string are set in the match data block so that they can be referenced by the substring extraction functions. After running a match, you must not free a compiled pattern (or a sub- ject string) until after all operations on the match data block have taken place. The options argument for pcre2_compile() contains various bit settings that affect the compilation. It should be zero if no options are required. The available options are described below. Some of them (in particular, those that are compatible with Perl, but some others as well) can also be set and unset from within the pattern (see the detailed description in the pcre2pattern documentation). For those options that can be different in different parts of the pat- tern, the contents of the options argument specifies their settings at the start of compilation. The PCRE2_ANCHORED and PCRE2_NO_UTF_CHECK options can be set at the time of matching as well as at compile time. Other, less frequently required compile-time parameters (for example, the newline setting) can be provided in a compile context (as described above). If errorcode or erroroffset is NULL, pcre2_compile() returns NULL imme- diately. Otherwise, the variables to which these point are set to an error code and an offset (number of code units) within the pattern, respectively, when pcre2_compile() returns NULL because a compilation error has occurred. The values are not defined when compilation is suc- cessful and pcre2_compile() returns a non-NULL value. The value returned in erroroffset is an indication of where in the pat- tern the error occurred. It is not necessarily the furthest point in the pattern that was read. For example, after the error "lookbehind assertion is not fixed length", the error offset points to the start of the failing assertion. The pcre2_get_error_message() function (see "Obtaining a textual error message" below) provides a textual message for each error code. Compi- lation errors have positive error codes; UTF formatting error codes are negative. For an invalid UTF-8 or UTF-16 string, the offset is that of the first code unit of the failing character. Some errors are not detected until the whole pattern has been scanned; in these cases, the offset passed back is the length of the pattern. Note that the offset is in code units, not characters, even in a UTF mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char- acter. This code fragment shows a typical straightforward call to pcre2_com- pile(): pcre2_code *re; PCRE2_SIZE erroffset; int errorcode; re = pcre2_compile( "^A.*Z", /* the pattern */ PCRE2_ZERO_TERMINATED, /* the pattern is zero-terminated */ 0, /* default options */ &errorcode, /* for error code */ &erroffset, /* for error offset */ NULL); /* no compile context */ The following names for option bits are defined in the pcre2.h header file: PCRE2_ANCHORED If this bit is set, the pattern is forced to be "anchored", that is, it is constrained to match only at the first matching point in the string that is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl. PCRE2_ALLOW_EMPTY_CLASS By default, for compatibility with Perl, a closing square bracket that immediately follows an opening one is treated as a data character for the class. When PCRE2_ALLOW_EMPTY_CLASS is set, it terminates the class, which therefore contains no characters and so can never match. PCRE2_ALT_BSUX This option request alternative handling of three escape sequences, which makes PCRE2's behaviour more like ECMAscript (aka JavaScript). When it is set: (1) \U matches an upper case "U" character; by default \U causes a com- pile time error (Perl uses \U to upper case subsequent characters). (2) \u matches a lower case "u" character unless it is followed by four hexadecimal digits, in which case the hexadecimal number defines the code point to match. By default, \u causes a compile time error (Perl uses it to upper case the following character). (3) \x matches a lower case "x" character unless it is followed by two hexadecimal digits, in which case the hexadecimal number defines the code point to match. By default, as in Perl, a hexadecimal number is always expected after \x, but it may have zero, one, or two digits (so, for example, \xz matches a binary zero character followed by z). PCRE2_ALT_CIRCUMFLEX In multiline mode (when PCRE2_MULTILINE is set), the circumflex metacharacter matches at the start of the subject (unless PCRE2_NOTBOL is set), and also after any internal newline. However, it does not match after a newline at the end of the subject, for compatibility with Perl. If you want a multiline circumflex also to match after a termi- nating newline, you must set PCRE2_ALT_CIRCUMFLEX. PCRE2_ALT_VERBNAMES By default, for compatibility with Perl, the name in any verb sequence such as (*MARK:NAME) is any sequence of characters that does not include a closing parenthesis. The name is not processed in any way, and it is not possible to include a closing parenthesis in the name. However, if the PCRE2_ALT_VERBNAMES option is set, normal backslash processing is applied to verb names and only an unescaped closing parenthesis terminates the name. A closing parenthesis can be included in a name either as \) or between \Q and \E. If the PCRE2_EXTENDED option is set, unescaped whitespace in verb names is skipped and #-com- ments are recognized, exactly as in the rest of the pattern. PCRE2_AUTO_CALLOUT If this bit is set, pcre2_compile() automatically inserts callout items, all with number 255, before each pattern item, except immedi- ately before or after a callout in the pattern. For discussion of the callout facility, see the pcre2callout documentation. PCRE2_CASELESS If this bit is set, letters in the pattern match both upper and lower case letters in the subject. It is equivalent to Perl's /i option, and it can be changed within a pattern by a (?i) option setting. PCRE2_DOLLAR_ENDONLY If this bit is set, a dollar metacharacter in the pattern matches only at the end of the subject string. Without this option, a dollar also matches immediately before a newline at the end of the string (but not before any other newlines). The PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set. There is no equivalent to this option in Perl, and no way to set it within a pattern. PCRE2_DOTALL If this bit is set, a dot metacharacter in the pattern matches any character, including one that indicates a newline. However, it only ever matches one character, even if newlines are coded as CRLF. Without this option, a dot does not match when the current position in the sub- ject is at a newline. This option is equivalent to Perl's /s option, and it can be changed within a pattern by a (?s) option setting. A neg- ative class such as [^a] always matches newline characters, independent of the setting of this option. PCRE2_DUPNAMES If this bit is set, names used to identify capturing subpatterns need not be unique. This can be helpful for certain types of pattern when it is known that only one instance of the named subpattern can ever be matched. There are more details of named subpatterns below; see also the pcre2pattern documentation. PCRE2_EXTENDED If this bit is set, most white space characters in the pattern are totally ignored except when escaped or inside a character class. How- ever, white space is not allowed within sequences such as (?> that introduce various parenthesized subpatterns, nor within numerical quan- tifiers such as {1,3}. Ignorable white space is permitted between an item and a following quantifier and between a quantifier and a follow- ing + that indicates possessiveness. PCRE2_EXTENDED also causes characters between an unescaped # outside a character class and the next newline, inclusive, to be ignored, which makes it possible to include comments inside complicated patterns. Note that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count. PCRE2_EXTENDED is equivalent to Perl's /x option, and it can be changed within a pattern by a (?x) option setting. Which characters are interpreted as newlines can be specified by a set- ting in the compile context that is passed to pcre2_compile() or by a special sequence at the start of the pattern, as described in the sec- tion entitled "Newline conventions" in the pcre2pattern documentation. A default is defined when PCRE2 is built. PCRE2_FIRSTLINE If this option is set, an unanchored pattern is required to match before or at the first newline in the subject string, though the matched text may continue over the newline. See also PCRE2_USE_OFF- SET_LIMIT, which provides a more general limiting facility. If PCRE2_FIRSTLINE is set with an offset limit, a match must occur in the first line and also within the offset limit. In other words, whichever limit comes first is used. PCRE2_MATCH_UNSET_BACKREF If this option is set, a back reference to an unset subpattern group matches an empty string (by default this causes the current matching alternative to fail). A pattern such as (\1)(a) succeeds when this option is set (assuming it can find an "a" in the subject), whereas it fails by default, for Perl compatibility. Setting this option makes PCRE2 behave more like ECMAscript (aka JavaScript). PCRE2_MULTILINE By default, for the purposes of matching "start of line" and "end of line", PCRE2 treats the subject string as consisting of a single line of characters, even if it actually contains newlines. The "start of line" metacharacter (^) matches only at the start of the string, and the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (except when PCRE2_DOL- LAR_ENDONLY is set). Note, however, that unless PCRE2_DOTALL is set, the "any character" metacharacter (.) does not match at a newline. This behaviour (for ^, $, and dot) is the same as Perl. When PCRE2_MULTILINE it is set, the "start of line" and "end of line" constructs match immediately following or immediately before internal newlines in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m option, and it can be changed within a pattern by a (?m) option setting. Note that the "start of line" metacharacter does not match after a newline at the end of the subject, for compatibility with Perl. However, you can change this by setting the PCRE2_ALT_CIRCUMFLEX option. If there are no newlines in a subject string, or no occurrences of ^ or $ in a pattern, setting PCRE2_MULTILINE has no effect. PCRE2_NEVER_BACKSLASH_C This option locks out the use of \C in the pattern that is being com- piled. This escape can cause unpredictable behaviour in UTF-8 or UTF-16 modes, because it may leave the current matching point in the middle of a multi-code-unit character. This option may be useful in applications that process patterns from external sources. Note that there is also a build-time option that permanently locks out the use of \C. PCRE2_NEVER_UCP This option locks out the use of Unicode properties for handling \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes, as described for the PCRE2_UCP option below. In particular, it prevents the creator of the pattern from enabling this facility by starting the pattern with (*UCP). This option may be useful in applications that process patterns from external sources. The option combination PCRE_UCP and PCRE_NEVER_UCP causes an error. PCRE2_NEVER_UTF This option locks out interpretation of the pattern as UTF-8, UTF-16, or UTF-32, depending on which library is in use. In particular, it pre- vents the creator of the pattern from switching to UTF interpretation by starting the pattern with (*UTF). This option may be useful in applications that process patterns from external sources. The combina- tion of PCRE2_UTF and PCRE2_NEVER_UTF causes an error. PCRE2_NO_AUTO_CAPTURE If this option is set, it disables the use of numbered capturing paren- theses in the pattern. Any opening parenthesis that is not followed by ? behaves as if it were followed by ?: but named parentheses can still be used for capturing (and they acquire numbers in the usual way). There is no equivalent of this option in Perl. Note that, if this option is set, references to capturing groups (back references or recursion/subroutine calls) may only refer to named groups, though the reference can be by name or by number. PCRE2_NO_AUTO_POSSESS If this option is set, it disables "auto-possessification", which is an optimization that, for example, turns a+b into a++b in order to avoid backtracks into a+ that can never be successful. However, if callouts are in use, auto-possessification means that some callouts are never taken. You can set this option if you want the matching functions to do a full unoptimized search and run all the callouts, but it is mainly provided for testing purposes. PCRE2_NO_DOTSTAR_ANCHOR If this option is set, it disables an optimization that is applied when .* is the first significant item in a top-level branch of a pattern, and all the other branches also start with .* or with \A or \G or ^. The optimization is automatically disabled for .* if it is inside an atomic group or a capturing group that is the subject of a back refer- ence, or if the pattern contains (*PRUNE) or (*SKIP). When the opti- mization is not disabled, such a pattern is automatically anchored if PCRE2_DOTALL is set for all the .* items and PCRE2_MULTILINE is not set for any ^ items. Otherwise, the fact that any match must start either at the start of the subject or following a newline is remembered. Like other optimizations, this can cause callouts to be skipped. PCRE2_NO_START_OPTIMIZE This is an option whose main effect is at matching time. It does not change what pcre2_compile() generates, but it does affect the output of the JIT compiler. There are a number of optimizations that may occur at the start of a match, in order to speed up the process. For example, if it is known that an unanchored match must start with a specific character, the matching code searches the subject for that character, and fails imme- diately if it cannot find it, without actually running the main match- ing function. This means that a special item such as (*COMMIT) at the start of a pattern is not considered until after a suitable starting point for the match has been found. Also, when callouts or (*MARK) items are in use, these "start-up" optimizations can cause them to be skipped if the pattern is never actually used. The start-up optimiza- tions are in effect a pre-scan of the subject that takes place before the pattern is run. The PCRE2_NO_START_OPTIMIZE option disables the start-up optimizations, possibly causing performance to suffer, but ensuring that in cases where the result is "no match", the callouts do occur, and that items such as (*COMMIT) and (*MARK) are considered at every possible starting position in the subject string. Setting PCRE2_NO_START_OPTIMIZE may change the outcome of a matching operation. Consider the pattern (*COMMIT)ABC When this is compiled, PCRE2 records the fact that a match must start with the character "A". Suppose the subject string is "DEFABC". The start-up optimization scans along the subject, finds "A" and runs the first match attempt from there. The (*COMMIT) item means that the pat- tern must match the current starting position, which in this case, it does. However, if the same match is run with PCRE2_NO_START_OPTIMIZE set, the initial scan along the subject string does not happen. The first match attempt is run starting from "D" and when this fails, (*COMMIT) prevents any further matches being tried, so the overall result is "no match". There are also other start-up optimizations. For example, a minimum length for the subject may be recorded. Consider the pattern (*MARK:A)(X|Y) The minimum length for a match is one character. If the subject is "ABC", there will be attempts to match "ABC", "BC", and "C". An attempt to match an empty string at the end of the subject does not take place, because PCRE2 knows that the subject is now too short, and so the (*MARK) is never encountered. In this case, the optimization does not affect the overall match result, which is still "no match", but it does affect the auxiliary information that is returned. PCRE2_NO_UTF_CHECK When PCRE2_UTF is set, the validity of the pattern as a UTF string is automatically checked. There are discussions about the validity of UTF-8 strings, UTF-16 strings, and UTF-32 strings in the pcre2unicode document. If an invalid UTF sequence is found, pcre2_compile() returns a negative error code. If you know that your pattern is valid, and you want to skip this check for performance reasons, you can set the PCRE2_NO_UTF_CHECK option. When it is set, the effect of passing an invalid UTF string as a pat- tern is undefined. It may cause your program to crash or loop. Note that this option can also be passed to pcre2_match() and pcre_dfa_match(), to suppress validity checking of the subject string. PCRE2_UCP This option changes the way PCRE2 processes \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes. By default, only ASCII characters are recognized, but if PCRE2_UCP is set, Unicode properties are used instead to classify characters. More details are given in the section on generic character types in the pcre2pattern page. If you set PCRE2_UCP, matching one of the items it affects takes much longer. The option is available only if PCRE2 has been compiled with Unicode sup- port. PCRE2_UNGREEDY This option inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by "?". It is not compatible with Perl. It can also be set by a (?U) option setting within the pattern. PCRE2_USE_OFFSET_LIMIT This option must be set for pcre2_compile() if pcre2_set_offset_limit() is going to be used to set a non-default offset limit in a match con- text for matches that use this pattern. An error is generated if an offset limit is set without this option. For more details, see the description of pcre2_set_offset_limit() in the section that describes match contexts. See also the PCRE2_FIRSTLINE option above. PCRE2_UTF This option causes PCRE2 to regard both the pattern and the subject strings that are subsequently processed as strings of UTF characters instead of single-code-unit strings. It is available when PCRE2 is built to include Unicode support (which is the default). If Unicode support is not available, the use of this option provokes an error. Details of how this option changes the behaviour of PCRE2 are given in the pcre2unicode page. COMPILATION ERROR CODES There are over 80 positive error codes that pcre2_compile() may return (via errorcode) if it finds an error in the pattern. There are also some negative error codes that are used for invalid UTF strings. These are the same as given by pcre2_match() and pcre2_dfa_match(), and are described in the pcre2unicode page. The pcre2_get_error_message() func- tion (see "Obtaining a textual error message" below) can be called to obtain a textual error message from any error code. JUST-IN-TIME (JIT) COMPILATION int pcre2_jit_compile(pcre2_code *code, uint32_t options); int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext); void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext); pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize, PCRE2_SIZE maxsize, pcre2_general_context *gcontext); void pcre2_jit_stack_assign(pcre2_match_context *mcontext, pcre2_jit_callback callback_function, void *callback_data); void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack); These functions provide support for JIT compilation, which, if the just-in-time compiler is available, further processes a compiled pat- tern into machine code that executes much faster than the pcre2_match() interpretive matching function. Full details are given in the pcre2jit documentation. JIT compilation is a heavyweight optimization. It can take some time for patterns to be analyzed, and for one-off matches and simple pat- terns the benefit of faster execution might be offset by a much slower compilation time. Most, but not all patterns can be optimized by the JIT compiler. LOCALE SUPPORT PCRE2 handles caseless matching, and determines whether characters are letters, digits, or whatever, by reference to a set of tables, indexed by character code point. This applies only to characters whose code points are less than 256. By default, higher-valued code points never match escapes such as \w or \d. However, if PCRE2 is built with UTF support, all characters can be tested with \p and \P, or, alterna- tively, the PCRE2_UCP option can be set when a pattern is compiled; this causes \w and friends to use Unicode property support instead of the built-in tables. The use of locales with Unicode is discouraged. If you are handling characters with code points greater than 128, you should either use Unicode support, or use locales, but not try to mix the two. PCRE2 contains an internal set of character tables that are used by default. These are sufficient for many applications. Normally, the internal tables recognize only ASCII characters. However, when PCRE2 is built, it is possible to cause the internal tables to be rebuilt in the default "C" locale of the local system, which may cause them to be dif- ferent. The internal tables can be overridden by tables supplied by the appli- cation that calls PCRE2. These may be created in a different locale from the default. As more and more applications change to using Uni- code, the need for this locale support is expected to die away. External tables are built by calling the pcre2_maketables() function, in the relevant locale. The result can be passed to pcre2_compile() as often as necessary, by creating a compile context and calling pcre2_set_character_tables() to set the tables pointer therein. For example, to build and use tables that are appropriate for the French locale (where accented characters with values greater than 128 are treated as letters), the following code could be used: setlocale(LC_CTYPE, "fr_FR"); tables = pcre2_maketables(NULL); ccontext = pcre2_compile_context_create(NULL); pcre2_set_character_tables(ccontext, tables); re = pcre2_compile(..., ccontext); The locale name "fr_FR" is used on Linux and other Unix-like systems; if you are using Windows, the name for the French locale is "french". It is the caller's responsibility to ensure that the memory containing the tables remains available for as long as it is needed. The pointer that is passed (via the compile context) to pcre2_compile() is saved with the compiled pattern, and the same tables are used by pcre2_match() and pcre_dfa_match(). Thus, for any single pattern, com- pilation, and matching all happen in the same locale, but different patterns can be processed in different locales. INFORMATION ABOUT A COMPILED PATTERN int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where); The pcre2_pattern_info() function returns general information about a compiled pattern. For information about callouts, see the next section. The first argument for pcre2_pattern_info() is a pointer to the com- piled pattern. The second argument specifies which piece of information is required, and the third argument is a pointer to a variable to receive the data. If the third argument is NULL, the first argument is ignored, and the function returns the size in bytes of the variable that is required for the information requested. Otherwise, The yield of the function is zero for success, or one of the following negative num- bers: PCRE2_ERROR_NULL the argument code was NULL PCRE2_ERROR_BADMAGIC the "magic number" was not found PCRE2_ERROR_BADOPTION the value of what was invalid PCRE2_ERROR_UNSET the requested field is not set The "magic number" is placed at the start of each compiled pattern as an simple check against passing an arbitrary memory pointer. Here is a typical call of pcre2_pattern_info(), to obtain the length of the com- piled pattern: int rc; size_t length; rc = pcre2_pattern_info( re, /* result of pcre2_compile() */ PCRE2_INFO_SIZE, /* what is required */ &length); /* where to put the data */ The possible values for the second argument are defined in pcre2.h, and are as follows: PCRE2_INFO_ALLOPTIONS PCRE2_INFO_ARGOPTIONS Return a copy of the pattern's options. The third argument should point to a uint32_t variable. PCRE2_INFO_ARGOPTIONS returns exactly the options that were passed to pcre2_compile(), whereas PCRE2_INFO_ALLOP- TIONS returns the compile options as modified by any top-level (*XXX) option settings such as (*UTF) at the start of the pattern itself. For example, if the pattern /(*UTF)abc/ is compiled with the PCRE2_EXTENDED option, the result for PCRE2_INFO_ALLOPTIONS is PCRE2_EXTENDED and PCRE2_UTF. Option settings such as (?i) that can change within a pattern do not affect the result of PCRE2_INFO_ALLOP- TIONS, even if they appear right at the start of the pattern. (This was different in some earlier releases.) A pattern compiled without PCRE2_ANCHORED is automatically anchored by PCRE2 if the first significant item in every top-level branch is one of the following: ^ unless PCRE2_MULTILINE is set \A always \G always .* sometimes - see below When .* is the first significant item, anchoring is possible only when all the following are true: .* is not in an atomic group .* is not in a capturing group that is the subject of a back reference PCRE2_DOTALL is in force for .* Neither (*PRUNE) nor (*SKIP) appears in the pattern. PCRE2_NO_DOTSTAR_ANCHOR is not set. For patterns that are auto-anchored, the PCRE2_ANCHORED bit is set in the options returned for PCRE2_INFO_ALLOPTIONS. PCRE2_INFO_BACKREFMAX Return the number of the highest back reference in the pattern. The third argument should point to an uint32_t variable. Named subpatterns acquire numbers as well as names, and these count towards the highest back reference. Back references such as \4 or \g{12} match the cap- tured characters of the given group, but in addition, the check that a capturing group is set in a conditional subpattern such as (?(3)a|b) is also a back reference. Zero is returned if there are no back refer- ences. PCRE2_INFO_BSR The output is a uint32_t whose value indicates what character sequences the \R escape sequence matches. A value of PCRE2_BSR_UNICODE means that \R matches any Unicode line ending sequence; a value of PCRE2_BSR_ANY- CRLF means that \R matches only CR, LF, or CRLF. PCRE2_INFO_CAPTURECOUNT Return the highest capturing subpattern number in the pattern. In pat- terns where (?| is not used, this is also the total number of capturing subpatterns. The third argument should point to an uint32_t variable. PCRE2_INFO_FIRSTBITMAP In the absence of a single first code unit for a non-anchored pattern, pcre2_compile() may construct a 256-bit table that defines a fixed set of values for the first code unit in any match. For example, a pattern that starts with [abc] results in a table with three bits set. When code unit values greater than 255 are supported, the flag bit for 255 means "any code unit of value 255 or above". If such a table was con- structed, a pointer to it is returned. Otherwise NULL is returned. The third argument should point to an const uint8_t * variable. PCRE2_INFO_FIRSTCODETYPE Return information about the first code unit of any matched string, for a non-anchored pattern. The third argument should point to an uint32_t variable. If there is a fixed first value, for example, the letter "c" from a pattern such as (cat|cow|coyote), 1 is returned, and the charac- ter value can be retrieved using PCRE2_INFO_FIRSTCODEUNIT. If there is no fixed first value, but it is known that a match can occur only at the start of the subject or following a newline in the subject, 2 is returned. Otherwise, and for anchored patterns, 0 is returned. PCRE2_INFO_FIRSTCODEUNIT Return the value of the first code unit of any matched string in the situation where PCRE2_INFO_FIRSTCODETYPE returns 1; otherwise return 0. The third argument should point to an uint32_t variable. In the 8-bit library, the value is always less than 256. In the 16-bit library the value can be up to 0xffff. In the 32-bit library in UTF-32 mode the value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32 mode. PCRE2_INFO_HASBACKSLASHC Return 1 if the pattern contains any instances of \C, otherwise 0. The third argument should point to an uint32_t variable. PCRE2_INFO_HASCRORLF Return 1 if the pattern contains any explicit matches for CR or LF characters, otherwise 0. The third argument should point to an uint32_t variable. An explicit match is either a literal CR or LF character, or \r or \n. PCRE2_INFO_JCHANGED Return 1 if the (?J) or (?-J) option setting is used in the pattern, otherwise 0. The third argument should point to an uint32_t variable. (?J) and (?-J) set and unset the local PCRE2_DUPNAMES option, respec- tively. PCRE2_INFO_JITSIZE If the compiled pattern was successfully processed by pcre2_jit_com- pile(), return the size of the JIT compiled code, otherwise return zero. The third argument should point to a size_t variable. PCRE2_INFO_LASTCODETYPE Returns 1 if there is a rightmost literal code unit that must exist in any matched string, other than at its start. The third argument should point to an uint32_t variable. If there is no such value, 0 is returned. When 1 is returned, the code unit value itself can be retrieved using PCRE2_INFO_LASTCODEUNIT. For anchored patterns, a last literal value is recorded only if it follows something of variable length. For example, for the pattern /^a\d+z\d+/ the returned value is 1 (with "z" returned from PCRE2_INFO_LASTCODEUNIT), but for /^a\dz\d/ the returned value is 0. PCRE2_INFO_LASTCODEUNIT Return the value of the rightmost literal data unit that must exist in any matched string, other than at its start, if such a value has been recorded. The third argument should point to an uint32_t variable. If there is no such value, 0 is returned. PCRE2_INFO_MATCHEMPTY Return 1 if the pattern might match an empty string, otherwise 0. The third argument should point to an uint32_t variable. When a pattern contains recursive subroutine calls it is not always possible to deter- mine whether or not it can match an empty string. PCRE2 takes a cau- tious approach and returns 1 in such cases. PCRE2_INFO_MATCHLIMIT If the pattern set a match limit by including an item of the form (*LIMIT_MATCH=nnnn) at the start, the value is returned. The third argument should point to an unsigned 32-bit integer. If no such value has been set, the call to pcre2_pattern_info() returns the error PCRE2_ERROR_UNSET. PCRE2_INFO_MAXLOOKBEHIND Return the number of characters (not code units) in the longest lookbe- hind assertion in the pattern. The third argument should point to an unsigned 32-bit integer. This information is useful when doing multi- segment matching using the partial matching facilities. Note that the simple assertions \b and \B require a one-character lookbehind. \A also registers a one-character lookbehind, though it does not actually inspect the previous character. This is to ensure that at least one character from the old segment is retained when a new segment is pro- cessed. Otherwise, if there are no lookbehinds in the pattern, \A might match incorrectly at the start of a new segment. PCRE2_INFO_MINLENGTH If a minimum length for matching subject strings was computed, its value is returned. Otherwise the returned value is 0. The value is a number of characters, which in UTF mode may be different from the num- ber of code units. The third argument should point to an uint32_t variable. The value is a lower bound to the length of any matching string. There may not be any strings of that length that do actually match, but every string that does match is at least that long. PCRE2_INFO_NAMECOUNT PCRE2_INFO_NAMEENTRYSIZE PCRE2_INFO_NAMETABLE PCRE2 supports the use of named as well as numbered capturing parenthe- ses. The names are just an additional way of identifying the parenthe- ses, which still acquire numbers. Several convenience functions such as pcre2_substring_get_byname() are provided for extracting captured sub- strings by name. It is also possible to extract the data directly, by first converting the name to a number in order to access the correct pointers in the output vector (described with pcre2_match() below). To do the conversion, you need to use the name-to-number map, which is described by these three values. The map consists of a number of fixed-size entries. PCRE2_INFO_NAME- COUNT gives the number of entries, and PCRE2_INFO_NAMEENTRYSIZE gives the size of each entry in code units; both of these return a uint32_t value. The entry size depends on the length of the longest name. PCRE2_INFO_NAMETABLE returns a pointer to the first entry of the table. This is a PCRE2_SPTR pointer to a block of code units. In the 8-bit library, the first two bytes of each entry are the number of the cap- turing parenthesis, most significant byte first. In the 16-bit library, the pointer points to 16-bit code units, the first of which contains the parenthesis number. In the 32-bit library, the pointer points to 32-bit code units, the first of which contains the parenthesis number. The rest of the entry is the corresponding name, zero terminated. The names are in alphabetical order. If (?| is used to create multiple groups with the same number, as described in the section on duplicate subpattern numbers in the pcre2pattern page, the groups may be given the same name, but there is only one entry in the table. Different names for groups of the same number are not permitted. Duplicate names for subpatterns with different numbers are permitted, but only if PCRE2_DUPNAMES is set. They appear in the table in the order in which they were found in the pattern. In the absence of (?| this is the order of increasing number; when (?| is used this is not necessarily the case because later subpatterns may have lower numbers. As a simple example of the name/number table, consider the following pattern after compilation by the 8-bit library (assume PCRE2_EXTENDED is set, so white space - including newlines - is ignored): (?<date> (?<year>(\d\d)?\d\d) - (?<month>\d\d) - (?<day>\d\d) ) There are four named subpatterns, so the table has four entries, and each entry in the table is eight bytes long. The table is as follows, with non-printing bytes shows in hexadecimal, and undefined bytes shown as ??: 00 01 d a t e 00 ?? 00 05 d a y 00 ?? ?? 00 04 m o n t h 00 00 02 y e a r 00 ?? When writing code to extract data from named subpatterns using the name-to-number map, remember that the length of the entries is likely to be different for each compiled pattern. PCRE2_INFO_NEWLINE The output is a uint32_t with one of the following values: PCRE2_NEWLINE_CR Carriage return (CR) PCRE2_NEWLINE_LF Linefeed (LF) PCRE2_NEWLINE_CRLF Carriage return, linefeed (CRLF) PCRE2_NEWLINE_ANY Any Unicode line ending PCRE2_NEWLINE_ANYCRLF Any of CR, LF, or CRLF This specifies the default character sequence that will be recognized as meaning "newline" while matching. PCRE2_INFO_RECURSIONLIMIT If the pattern set a recursion limit by including an item of the form (*LIMIT_RECURSION=nnnn) at the start, the value is returned. The third argument should point to an unsigned 32-bit integer. If no such value has been set, the call to pcre2_pattern_info() returns the error PCRE2_ERROR_UNSET. PCRE2_INFO_SIZE Return the size of the compiled pattern in bytes (for all three libraries). The third argument should point to a size_t variable. This value includes the size of the general data block that precedes the code units of the compiled pattern itself. The value that is used when pcre2_compile() is getting memory in which to place the compiled pat- tern may be slightly larger than the value returned by this option, because there are cases where the code that calculates the size has to over-estimate. Processing a pattern with the JIT compiler does not alter the value returned by this option. INFORMATION ABOUT A PATTERN'S CALLOUTS int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data); A script language that supports the use of string arguments in callouts might like to scan all the callouts in a pattern before running the match. This can be done by calling pcre2_callout_enumerate(). The first argument is a pointer to a compiled pattern, the second points to a callback function, and the third is arbitrary user data. The callback function is called for every callout in the pattern in the order in which they appear. Its first argument is a pointer to a callout enumer- ation block, and its second argument is the user_data value that was passed to pcre2_callout_enumerate(). The contents of the callout enu- meration block are described in the pcre2callout documentation, which also gives further details about callouts. SERIALIZATION AND PRECOMPILING It is possible to save compiled patterns on disc or elsewhere, and reload them later, subject to a number of restrictions. The functions whose names begin with pcre2_serialize_ are used for this purpose. They are described in the pcre2serialize documentation. THE MATCH DATA BLOCK pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize, pcre2_general_context *gcontext); pcre2_match_data *pcre2_match_data_create_from_pattern( const pcre2_code *code, pcre2_general_context *gcontext); void pcre2_match_data_free(pcre2_match_data *match_data); Information about a successful or unsuccessful match is placed in a match data block, which is an opaque structure that is accessed by function calls. In particular, the match data block contains a vector of offsets into the subject string that define the matched part of the subject and any substrings that were captured. This is known as the ovector. Before calling pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match() you must create a match data block by calling one of the creation func- tions above. For pcre2_match_data_create(), the first argument is the number of pairs of offsets in the ovector. One pair of offsets is required to identify the string that matched the whole pattern, with another pair for each captured substring. For example, a value of 4 creates enough space to record the matched portion of the subject plus three captured substrings. A minimum of at least 1 pair is imposed by pcre2_match_data_create(), so it is always possible to return the over- all matched string. The second argument of pcre2_match_data_create() is a pointer to a gen- eral context, which can specify custom memory management for obtaining the memory for the match data block. If you are not using custom memory management, pass NULL, which causes malloc() to be used. For pcre2_match_data_create_from_pattern(), the first argument is a pointer to a compiled pattern. The ovector is created to be exactly the right size to hold all the substrings a pattern might capture. The sec- ond argument is again a pointer to a general context, but in this case if NULL is passed, the memory is obtained using the same allocator that was used for the compiled pattern (custom or default). A match data block can be used many times, with the same or different compiled patterns. You can extract information from a match data block after a match operation has finished, using functions that are described in the sections on matched strings and other match data below. When a call of pcre2_match() fails, valid data is available in the match block only when the error is PCRE2_ERROR_NOMATCH, PCRE2_ERROR_PARTIAL, or one of the error codes for an invalid UTF string. Exactly what is available depends on the error, and is detailed below. When one of the matching functions is called, pointers to the compiled pattern and the subject string are set in the match data block so that they can be referenced by the extraction functions. After running a match, you must not free a compiled pattern or a subject string until after all operations on the match data block (for that match) have taken place. When a match data block itself is no longer needed, it should be freed by calling pcre2_match_data_free(). MATCHING A PATTERN: THE TRADITIONAL FUNCTION int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext); The function pcre2_match() is called to match a subject string against a compiled pattern, which is passed in the code argument. You can call pcre2_match() with the same code argument as many times as you like, in order to find multiple matches in the subject string or to match dif- ferent subject strings with the same pattern. This function is the main matching facility of the library, and it operates in a Perl-like manner. For specialist use there is also an alternative matching function, which is described below in the section about the pcre2_dfa_match() function. Here is an example of a simple call to pcre2_match(): pcre2_match_data *md = pcre2_match_data_create(4, NULL); int rc = pcre2_match( re, /* result of pcre2_compile() */ "some string", /* the subject string */ 11, /* the length of the subject string */ 0, /* start at offset 0 in the subject */ 0, /* default options */ match_data, /* the match data block */ NULL); /* a match context; NULL means use defaults */ If the subject string is zero-terminated, the length can be given as PCRE2_ZERO_TERMINATED. A match context must be provided if certain less common matching parameters are to be changed. For details, see the sec- tion on the match context above. The string to be matched by pcre2_match() The subject string is passed to pcre2_match() as a pointer in subject, a length in length, and a starting offset in startoffset. The length and offset are in code units, not characters. That is, they are in bytes for the 8-bit library, 16-bit code units for the 16-bit library, and 32-bit code units for the 32-bit library, whether or not UTF pro- cessing is enabled. If startoffset is greater than the length of the subject, pcre2_match() returns PCRE2_ERROR_BADOFFSET. When the starting offset is zero, the search for a match starts at the beginning of the subject, and this is by far the most common case. In UTF-8 or UTF-16 mode, the starting off- set must point to the start of a character, or to the end of the sub- ject (in UTF-32 mode, one code unit equals one character, so all off- sets are valid). Like the pattern string, the subject may contain binary zeroes. A non-zero starting offset is useful when searching for another match in the same subject by calling pcre2_match() again after a previous success. Setting startoffset differs from passing over a shortened string and setting PCRE2_NOTBOL in the case of a pattern that begins with any kind of lookbehind. For example, consider the pattern \Biss\B which finds occurrences of "iss" in the middle of words. (\B matches only if the current position in the subject is not a word boundary.) When applied to the string "Mississipi" the first call to pcre2_match() finds the first occurrence. If pcre2_match() is called again with just the remainder of the subject, namely "issipi", it does not match, because \B is always false at the start of the subject, which is deemed to be a word boundary. However, if pcre2_match() is passed the entire string again, but with startoffset set to 4, it finds the second occur- rence of "iss" because it is able to look behind the starting point to discover that it is preceded by a letter. Finding all the matches in a subject is tricky when the pattern can match an empty string. It is possible to emulate Perl's /g behaviour by first trying the match again at the same offset, with the PCRE2_NOTEMPTY_ATSTART and PCRE2_ANCHORED options, and then if that fails, advancing the starting offset and trying an ordinary match again. There is some code that demonstrates how to do this in the pcre2demo sample program. In the most general case, you have to check to see if the newline convention recognizes CRLF as a newline, and if so, and the current character is CR followed by LF, advance the start- ing offset by two characters instead of one. If a non-zero starting offset is passed when the pattern is anchored, one attempt to match at the given offset is made. This can only succeed if the pattern does not require the match to be at the start of the subject. Option bits for pcre2_match() The unused bits of the options argument for pcre2_match() must be zero. The only bits that may be set are PCRE2_ANCHORED, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_JIT, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. Their action is described below. Setting PCRE2_ANCHORED at match time is not supported by the just-in- time (JIT) compiler. If it is set, JIT matching is disabled and the normal interpretive code in pcre2_match() is run. Apart from PCRE2_NO_JIT (obviously), the remaining options are supported for JIT matching. PCRE2_ANCHORED The PCRE2_ANCHORED option limits pcre2_match() to matching at the first matching position. If a pattern was compiled with PCRE2_ANCHORED, or turned out to be anchored by virtue of its contents, it cannot be made unachored at matching time. Note that setting the option at match time disables JIT matching. PCRE2_NOTBOL This option specifies that first character of the subject string is not the beginning of a line, so the circumflex metacharacter should not match before it. Setting this without having set PCRE2_MULTILINE at compile time causes circumflex never to match. This option affects only the behaviour of the circumflex metacharacter. It does not affect \A. PCRE2_NOTEOL This option specifies that the end of the subject string is not the end of a line, so the dollar metacharacter should not match it nor (except in multiline mode) a newline immediately before it. Setting this with- out having set PCRE2_MULTILINE at compile time causes dollar never to match. This option affects only the behaviour of the dollar metacharac- ter. It does not affect \Z or \z. PCRE2_NOTEMPTY An empty string is not considered to be a valid match if this option is set. If there are alternatives in the pattern, they are tried. If all the alternatives match the empty string, the entire match fails. For example, if the pattern a?b? is applied to a string not beginning with "a" or "b", it matches an empty string at the start of the subject. With PCRE2_NOTEMPTY set, this match is not valid, so pcre2_match() searches further into the string for occurrences of "a" or "b". PCRE2_NOTEMPTY_ATSTART This is like PCRE2_NOTEMPTY, except that it locks out an empty string match only at the first matching position, that is, at the start of the subject plus the starting offset. An empty string match later in the subject is permitted. If the pattern is anchored, such a match can occur only if the pattern contains \K. PCRE2_NO_JIT By default, if a pattern has been successfully processed by pcre2_jit_compile(), JIT is automatically used when pcre2_match() is called with options that JIT supports. Setting PCRE2_NO_JIT disables the use of JIT; it forces matching to be done by the interpreter. PCRE2_NO_UTF_CHECK When PCRE2_UTF is set at compile time, the validity of the subject as a UTF string is checked by default when pcre2_match() is subsequently called. If a non-zero starting offset is given, the check is applied only to that part of the subject that could be inspected during match- ing, and there is a check that the starting offset points to the first code unit of a character or to the end of the subject. If there are no lookbehind assertions in the pattern, the check starts at the starting offset. Otherwise, it starts at the length of the longest lookbehind before the starting offset, or at the start of the subject if there are not that many characters before the starting offset. Note that the sequences \b and \B are one-character lookbehinds. The check is carried out before any other processing takes place, and a negative error code is returned if the check fails. There are several UTF error codes for each code unit width, corresponding to different problems with the code unit sequence. There are discussions about the validity of UTF-8 strings, UTF-16 strings, and UTF-32 strings in the pcre2unicode page. If you know that your subject is valid, and you want to skip these checks for performance reasons, you can set the PCRE2_NO_UTF_CHECK option when calling pcre2_match(). You might want to do this for the second and subsequent calls to pcre2_match() if you are making repeated calls to find all the matches in a single subject string. NOTE: When PCRE2_NO_UTF_CHECK is set, the effect of passing an invalid string as a subject, or an invalid value of startoffset, is undefined. Your program may crash or loop indefinitely. PCRE2_PARTIAL_HARD PCRE2_PARTIAL_SOFT These options turn on the partial matching feature. A partial match occurs if the end of the subject string is reached successfully, but there are not enough subject characters to complete the match. If this happens when PCRE2_PARTIAL_SOFT (but not PCRE2_PARTIAL_HARD) is set, matching continues by testing any remaining alternatives. Only if no complete match can be found is PCRE2_ERROR_PARTIAL returned instead of PCRE2_ERROR_NOMATCH. In other words, PCRE2_PARTIAL_SOFT specifies that the caller is prepared to handle a partial match, but only if no com- plete match can be found. If PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In this case, if a partial match is found, pcre2_match() immediately returns PCRE2_ERROR_PARTIAL, without considering any other alternatives. In other words, when PCRE2_PARTIAL_HARD is set, a partial match is consid- ered to be more important that an alternative complete match. There is a more detailed discussion of partial and multi-segment match- ing, with examples, in the pcre2partial documentation. NEWLINE HANDLING WHEN MATCHING When PCRE2 is built, a default newline convention is set; this is usu- ally the standard convention for the operating system. The default can be overridden in a compile context by calling pcre2_set_newline(). It can also be overridden by starting a pattern string with, for example, (*CRLF), as described in the section on newline conventions in the pcre2pattern page. During matching, the newline choice affects the be- haviour of the dot, circumflex, and dollar metacharacters. It may also alter the way the match starting position is advanced after a match failure for an unanchored pattern. When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is set as the newline convention, and a match attempt for an unanchored pattern fails when the current starting position is at a CRLF sequence, and the pattern contains no explicit matches for CR or LF characters, the match position is advanced by two characters instead of one, in other words, to after the CRLF. The above rule is a compromise that makes the most common cases work as expected. For example, if the pattern is .+A (and the PCRE2_DOTALL option is not set), it does not match the string "\r\nA" because, after failing at the start, it skips both the CR and the LF before retrying. However, the pattern [\r\n]A does match that string, because it con- tains an explicit CR or LF reference, and so advances only by one char- acter after the first failure. An explicit match for CR of LF is either a literal appearance of one of those characters in the pattern, or one of the \r or \n escape sequences. Implicit matches such as [^X] do not count, nor does \s, even though it includes CR and LF in the characters that it matches. Notwithstanding the above, anomalous effects may still occur when CRLF is a valid newline sequence and explicit \r or \n escapes appear in the pattern. HOW PCRE2_MATCH() RETURNS A STRING AND CAPTURED SUBSTRINGS uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data); PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data); In general, a pattern matches a certain portion of the subject, and in addition, further substrings from the subject may be picked out by parenthesized parts of the pattern. Following the usage in Jeffrey Friedl's book, this is called "capturing" in what follows, and the phrase "capturing subpattern" or "capturing group" is used for a frag- ment of a pattern that picks out a substring. PCRE2 supports several other kinds of parenthesized subpattern that do not cause substrings to be captured. The pcre2_pattern_info() function can be used to find out how many capturing subpatterns there are in a compiled pattern. You can use auxiliary functions for accessing captured substrings by number or by name, as described in sections below. Alternatively, you can make direct use of the vector of PCRE2_SIZE val- ues, called the ovector, which contains the offsets of captured strings. It is part of the match data block. The function pcre2_get_ovector_pointer() returns the address of the ovector, and pcre2_get_ovector_count() returns the number of pairs of values it con- tains. Within the ovector, the first in each pair of values is set to the off- set of the first code unit of a substring, and the second is set to the offset of the first code unit after the end of a substring. These val- ues are always code unit offsets, not character offsets. That is, they are byte offsets in the 8-bit library, 16-bit offsets in the 16-bit library, and 32-bit offsets in the 32-bit library. After a partial match (error return PCRE2_ERROR_PARTIAL), only the first pair of offsets (that is, ovector[0] and ovector[1]) are set. They identify the part of the subject that was partially matched. See the pcre2partial documentation for details of partial matching. After a successful match, the first pair of offsets identifies the por- tion of the subject string that was matched by the entire pattern. The next pair is used for the first capturing subpattern, and so on. The value returned by pcre2_match() is one more than the highest numbered pair that has been set. For example, if two substrings have been cap- tured, the returned value is 3. If there are no capturing subpatterns, the return value from a successful match is 1, indicating that just the first pair of offsets has been set. If a pattern uses the \K escape sequence within a positive assertion, the reported start of a successful match can be greater than the end of the match. For example, if the pattern (?=ab\K) is matched against "ab", the start and end offset values for the match are 2 and 0. If a capturing subpattern group is matched repeatedly within a single match operation, it is the last portion of the subject that it matched that is returned. If the ovector is too small to hold all the captured substring offsets, as much as possible is filled in, and the function returns a value of zero. If captured substrings are not of interest, pcre2_match() may be called with a match data block whose ovector is of minimum length (that is, one pair). However, if the pattern contains back references and the ovector is not big enough to remember the related substrings, PCRE2 has to get additional memory for use during matching. Thus it is usually advisable to set up a match data block containing an ovector of reason- able size. It is possible for capturing subpattern number n+1 to match some part of the subject when subpattern n has not been used at all. For example, if the string "abc" is matched against the pattern (a|(z))(bc) the return from the function is 4, and subpatterns 1 and 3 are matched, but 2 is not. When this happens, both values in the offset pairs corre- sponding to unused subpatterns are set to PCRE2_UNSET. Offset values that correspond to unused subpatterns at the end of the expression are also set to PCRE2_UNSET. For example, if the string "abc" is matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not matched. The return from the function is 2, because the high- est used capturing subpattern number is 1. The offsets for for the sec- ond and third capturing subpatterns (assuming the vector is large enough, of course) are set to PCRE2_UNSET. Elements in the ovector that do not correspond to capturing parentheses in the pattern are never changed. That is, if a pattern contains n cap- turing parentheses, no more than ovector[0] to ovector[2n+1] are set by pcre2_match(). The other elements retain whatever values they previ- ously had. OTHER INFORMATION ABOUT A MATCH PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data); PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data); As well as the offsets in the ovector, other information about a match is retained in the match data block and can be retrieved by the above functions in appropriate circumstances. If they are called at other times, the result is undefined. After a successful match, a partial match (PCRE2_ERROR_PARTIAL), or a failure to match (PCRE2_ERROR_NOMATCH), a (*MARK) name may be avail- able, and pcre2_get_mark() can be called. It returns a pointer to the zero-terminated name, which is within the compiled pattern. Otherwise NULL is returned. The length of the (*MARK) name (excluding the termi- nating zero) is stored in the code unit that preceeds the name. You should use this instead of relying on the terminating zero if the (*MARK) name might contain a binary zero. After a successful match, the (*MARK) name that is returned is the last one encountered on the matching path through the pattern. After a "no match" or a partial match, the last encountered (*MARK) name is returned. For example, consider this pattern: ^(*MARK:A)((*MARK:B)a|b)c When it matches "bc", the returned mark is A. The B mark is "seen" in the first branch of the group, but it is not on the matching path. On the other hand, when this pattern fails to match "bx", the returned mark is B. After a successful match, a partial match, or one of the invalid UTF errors (for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar() can be called. After a successful or partial match it returns the code unit offset of the character at which the match started. For a non-partial match, this can be different to the value of ovector[0] if the pattern contains the \K escape sequence. After a partial match, however, this value is always the same as ovector[0] because \K does not affect the result of a partial match. After a UTF check failure, pcre2_get_startchar() can be used to obtain the code unit offset of the invalid UTF character. Details are given in the pcre2unicode page. ERROR RETURNS FROM pcre2_match() If pcre2_match() fails, it returns a negative number. This can be con- verted to a text string by calling the pcre2_get_error_message() func- tion (see "Obtaining a textual error message" below). Negative error codes are also returned by other functions, and are documented with them. The codes are given names in the header file. If UTF checking is in force and an invalid UTF subject string is detected, one of a number of UTF-specific negative error codes is returned. Details are given in the pcre2unicode page. The following are the other errors that may be returned by pcre2_match(): PCRE2_ERROR_NOMATCH The subject string did not match the pattern. PCRE2_ERROR_PARTIAL The subject string did not match, but it did match partially. See the pcre2partial documentation for details of partial matching. PCRE2_ERROR_BADMAGIC PCRE2 stores a 4-byte "magic number" at the start of the compiled code, to catch the case when it is passed a junk pointer. This is the error that is returned when the magic number is not present. PCRE2_ERROR_BADMODE This error is given when a pattern that was compiled by the 8-bit library is passed to a 16-bit or 32-bit library function, or vice versa. PCRE2_ERROR_BADOFFSET The value of startoffset was greater than the length of the subject. PCRE2_ERROR_BADOPTION An unrecognized bit was set in the options argument. PCRE2_ERROR_BADUTFOFFSET The UTF code unit sequence that was passed as a subject was checked and found to be valid (the PCRE2_NO_UTF_CHECK option was not set), but the value of startoffset did not point to the beginning of a UTF character or the end of the subject. PCRE2_ERROR_CALLOUT This error is never generated by pcre2_match() itself. It is provided for use by callout functions that want to cause pcre2_match() or pcre2_callout_enumerate() to return a distinctive error code. See the pcre2callout documentation for details. PCRE2_ERROR_INTERNAL An unexpected internal error has occurred. This error could be caused by a bug in PCRE2 or by overwriting of the compiled pattern. PCRE2_ERROR_JIT_BADOPTION This error is returned when a pattern that was successfully studied using JIT is being matched, but the matching mode (partial or complete match) does not correspond to any JIT compilation mode. When the JIT fast path function is used, this error may be also given for invalid options. See the pcre2jit documentation for more details. PCRE2_ERROR_JIT_STACKLIMIT This error is returned when a pattern that was successfully studied using JIT is being matched, but the memory available for the just-in- time processing stack is not large enough. See the pcre2jit documenta- tion for more details. PCRE2_ERROR_MATCHLIMIT The backtracking limit was reached. PCRE2_ERROR_NOMEMORY If a pattern contains back references, but the ovector is not big enough to remember the referenced substrings, PCRE2 gets a block of memory at the start of matching to use for this purpose. There are some other special cases where extra memory is needed during matching. This error is given when memory cannot be obtained. PCRE2_ERROR_NULL Either the code, subject, or match_data argument was passed as NULL. PCRE2_ERROR_RECURSELOOP This error is returned when pcre2_match() detects a recursion loop within the pattern. Specifically, it means that either the whole pat- tern or a subpattern has been called recursively for the second time at the same position in the subject string. Some simple patterns that might do this are detected and faulted at compile time, but more com- plicated cases, in particular mutual recursions between two different subpatterns, cannot be detected until matching is attempted. PCRE2_ERROR_RECURSIONLIMIT The internal recursion limit was reached. OBTAINING A TEXTUAL ERROR MESSAGE int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer, PCRE2_SIZE bufflen); A text message for an error code from any PCRE2 function (compile, match, or auxiliary) can be obtained by calling pcre2_get_error_mes- sage(). The code is passed as the first argument, with the remaining two arguments specifying a code unit buffer and its length, into which the text message is placed. Note that the message is returned in code units of the appropriate width for the library that is being used. The returned message is terminated with a trailing zero, and the func- tion returns the number of code units used, excluding the trailing zero. If the error number is unknown, the negative error code PCRE2_ERROR_BADDATA is returned. If the buffer is too small, the mes- sage is truncated (but still with a trailing zero), and the negative error code PCRE2_ERROR_NOMEMORY is returned. None of the messages are very long; a buffer size of 120 code units is ample. EXTRACTING CAPTURED SUBSTRINGS BY NUMBER int pcre2_substring_length_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_SIZE *length); int pcre2_substring_copy_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen); int pcre2_substring_get_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen); void pcre2_substring_free(PCRE2_UCHAR *buffer); Captured substrings can be accessed directly by using the ovector as described above. For convenience, auxiliary functions are provided for extracting captured substrings as new, separate, zero-terminated strings. A substring that contains a binary zero is correctly extracted and has a further zero added on the end, but the result is not, of course, a C string. The functions in this section identify substrings by number. The number zero refers to the entire matched substring, with higher numbers refer- ring to substrings captured by parenthesized groups. After a partial match, only substring zero is available. An attempt to extract any other substring gives the error PCRE2_ERROR_PARTIAL. The next section describes similar functions for extracting captured substrings by name. If a pattern uses the \K escape sequence within a positive assertion, the reported start of a successful match can be greater than the end of the match. For example, if the pattern (?=ab\K) is matched against "ab", the start and end offset values for the match are 2 and 0. In this situation, calling these functions with a zero substring number extracts a zero-length empty string. You can find the length in code units of a captured substring without extracting it by calling pcre2_substring_length_bynumber(). The first argument is a pointer to the match data block, the second is the group number, and the third is a pointer to a variable into which the length is placed. If you just want to know whether or not the substring has been captured, you can pass the third argument as NULL. The pcre2_substring_copy_bynumber() function copies a captured sub- string into a supplied buffer, whereas pcre2_substring_get_bynumber() copies it into new memory, obtained using the same memory allocation function that was used for the match data block. The first two argu- ments of these functions are a pointer to the match data block and a capturing group number. The final arguments of pcre2_substring_copy_bynumber() are a pointer to the buffer and a pointer to a variable that contains its length in code units. This is updated to contain the actual number of code units used for the extracted substring, excluding the terminating zero. For pcre2_substring_get_bynumber() the third and fourth arguments point to variables that are updated with a pointer to the new memory and the number of code units that comprise the substring, again excluding the terminating zero. When the substring is no longer needed, the memory should be freed by calling pcre2_substring_free(). The return value from all these functions is zero for success, or a negative error code. If the pattern match failed, the match failure code is returned. If a substring number greater than zero is used after a partial match, PCRE2_ERROR_PARTIAL is returned. Other possible error codes are: PCRE2_ERROR_NOMEMORY The buffer was too small for pcre2_substring_copy_bynumber(), or the attempt to get memory failed for pcre2_substring_get_bynumber(). PCRE2_ERROR_NOSUBSTRING There is no substring with that number in the pattern, that is, the number is greater than the number of capturing parentheses. PCRE2_ERROR_UNAVAILABLE The substring number, though not greater than the number of captures in the pattern, is greater than the number of slots in the ovector, so the substring could not be captured. PCRE2_ERROR_UNSET The substring did not participate in the match. For example, if the pattern is (abc)|(def) and the subject is "def", and the ovector con- tains at least two capturing slots, substring number 1 is unset. EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS int pcre2_substring_list_get(pcre2_match_data *match_data, PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr); void pcre2_substring_list_free(PCRE2_SPTR *list); The pcre2_substring_list_get() function extracts all available sub- strings and builds a list of pointers to them. It also (optionally) builds a second list that contains their lengths (in code units), excluding a terminating zero that is added to each of them. All this is done in a single block of memory that is obtained using the same memory allocation function that was used to get the match data block. This function must be called only after a successful match. If called after a partial match, the error code PCRE2_ERROR_PARTIAL is returned. The address of the memory block is returned via listptr, which is also the start of the list of string pointers. The end of the list is marked by a NULL pointer. The address of the list of lengths is returned via lengthsptr. If your strings do not contain binary zeros and you do not therefore need the lengths, you may supply NULL as the lengthsptr argu- ment to disable the creation of a list of lengths. The yield of the function is zero if all went well, or PCRE2_ERROR_NOMEMORY if the mem- ory block could not be obtained. When the list is no longer needed, it should be freed by calling pcre2_substring_list_free(). If this function encounters a substring that is unset, which can happen when capturing subpattern number n+1 matches some part of the subject, but subpattern n has not been used at all, it returns an empty string. This can be distinguished from a genuine zero-length substring by inspecting the appropriate offset in the ovector, which contain PCRE2_UNSET for unset substrings, or by calling pcre2_sub- string_length_bynumber(). EXTRACTING CAPTURED SUBSTRINGS BY NAME int pcre2_substring_number_from_name(const pcre2_code *code, PCRE2_SPTR name); int pcre2_substring_length_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_SIZE *length); int pcre2_substring_copy_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen); int pcre2_substring_get_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen); void pcre2_substring_free(PCRE2_UCHAR *buffer); To extract a substring by name, you first have to find associated num- ber. For example, for this pattern: (a+)b(?<xxx>\d+)... the number of the subpattern called "xxx" is 2. If the name is known to be unique (PCRE2_DUPNAMES was not set), you can find the number from the name by calling pcre2_substring_number_from_name(). The first argu- ment is the compiled pattern, and the second is the name. The yield of the function is the subpattern number, PCRE2_ERROR_NOSUBSTRING if there is no subpattern of that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if there is more than one subpattern of that name. Given the number, you can extract the substring directly, or use one of the functions described above. For convenience, there are also "byname" functions that correspond to the "bynumber" functions, the only difference being that the second argument is a name instead of a number. If PCRE2_DUPNAMES is set and there are duplicate names, these functions scan all the groups with the given name, and return the first named string that is set. If there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING is returned. If all groups with the name have numbers that are greater than the number of slots in the ovector, PCRE2_ERROR_UNAVAILABLE is returned. If there is at least one group with a slot in the ovector, but no group is found to be set, PCRE2_ERROR_UNSET is returned. Warning: If the pattern uses the (?| feature to set up multiple subpat- terns with the same number, as described in the section on duplicate subpattern numbers in the pcre2pattern page, you cannot use names to distinguish the different subpatterns, because names are not included in the compiled code. The matching process uses only numbers. For this reason, the use of different names for subpatterns of the same number causes an error at compile time. CREATING A NEW STRING WITH SUBSTITUTIONS int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext, PCRE2_SPTR replacement, PCRE2_SIZE rlength, PCRE2_UCHAR *outputbufferP, PCRE2_SIZE *outlengthptr); This function calls pcre2_match() and then makes a copy of the subject string in outputbuffer, replacing the part that was matched with the replacement string, whose length is supplied in rlength. This can be given as PCRE2_ZERO_TERMINATED for a zero-terminated string. Matches in which a \K item in a lookahead in the pattern causes the match to end before it starts are not supported, and give rise to an error return. The first seven arguments of pcre2_substitute() are the same as for pcre2_match(), except that the partial matching options are not permit- ted, and match_data may be passed as NULL, in which case a match data block is obtained and freed within this function, using memory manage- ment functions from the match context, if provided, or else those that were used to allocate memory for the compiled code. The outlengthptr argument must point to a variable that contains the length, in code units, of the output buffer. If the function is suc- cessful, the value is updated to contain the length of the new string, excluding the trailing zero that is automatically added. If the function is not successful, the value set via outlengthptr depends on the type of error. For syntax errors in the replacement string, the value is the offset in the replacement string where the error was detected. For other errors, the value is PCRE2_UNSET by default. This includes the case of the output buffer being too small, unless PCRE2_SUBSTITUTE_OVERFLOW_LENGTH is set (see below), in which case the value is the minimum length needed, including space for the trailing zero. Note that in order to compute the required length, pcre2_substitute() has to simulate all the matching and copying, instead of giving an error return as soon as the buffer overflows. Note also that the length is in code units, not bytes. In the replacement string, which is interpreted as a UTF string in UTF mode, and is checked for UTF validity unless the PCRE2_NO_UTF_CHECK option is set, a dollar character is an escape character that can spec- ify the insertion of characters from capturing groups or (*MARK) items in the pattern. The following forms are always recognized: $$ insert a dollar character $<n> or ${<n>} insert the contents of group <n> $*MARK or ${*MARK} insert the name of the last (*MARK) encountered Either a group number or a group name can be given for <n>. Curly brackets are required only if the following character would be inter- preted as part of the number or name. The number may be zero to include the entire matched string. For example, if the pattern a(b)c is matched with "=abc=" and the replacement string "+$1$0$1+", the result is "=+babcb+=". The facility for inserting a (*MARK) name can be used to perform simple simultaneous substitutions, as this pcre2test example shows: /(*:pear)apple|(*:orange)lemon/g,replace=${*MARK} apple lemon 2: pear orange As well as the usual options for pcre2_match(), a number of additional options can be set in the options argument. PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject string, replacing every matching substring. If this is not set, only the first matching substring is replaced. If any matched substring has zero length, after the substitution has happened, an attempt to find a non-empty match at the same position is performed. If this is not suc- cessful, the current position is advanced by one character except when CRLF is a valid newline sequence and the next two characters are CR, LF. In this case, the current position is advanced by two characters. PCRE2_SUBSTITUTE_OVERFLOW_LENGTH changes what happens when the output buffer is too small. The default action is to return PCRE2_ERROR_NOMEM- ORY immediately. If this option is set, however, pcre2_substitute() continues to go through the motions of matching and substituting (with- out, of course, writing anything) in order to compute the size of buf- fer that is needed. This value is passed back via the outlengthptr variable, with the result of the function still being PCRE2_ERROR_NOMEMORY. Passing a buffer size of zero is a permitted way of finding out how much memory is needed for given substitution. However, this does mean that the entire operation is carried out twice. Depending on the appli- cation, it may be more efficient to allocate a large buffer and free the excess afterwards, instead of using PCRE2_SUBSTITUTE_OVER- FLOW_LENGTH. PCRE2_SUBSTITUTE_UNKNOWN_UNSET causes references to capturing groups that do not appear in the pattern to be treated as unset groups. This option should be used with care, because it means that a typo in a group name or number no longer causes the PCRE2_ERROR_NOSUBSTRING error. PCRE2_SUBSTITUTE_UNSET_EMPTY causes unset capturing groups (including unknown groups when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) to be treated as empty strings when inserted as described above. If this option is not set, an attempt to insert an unset group causes the PCRE2_ERROR_UNSET error. This option does not influence the extended substitution syntax described below. PCRE2_SUBSTITUTE_EXTENDED causes extra processing to be applied to the replacement string. Without this option, only the dollar character is special, and only the group insertion forms listed above are valid. When PCRE2_SUBSTITUTE_EXTENDED is set, two things change: Firstly, backslash in a replacement string is interpreted as an escape character. The usual forms such as \n or \x{ddd} can be used to specify particular character codes, and backslash followed by any non-alphanu- meric character quotes that character. Extended quoting can be coded using \Q...\E, exactly as in pattern strings. There are also four escape sequences for forcing the case of inserted letters. The insertion mechanism has three states: no case forcing, force upper case, and force lower case. The escape sequences change the current state: \U and \L change to upper or lower case forcing, respec- tively, and \E (when not terminating a \Q quoted sequence) reverts to no case forcing. The sequences \u and \l force the next character (if it is a letter) to upper or lower case, respectively, and then the state automatically reverts to no case forcing. Case forcing applies to all inserted characters, including those from captured groups and let- ters within \Q...\E quoted sequences. Note that case forcing sequences such as \U...\E do not nest. For exam- ple, the result of processing "\Uaa\LBB\Ecc\E" is "AAbbcc"; the final \E has no effect. The second effect of setting PCRE2_SUBSTITUTE_EXTENDED is to add more flexibility to group substitution. The syntax is similar to that used by Bash: ${<n>:-<string>} ${<n>:+<string1>:<string2>} As before, <n> may be a group number or a name. The first form speci- fies a default value. If group <n> is set, its value is inserted; if not, <string> is expanded and the result inserted. The second form specifies strings that are expanded and inserted when group <n> is set or unset, respectively. The first form is just a convenient shorthand for ${<n>:+${<n>}:<string>} Backslash can be used to escape colons and closing curly brackets in the replacement strings. A change of the case forcing state within a replacement string remains in force afterwards, as shown in this pcre2test example: /(some)?(body)/substitute_extended,replace=${1:+\U:\L}HeLLo body 1: hello somebody 1: HELLO The PCRE2_SUBSTITUTE_UNSET_EMPTY option does not affect these extended substitutions. However, PCRE2_SUBSTITUTE_UNKNOWN_UNSET does cause unknown groups in the extended syntax forms to be treated as unset. If successful, pcre2_substitute() returns the number of replacements that were made. This may be zero if no matches were found, and is never greater than 1 unless PCRE2_SUBSTITUTE_GLOBAL is set. In the event of an error, a negative error code is returned. Except for PCRE2_ERROR_NOMATCH (which is never returned), errors from pcre2_match() are passed straight back. PCRE2_ERROR_NOSUBSTRING is returned for a non-existent substring inser- tion, unless PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set. PCRE2_ERROR_UNSET is returned for an unset substring insertion (includ- ing an unknown substring when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) when the simple (non-extended) syntax is used and PCRE2_SUBSTI- TUTE_UNSET_EMPTY is not set. PCRE2_ERROR_NOMEMORY is returned if the output buffer is not big enough. If the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option is set, the size of buffer that is needed is returned via outlengthptr. Note that this does not happen by default. PCRE2_ERROR_BADREPLACEMENT is used for miscellaneous syntax errors in the replacement string, with more particular errors being PCRE2_ERROR_BADREPESCAPE (invalid escape sequence), PCRE2_ERROR_REP- MISSING_BRACE (closing curly bracket not found), PCRE2_BADSUBSTITUTION (syntax error in extended group substitution), and PCRE2_BADSUBPATTERN (the pattern match ended before it started, which can happen if \K is used in an assertion). As for all PCRE2 errors, a text message that describes the error can be obtained by calling the pcre2_get_error_message() function (see "Obtaining a textual error message" above). DUPLICATE SUBPATTERN NAMES int pcre2_substring_nametable_scan(const pcre2_code *code, PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last); When a pattern is compiled with the PCRE2_DUPNAMES option, names for subpatterns are not required to be unique. Duplicate names are always allowed for subpatterns with the same number, created by using the (?| feature. Indeed, if such subpatterns are named, they are required to use the same names. Normally, patterns with duplicate names are such that in any one match, only one of the named subpatterns participates. An example is shown in the pcre2pattern documentation. When duplicates are present, pcre2_substring_copy_byname() and pcre2_substring_get_byname() return the first substring corresponding to the given name that is set. Only if none are set is PCRE2_ERROR_UNSET is returned. The pcre2_substring_number_from_name() function returns the error PCRE2_ERROR_NOUNIQUESUBSTRING when there are duplicate names. If you want to get full details of all captured substrings for a given name, you must use the pcre2_substring_nametable_scan() function. The first argument is the compiled pattern, and the second is the name. If the third and fourth arguments are NULL, the function returns a group number for a unique name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise. When the third and fourth arguments are not NULL, they must be pointers to variables that are updated by the function. After it has run, they point to the first and last entries in the name-to-number table for the given name, and the function returns the length of each entry in code units. In both cases, PCRE2_ERROR_NOSUBSTRING is returned if there are no entries for the given name. The format of the name table is described above in the section entitled Information about a pattern. Given all the relevant entries for the name, you can extract each of their numbers, and hence the captured data. FINDING ALL POSSIBLE MATCHES AT ONE POSITION The traditional matching function uses a similar algorithm to Perl, which stops when it finds the first match at a given point in the sub- ject. If you want to find all possible matches, or the longest possible match at a given position, consider using the alternative matching function (see below) instead. If you cannot use the alternative func- tion, you can kludge it up by making use of the callout facility, which is described in the pcre2callout documentation. What you have to do is to insert a callout right at the end of the pat- tern. When your callout function is called, extract and save the cur- rent matched substring. Then return 1, which forces pcre2_match() to backtrack and try other alternatives. Ultimately, when it runs out of matches, pcre2_match() will yield PCRE2_ERROR_NOMATCH. MATCHING A PATTERN: THE ALTERNATIVE FUNCTION int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext, int *workspace, PCRE2_SIZE wscount); The function pcre2_dfa_match() is called to match a subject string against a compiled pattern, using a matching algorithm that scans the subject string just once, and does not backtrack. This has different characteristics to the normal algorithm, and is not compatible with Perl. Some of the features of PCRE2 patterns are not supported. Never- theless, there are times when this kind of matching can be useful. For a discussion of the two matching algorithms, and a list of features that pcre2_dfa_match() does not support, see the pcre2matching documen- tation. The arguments for the pcre2_dfa_match() function are the same as for pcre2_match(), plus two extras. The ovector within the match data block is used in a different way, and this is described below. The other com- mon arguments are used in the same way as for pcre2_match(), so their description is not repeated here. The two additional arguments provide workspace for the function. The workspace vector should contain at least 20 elements. It is used for keeping track of multiple paths through the pattern tree. More workspace is needed for patterns and subjects where there are a lot of potential matches. Here is an example of a simple call to pcre2_dfa_match(): int wspace[20]; pcre2_match_data *md = pcre2_match_data_create(4, NULL); int rc = pcre2_dfa_match( re, /* result of pcre2_compile() */ "some string", /* the subject string */ 11, /* the length of the subject string */ 0, /* start at offset 0 in the subject */ 0, /* default options */ match_data, /* the match data block */ NULL, /* a match context; NULL means use defaults */ wspace, /* working space vector */ 20); /* number of elements (NOT size in bytes) */ Option bits for pcre_dfa_match() The unused bits of the options argument for pcre2_dfa_match() must be zero. The only bits that may be set are PCRE2_ANCHORED, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, PCRE2_PARTIAL_SOFT, PCRE2_DFA_SHORTEST, and PCRE2_DFA_RESTART. All but the last four of these are exactly the same as for pcre2_match(), so their description is not repeated here. PCRE2_PARTIAL_HARD PCRE2_PARTIAL_SOFT These have the same general effect as they do for pcre2_match(), but the details are slightly different. When PCRE2_PARTIAL_HARD is set for pcre2_dfa_match(), it returns PCRE2_ERROR_PARTIAL if the end of the subject is reached and there is still at least one matching possibility that requires additional characters. This happens even if some complete matches have already been found. When PCRE2_PARTIAL_SOFT is set, the return code PCRE2_ERROR_NOMATCH is converted into PCRE2_ERROR_PARTIAL if the end of the subject is reached, there have been no complete matches, but there is still at least one matching possibility. The por- tion of the string that was inspected when the longest partial match was found is set as the first matching string in both cases. There is a more detailed discussion of partial and multi-segment matching, with examples, in the pcre2partial documentation. PCRE2_DFA_SHORTEST Setting the PCRE2_DFA_SHORTEST option causes the matching algorithm to stop as soon as it has found one match. Because of the way the alterna- tive algorithm works, this is necessarily the shortest possible match at the first possible matching point in the subject string. PCRE2_DFA_RESTART When pcre2_dfa_match() returns a partial match, it is possible to call it again, with additional subject characters, and have it continue with the same match. The PCRE2_DFA_RESTART option requests this action; when it is set, the workspace and wscount options must reference the same vector as before because data about the match so far is left in them after a partial match. There is more discussion of this facility in the pcre2partial documentation. Successful returns from pcre2_dfa_match() When pcre2_dfa_match() succeeds, it may have matched more than one sub- string in the subject. Note, however, that all the matches from one run of the function start at the same point in the subject. The shorter matches are all initial substrings of the longer matches. For example, if the pattern <.*> is matched against the string This is <something> <something else> <something further> no more the three matched strings are <something> <something else> <something further> <something> <something else> <something> On success, the yield of the function is a number greater than zero, which is the number of matched substrings. The offsets of the sub- strings are returned in the ovector, and can be extracted by number in the same way as for pcre2_match(), but the numbers bear no relation to any capturing groups that may exist in the pattern, because DFA match- ing does not support group capture. Calls to the convenience functions that extract substrings by name return the error PCRE2_ERROR_DFA_UFUNC (unsupported function) if used after a DFA match. The convenience functions that extract substrings by number never return PCRE2_ERROR_NOSUBSTRING, and the meanings of some other errors are slightly different: PCRE2_ERROR_UNAVAILABLE The ovector is not big enough to include a slot for the given substring number. PCRE2_ERROR_UNSET There is a slot in the ovector for this substring, but there were insufficient matches to fill it. The matched strings are stored in the ovector in reverse order of length; that is, the longest matching string is first. If there were too many matches to fit into the ovector, the yield of the function is zero, and the vector is filled with the longest matches. NOTE: PCRE2's "auto-possessification" optimization usually applies to character repeats at the end of a pattern (as well as internally). For example, the pattern "a\d+" is compiled as if it were "a\d++". For DFA matching, this means that only one possible match is found. If you really do want multiple matches in such cases, either use an ungreedy repeat auch as "a\d+?" or set the PCRE2_NO_AUTO_POSSESS option when compiling. Error returns from pcre2_dfa_match() The pcre2_dfa_match() function returns a negative number when it fails. Many of the errors are the same as for pcre2_match(), as described above. There are in addition the following errors that are specific to pcre2_dfa_match(): PCRE2_ERROR_DFA_UITEM This return is given if pcre2_dfa_match() encounters an item in the pattern that it does not support, for instance, the use of \C in a UTF mode or a back reference. PCRE2_ERROR_DFA_UCOND This return is given if pcre2_dfa_match() encounters a condition item that uses a back reference for the condition, or a test for recursion in a specific group. These are not supported. PCRE2_ERROR_DFA_WSSIZE This return is given if pcre2_dfa_match() runs out of space in the workspace vector. PCRE2_ERROR_DFA_RECURSE When a recursive subpattern is processed, the matching function calls itself recursively, using private memory for the ovector and workspace. This error is given if the internal ovector is not large enough. This should be extremely rare, as a vector of size 1000 is used. PCRE2_ERROR_DFA_BADRESTART When pcre2_dfa_match() is called with the PCRE2_DFA_RESTART option, some plausibility checks are made on the contents of the workspace, which should contain data about the previous partial match. If any of these checks fail, this error is given. SEE ALSO pcre2build(3), pcre2callout(3), pcre2demo(3), pcre2matching(3), pcre2partial(3), pcre2posix(3), pcre2sample(3), pcre2stack(3), pcre2unicode(3). AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 23 December 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2BUILD(3) Library Functions Manual PCRE2BUILD(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) BUILDING PCRE2 PCRE2 is distributed with a configure script that can be used to build the library in Unix-like environments using the applications known as Autotools. Also in the distribution are files to support building using CMake instead of configure. The text file README contains general information about building with Autotools (some of which is repeated below), and also has some comments about building on various operating systems. There is a lot more information about building PCRE2 without using Autotools (including information about using CMake and building "by hand") in the text file called NON-AUTOTOOLS-BUILD. You should consult this file as well as the README file if you are building in a non-Unix-like environment. PCRE2 BUILD-TIME OPTIONS The rest of this document describes the optional features of PCRE2 that can be selected when the library is compiled. It assumes use of the configure script, where the optional features are selected or dese- lected by providing options to configure before running the make com- mand. However, the same options can be selected in both Unix-like and non-Unix-like environments if you are using CMake instead of configure to build PCRE2. If you are not using Autotools or CMake, option selection can be done by editing the config.h file, or by passing parameter settings to the compiler, as described in NON-AUTOTOOLS-BUILD. The complete list of options for configure (which includes the standard ones such as the selection of the installation directory) can be obtained by running ./configure --help The following sections include descriptions of options whose names begin with --enable or --disable. These settings specify changes to the defaults for the configure command. Because of the way that configure works, --enable and --disable always come in pairs, so the complemen- tary option always exists as well, but as it specifies the default, it is not described. BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES By default, a library called libpcre2-8 is built, containing functions that take string arguments contained in vectors of bytes, interpreted either as single-byte characters, or UTF-8 strings. You can also build two other libraries, called libpcre2-16 and libpcre2-32, which process strings that are contained in vectors of 16-bit and 32-bit code units, respectively. These can be interpreted either as single-unit characters or UTF-16/UTF-32 strings. To build these additional libraries, add one or both of the following to the configure command: --enable-pcre2-16 --enable-pcre2-32 If you do not want the 8-bit library, add --disable-pcre2-8 as well. At least one of the three libraries must be built. Note that the POSIX wrapper is for the 8-bit library only, and that pcre2grep is an 8-bit program. Neither of these are built if you select only the 16-bit or 32-bit libraries. BUILDING SHARED AND STATIC LIBRARIES The Autotools PCRE2 building process uses libtool to build both shared and static libraries by default. You can suppress an unwanted library by adding one of --disable-shared --disable-static to the configure command. UNICODE AND UTF SUPPORT By default, PCRE2 is built with support for Unicode and UTF character strings. To build it without Unicode support, add --disable-unicode to the configure command. This setting applies to all three libraries. It is not possible to build one library with Unicode support, and another without, in the same configuration. Of itself, Unicode support does not make PCRE2 treat strings as UTF-8, UTF-16 or UTF-32. To do that, applications that use the library can set the PCRE2_UTF option when they call pcre2_compile() to compile a pat- tern. Alternatively, patterns may be started with (*UTF) unless the application has locked this out by setting PCRE2_NEVER_UTF. UTF support allows the libraries to process character code points up to 0x10ffff in the strings that they handle. It also provides support for accessing the Unicode properties of such characters, using pattern escapes such as \P, \p, and \X. Only the general category properties such as Lu and Nd are supported. Details are given in the pcre2pattern documentation. Pattern escapes such as \d and \w do not by default make use of Unicode properties. The application can request that they do by setting the PCRE2_UCP option. Unless the application has set PCRE2_NEVER_UCP, a pattern may also request this by starting with (*UCP). DISABLING THE USE OF \C The \C escape sequence, which matches a single code unit, even in a UTF mode, can cause unpredictable behaviour because it may leave the cur- rent matching point in the middle of a multi-code-unit character. The application can lock it out by setting the PCRE2_NEVER_BACKSLASH_C option when calling pcre2_compile(). There is also a build-time option --enable-never-backslash-C (note the upper case C) which locks out the use of \C entirely. JUST-IN-TIME COMPILER SUPPORT Just-in-time compiler support is included in the build by specifying --enable-jit This support is available only for certain hardware architectures. If this option is set for an unsupported architecture, a building error occurs. See the pcre2jit documentation for a discussion of JIT usage. When JIT support is enabled, pcre2grep automatically makes use of it, unless you add --disable-pcre2grep-jit to the "configure" command. NEWLINE RECOGNITION By default, PCRE2 interprets the linefeed (LF) character as indicating the end of a line. This is the normal newline character on Unix-like systems. You can compile PCRE2 to use carriage return (CR) instead, by adding --enable-newline-is-cr to the configure command. There is also an --enable-newline-is-lf option, which explicitly specifies linefeed as the newline character. Alternatively, you can specify that line endings are to be indicated by the two-character sequence CRLF (CR immediately followed by LF). If you want this, add --enable-newline-is-crlf to the configure command. There is a fourth option, specified by --enable-newline-is-anycrlf which causes PCRE2 to recognize any of the three sequences CR, LF, or CRLF as indicating a line ending. Finally, a fifth option, specified by --enable-newline-is-any causes PCRE2 to recognize any Unicode newline sequence. The Unicode newline sequences are the three just mentioned, plus the single charac- ters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029). Whatever default line ending convention is selected when PCRE2 is built can be overridden by applications that use the library. At build time it is conventional to use the standard for your operating system. WHAT \R MATCHES By default, the sequence \R in a pattern matches any Unicode newline sequence, independently of what has been selected as the line ending sequence. If you specify --enable-bsr-anycrlf the default is changed so that \R matches only CR, LF, or CRLF. What- ever is selected when PCRE2 is built can be overridden by applications that use the called. HANDLING VERY LARGE PATTERNS Within a compiled pattern, offset values are used to point from one part to another (for example, from an opening parenthesis to an alter- nation metacharacter). By default, in the 8-bit and 16-bit libraries, two-byte values are used for these offsets, leading to a maximum size for a compiled pattern of around 64K code units. This is sufficient to handle all but the most gigantic patterns. Nevertheless, some people do want to process truly enormous patterns, so it is possible to compile PCRE2 to use three-byte or four-byte offsets by adding a setting such as --with-link-size=3 to the configure command. The value given must be 2, 3, or 4. For the 16-bit library, a value of 3 is rounded up to 4. In these libraries, using longer offsets slows down the operation of PCRE2 because it has to load additional data when handling them. For the 32-bit library the value is always 4 and cannot be overridden; the value of --with-link- size is ignored. AVOIDING EXCESSIVE STACK USAGE When matching with the pcre2_match() function, PCRE2 implements back- tracking by making recursive calls to an internal function called match(). In environments where the size of the stack is limited, this can severely limit PCRE2's operation. (The Unix environment does not usually suffer from this problem, but it may sometimes be necessary to increase the maximum stack size. There is a discussion in the pcre2stack documentation.) An alternative approach to recursion that uses memory from the heap to remember data, instead of using recursive function calls, has been implemented to work round the problem of lim- ited stack size. If you want to build a version of PCRE2 that works this way, add --disable-stack-for-recursion to the configure command. By default, the system functions malloc() and free() are called to manage the heap memory that is required, but cus- tom memory management functions can be called instead. PCRE2 runs noticeably more slowly when built in this way. This option affects only the pcre2_match() function; it is not relevant for pcre2_dfa_match(). LIMITING PCRE2 RESOURCE USAGE Internally, PCRE2 has a function called match(), which it calls repeat- edly (sometimes recursively) when matching a pattern with the pcre2_match() function. By controlling the maximum number of times this function may be called during a single matching operation, a limit can be placed on the resources used by a single call to pcre2_match(). The limit can be changed at run time, as described in the pcre2api documen- tation. The default is 10 million, but this can be changed by adding a setting such as --with-match-limit=500000 to the configure command. This setting has no effect on the pcre2_dfa_match() matching function. In some environments it is desirable to limit the depth of recursive calls of match() more strictly than the total number of calls, in order to restrict the maximum amount of stack (or heap, if --disable-stack- for-recursion is specified) that is used. A second limit controls this; it defaults to the value that is set for --with-match-limit, which imposes no additional constraints. However, you can set a lower limit by adding, for example, --with-match-limit-recursion=10000 to the configure command. This value can also be overridden at run time. CREATING CHARACTER TABLES AT BUILD TIME PCRE2 uses fixed tables for processing characters whose code points are less than 256. By default, PCRE2 is built with a set of tables that are distributed in the file src/pcre2_chartables.c.dist. These tables are for ASCII codes only. If you add --enable-rebuild-chartables to the configure command, the distributed tables are no longer used. Instead, a program called dftables is compiled and run. This outputs the source for new set of tables, created in the default locale of your C run-time system. (This method of replacing the tables does not work if you are cross compiling, because dftables is run on the local host. If you need to create alternative tables when cross compiling, you will have to do so "by hand".) USING EBCDIC CODE PCRE2 assumes by default that it will run in an environment where the character code is ASCII or Unicode, which is a superset of ASCII. This is the case for most computer operating systems. PCRE2 can, however, be compiled to run in an 8-bit EBCDIC environment by adding --enable-ebcdic --disable-unicode to the configure command. This setting implies --enable-rebuild-charta- bles. You should only use it if you know that you are in an EBCDIC environment (for example, an IBM mainframe operating system). It is not possible to support both EBCDIC and UTF-8 codes in the same version of the library. Consequently, --enable-unicode and --enable- ebcdic are mutually exclusive. The EBCDIC character that corresponds to an ASCII LF is assumed to have the value 0x15 by default. However, in some EBCDIC environments, 0x25 is used. In such an environment you should use --enable-ebcdic-nl25 as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR has the same value as in ASCII, namely, 0x0d. Whichever of 0x15 and 0x25 is not chosen as LF is made to correspond to the Unicode NEL char- acter (which, in Unicode, is 0x85). The options that select newline behaviour, such as --enable-newline-is- cr, and equivalent run-time options, refer to these character values in an EBCDIC environment. PCRE2GREP SUPPORT FOR EXTERNAL SCRIPTS By default, on non-Windows systems, pcre2grep supports the use of call- outs with string arguments within the patterns it is matching, in order to run external scripts. For details, see the pcre2grep documentation. This support can be disabled by adding --disable-pcre2grep-callout to the configure command. PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT By default, pcre2grep reads all files as plain text. You can build it so that it recognizes files whose names end in .gz or .bz2, and reads them with libz or libbz2, respectively, by adding one or both of --enable-pcre2grep-libz --enable-pcre2grep-libbz2 to the configure command. These options naturally require that the rel- evant libraries are installed on your system. Configuration will fail if they are not. PCRE2GREP BUFFER SIZE pcre2grep uses an internal buffer to hold a "window" on the file it is scanning, in order to be able to output "before" and "after" lines when it finds a match. The starting size of the buffer is controlled by a parameter whose default value is 20K. The buffer itself is three times this size, but because of the way it is used for holding "before" lines, the longest line that is guaranteed to be processable is the parameter size. If a longer line is encountered, pcre2grep automati- cally expands the buffer, up to a specified maximum size, whose default is 1M or the starting size, whichever is the larger. You can change the default parameter values by adding, for example, --with-pcre2grep-bufsize=51200 --with-pcre2grep-max-bufsize=2097152 to the configure command. The caller of pcre2grep can override these values by using --buffer-size and --max-buffer-size on the command line. PCRE2TEST OPTION FOR LIBREADLINE SUPPORT If you add one of --enable-pcre2test-libreadline --enable-pcre2test-libedit to the configure command, pcre2test is linked with the libreadline orlibedit library, respectively, and when its input is from a terminal, it reads it using the readline() function. This provides line-editing and history facilities. Note that libreadline is GPL-licensed, so if you distribute a binary of pcre2test linked in this way, there may be licensing issues. These can be avoided by linking instead with libedit, which has a BSD licence. Setting --enable-pcre2test-libreadline causes the -lreadline option to be added to the pcre2test build. In many operating environments with a sytem-installed readline library this is sufficient. However, in some environments (e.g. if an unmodified distribution version of readline is in use), some extra configuration may be necessary. The INSTALL file for libreadline says this: "Readline uses the termcap functions, but does not link with the termcap or curses library itself, allowing applications which link with readline the to choose an appropriate library." If your environment has not been set up so that an appropriate library is automatically included, you may need to add something like LIBS="-ncurses" immediately before the configure command. INCLUDING DEBUGGING CODE If you add --enable-debug to the configure command, additional debugging code is included in the build. This feature is intended for use by the PCRE2 maintainers. DEBUGGING WITH VALGRIND SUPPORT If you add --enable-valgrind to the configure command, PCRE2 will use valgrind annotations to mark certain memory regions as unaddressable. This allows it to detect invalid memory accesses, and is mostly useful for debugging PCRE2 itself. CODE COVERAGE REPORTING If your C compiler is gcc, you can build a version of PCRE2 that can generate a code coverage report for its test suite. To enable this, you must install lcov version 1.6 or above. Then specify --enable-coverage to the configure command and build PCRE2 in the usual way. Note that using ccache (a caching C compiler) is incompatible with code coverage reporting. If you have configured ccache to run automatically on your system, you must set the environment variable CCACHE_DISABLE=1 before running make to build PCRE2, so that ccache is not used. When --enable-coverage is used, the following addition targets are added to the Makefile: make coverage This creates a fresh coverage report for the PCRE2 test suite. It is equivalent to running "make coverage-reset", "make coverage-baseline", "make check", and then "make coverage-report". make coverage-reset This zeroes the coverage counters, but does nothing else. make coverage-baseline This captures baseline coverage information. make coverage-report This creates the coverage report. make coverage-clean-report This removes the generated coverage report without cleaning the cover- age data itself. make coverage-clean-data This removes the captured coverage data without removing the coverage files created at compile time (*.gcno). make coverage-clean This cleans all coverage data including the generated coverage report. For more information about code coverage, see the gcov and lcov docu- mentation. SUPPORT FOR FUZZERS There is a special option for use by people who want to run fuzzing tests on PCRE2: --enable-fuzz-support At present this applies only to the 8-bit library. If set, it causes an extra library called libpcre2-fuzzsupport.a to be built, but not installed. This contains a single function called LLVMFuzzerTestOneIn- put() whose arguments are a pointer to a string and the length of the string. When called, this function tries to compile the string as a pattern, and if that succeeds, to match it. This is done both with no options and with some random options bits that are generated from the string. Setting --enable-fuzz-support also causes a binary called pcre2fuzzcheck to be created. This is normally run under valgrind or used when PCRE2 is compiled with address sanitizing enabled. It calls the fuzzing function and outputs information about it is doing. The input strings are specified by arguments: if an argument starts with "=" the rest of it is a literal input string. Otherwise, it is assumed to be a file name, and the contents of the file are the test string. SEE ALSO pcre2api(3), pcre2-config(3). AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 01 November 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2CALLOUT(3) Library Functions Manual PCRE2CALLOUT(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) SYNOPSIS #include <pcre2.h> int (*pcre2_callout)(pcre2_callout_block *, void *); int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data); DESCRIPTION PCRE2 provides a feature called "callout", which is a means of tempo- rarily passing control to the caller of PCRE2 in the middle of pattern matching. The caller of PCRE2 provides an external function by putting its entry point in a match context (see pcre2_set_callout() in the pcre2api documentation). Within a regular expression, (?C<arg>) indicates a point at which the external function is to be called. Different callout points can be identified by putting a number less than 256 after the letter C. The default value is zero. Alternatively, the argument may be a delimited string. The starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is the same as the start, except for {, where the end- ing delimiter is }. If the ending delimiter is needed within the string, it must be doubled. For example, this pattern has two callout points: (?C1)abc(?C"some ""arbitrary"" text")def If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE2 automatically inserts callouts, all with number 255, before each item in the pattern except for immediately before or after a callout item in the pattern. For example, if PCRE2_AUTO_CALLOUT is used with the pattern A(?C3)B it is processed as if it were (?C255)A(?C3)B(?C255) Here is a more complicated example: A(\d{2}|--) With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) Notice that there is a callout before and after each parenthesis and alternation bar. If the pattern contains a conditional group whose con- dition is an assertion, an automatic callout is inserted immediately before the condition. Such a callout may also be inserted explicitly, for example: (?(?C9)(?=a)ab|de) (?(?C%text%)(?!=d)ab|de) This applies only to assertion conditions (because they are themselves independent groups). Callouts can be useful for tracking the progress of pattern matching. The pcre2test program has a pattern qualifier (/auto_callout) that sets automatic callouts. When any callouts are present, the output from pcre2test indicates how the pattern is being matched. This is useful information when you are trying to optimize the performance of a par- ticular pattern. MISSING CALLOUTS You should be aware that, because of optimizations in the way PCRE2 compiles and matches patterns, callouts sometimes do not happen exactly as you might expect. Auto-possessification At compile time, PCRE2 "auto-possessifies" repeated items when it knows that what follows cannot be part of the repeat. For example, a+[bc] is compiled as if it were a++[bc]. The pcre2test output when this pattern is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied to the string "aaaa" is: --->aaaa +0 ^ a+ +2 ^ ^ [bc] No match This indicates that when matching [bc] fails, there is no backtracking into a+ (because it is being treated as a++) and therefore the callouts that would be taken for the backtracks do not occur. You can disable the auto-possessify feature by passing PCRE2_NO_AUTO_POSSESS to pcre2_compile(), or starting the pattern with (*NO_AUTO_POSSESS). In this case, the output changes to this: --->aaaa +0 ^ a+ +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^^ [bc] No match This time, when matching [bc] fails, the matcher backtracks into a+ and tries again, repeatedly, until a+ itself fails. Automatic .* anchoring By default, an optimization is applied when .* is the first significant item in a pattern. If PCRE2_DOTALL is set, so that the dot can match any character, the pattern is automatically anchored. If PCRE2_DOTALL is not set, a match can start only after an internal newline or at the beginning of the subject, and pcre2_compile() remembers this. This optimization is disabled, however, if .* is in an atomic group or if there is a back reference to the capturing group in which it appears. It is also disabled if the pattern contains (*PRUNE) or (*SKIP). How- ever, the presence of callouts does not affect it. For example, if the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT and applied to the string "aa", the pcre2test output is: --->aa +0 ^ .* +2 ^ ^ \d +2 ^^ \d +2 ^ \d No match This shows that all match attempts start at the beginning of the sub- ject. In other words, the pattern is anchored. You can disable this optimization by passing PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(), or starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the out- put changes to: --->aa +0 ^ .* +2 ^ ^ \d +2 ^^ \d +2 ^ \d +0 ^ .* +2 ^^ \d +2 ^ \d No match This shows more match attempts, starting at the second subject charac- ter. Another optimization, described in the next section, means that there is no subsequent attempt to match with an empty subject. If a pattern has more than one top-level branch, automatic anchoring occurs if all branches are anchorable. Other optimizations Other optimizations that provide fast "no match" results also affect callouts. For example, if the pattern is ab(?C4)cd PCRE2 knows that any matching string must contain the letter "d". If the subject string is "abyz", the lack of "d" means that matching doesn't ever start, and the callout is never reached. However, with "abyd", though the result is still no match, the callout is obeyed. PCRE2 also knows the minimum length of a matching string, and will immediately give a "no match" return without actually running a match if the subject is not long enough, or, for unanchored patterns, if it has been scanned far enough. You can disable these optimizations by passing the PCRE2_NO_START_OPTI- MIZE option to pcre2_compile(), or by starting the pattern with (*NO_START_OPT). This slows down the matching process, but does ensure that callouts such as the example above are obeyed. THE CALLOUT INTERFACE During matching, when PCRE2 reaches a callout point, if an external function is set in the match context, it is called. This applies to both normal and DFA matching. The first argument to the callout func- tion is a pointer to a pcre2_callout block. The second argument is the void * callout data that was supplied when the callout was set up by calling pcre2_set_callout() (see the pcre2api documentation). The call- out block structure contains the following fields: uint32_t version; uint32_t callout_number; uint32_t capture_top; uint32_t capture_last; PCRE2_SIZE *offset_vector; PCRE2_SPTR mark; PCRE2_SPTR subject; PCRE2_SIZE subject_length; PCRE2_SIZE start_match; PCRE2_SIZE current_position; PCRE2_SIZE pattern_position; PCRE2_SIZE next_item_length; PCRE2_SIZE callout_string_offset; PCRE2_SIZE callout_string_length; PCRE2_SPTR callout_string; The version field contains the version number of the block format. The current version is 1; the three callout string fields were added for this version. If you are writing an application that might use an ear- lier release of PCRE2, you should check the version number before accessing any of these fields. The version number will increase in future if more fields are added, but the intention is never to remove any of the existing fields. Fields for numerical callouts For a numerical callout, callout_string is NULL, and callout_number contains the number of the callout, in the range 0-255. This is the number that follows (?C for callouts that part of the pattern; it is 255 for automatically generated callouts. Fields for string callouts For callouts with string arguments, callout_number is always zero, and callout_string points to the string that is contained within the com- piled pattern. Its length is given by callout_string_length. Duplicated ending delimiters that were present in the original pattern string have been turned into single characters, but there is no other processing of the callout string argument. An additional code unit containing binary zero is present after the string, but is not included in the length. The delimiter that was used to start the string is also stored within the pattern, immediately before the string itself. You can access this delimiter as callout_string[-1] if you need it. The callout_string_offset field is the code unit offset to the start of the callout argument string within the original pattern string. This is provided for the benefit of applications such as script languages that might need to report errors in the callout string within the pattern. Fields for all callouts The remaining fields in the callout block are the same for both kinds of callout. The offset_vector field is a pointer to the vector of capturing offsets (the "ovector") that was passed to the matching function in the match data block. When pcre2_match() is used, the contents can be inspected in order to extract substrings that have been matched so far, in the same way as for extracting substrings after a match has completed. For the DFA matching function, this field is not useful. The subject and subject_length fields contain copies of the values that were passed to the matching function. The start_match field normally contains the offset within the subject at which the current match attempt started. However, if the escape sequence \K has been encountered, this value is changed to reflect the modified starting point. If the pattern is not anchored, the callout function may be called several times from the same point in the pattern for different starting points in the subject. The current_position field contains the offset within the subject of the current match pointer. When the pcre2_match() is used, the capture_top field contains one more than the number of the highest numbered captured substring so far. If no substrings have been captured, the value of capture_top is one. This is always the case when the DFA functions are used, because they do not support captured substrings. The capture_last field contains the number of the most recently cap- tured substring. However, when a recursion exits, the value reverts to what it was outside the recursion, as do the values of all captured substrings. If no substrings have been captured, the value of cap- ture_last is 0. This is always the case for the DFA matching functions. The pattern_position field contains the offset in the pattern string to the next item to be matched. The next_item_length field contains the length of the next item to be processed in the pattern string. When the callout is at the end of the pattern, the length is zero. When the callout precedes an opening parenthesis, the length includes meta characters that follow the paren- thesis. For example, in a callout before an assertion such as (?=ab) the length is 3. For an an alternation bar or a closing parenthesis, the length is one, unless a closing parenthesis is followed by a quan- tifier, in which case its length is included. (This changed in release 10.23. In earlier releases, before an opening parenthesis the length was that of the entire subpattern, and before an alternation bar or a closing parenthesis the length was zero.) The pattern_position and next_item_length fields are intended to help in distinguishing between different automatic callouts, which all have the same callout number. However, they are set for all callouts, and are used by pcre2test to show the next item to be matched when display- ing callout information. In callouts from pcre2_match() the mark field contains a pointer to the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In callouts from the DFA matching function this field always contains NULL. RETURN VALUES FROM CALLOUTS The external callout function returns an integer to PCRE2. If the value is zero, matching proceeds as normal. If the value is greater than zero, matching fails at the current point, but the testing of other matching possibilities goes ahead, just as if a lookahead assertion had failed. If the value is less than zero, the match is abandoned, and the matching function returns the negative value. Negative values should normally be chosen from the set of PCRE2_ERROR_xxx values. In particular, PCRE2_ERROR_NOMATCH forces a standard "no match" failure. The error number PCRE2_ERROR_CALLOUT is reserved for use by callout functions; it will never be used by PCRE2 itself. CALLOUT ENUMERATION int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data); A script language that supports the use of string arguments in callouts might like to scan all the callouts in a pattern before running the match. This can be done by calling pcre2_callout_enumerate(). The first argument is a pointer to a compiled pattern, the second points to a callback function, and the third is arbitrary user data. The callback function is called for every callout in the pattern in the order in which they appear. Its first argument is a pointer to a callout enumer- ation block, and its second argument is the user_data value that was passed to pcre2_callout_enumerate(). The data block contains the fol- lowing fields: version Block version number pattern_position Offset to next item in pattern next_item_length Length of next item in pattern callout_number Number for numbered callouts callout_string_offset Offset to string within pattern callout_string_length Length of callout string callout_string Points to callout string or is NULL The version number is currently 0. It will increase if new fields are ever added to the block. The remaining fields are the same as their namesakes in the pcre2_callout block that is used for callouts during matching, as described above. Note that the value of pattern_position is unique for each callout. However, if a callout occurs inside a group that is quantified with a non-zero minimum or a fixed maximum, the group is replicated inside the compiled pattern. For example, a pattern such as /(a){2}/ is compiled as if it were /(a)(a)/. This means that the callout will be enumerated more than once, but with the same value for pattern_position in each case. The callback function should normally return zero. If it returns a non- zero value, scanning the pattern stops, and that value is returned from pcre2_callout_enumerate(). AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 29 September 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2COMPAT(3) Library Functions Manual PCRE2COMPAT(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) DIFFERENCES BETWEEN PCRE2 AND PERL This document describes the differences in the ways that PCRE2 and Perl handle regular expressions. The differences described here are with respect to Perl versions 5.10 and above. 1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does have are given in the pcre2unicode page. 2. PCRE2 allows repeat quantifiers only on parenthesized assertions, but they do not mean what you might think. For example, (?!a){3} does not assert that the next three characters are not "a". It just asserts that the next character is not "a" three times (in principle: PCRE2 optimizes this to run the assertion just once). Perl allows repeat quantifiers on other assertions such as \b, but these do not seem to have any use. 3. Capturing subpatterns that occur inside negative lookahead asser- tions are counted, but their entries in the offsets vector are never set. Perl sometimes (but not always) sets its numerical variables from inside negative assertions. 4. The following Perl escape sequences are not supported: \l, \u, \L, \U, and \N when followed by a character name or Unicode value. (\N on its own, matching a non-newline character, is supported.) In fact these are implemented by Perl's general string-handling and are not part of its pattern matching engine. If any of these are encountered by PCRE2, an error is generated by default. However, if the PCRE2_ALT_BSUX option is set, \U and \u are interpreted as ECMAScript interprets them. 5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 is built with Unicode support. The properties that can be tested with \p and \P are limited to the general category properties such as Lu and Nd, script names such as Greek or Han, and the derived properties Any and L&. PCRE2 does support the Cs (surrogate) property, which Perl does not; the Perl documentation says "Because Perl hides the need for the user to understand the internal representation of Unicode characters, there is no need to implement the somewhat messy concept of surro- gates." 6. PCRE2 does support the \Q...\E escape for quoting substrings. Char- acters in between are treated as literals. This is slightly different from Perl in that $ and @ are also handled as literals inside the quotes. In Perl, they cause variable interpolation (but of course PCRE2 does not have variables). Note the following examples: Pattern PCRE2 matches Perl matches \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyz The \Q...\E sequence is recognized both inside and outside character classes. 7. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code}) constructions. However, there is support for recursive pat- terns. This is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE2 "callout" feature allows an external function to be called during pattern matching. See the pcre2callout documentation for details. 8. Subroutine calls (whether recursive or not) are treated as atomic groups. Atomic recursion is like Python, but unlike Perl. Captured values that are set outside a subroutine call can be referenced from inside in PCRE2, but not in Perl. There is a discussion that explains these differences in more detail in the section on recursion differ- ences from Perl in the pcre2pattern page. 9. If any of the backtracking control verbs are used in a subpattern that is called as a subroutine (whether or not recursively), their effect is confined to that subpattern; it does not extend to the sur- rounding pattern. This is not always the case in Perl. In particular, if (*THEN) is present in a group that is called as a subroutine, its action is limited to that group, even if the group does not contain any | characters. Note that such subpatterns are processed as anchored at the point where they are tested. 10. If a pattern contains more than one backtracking control verb, the first one that is backtracked onto acts. For example, in the pattern A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the same as PCRE2, but there are cases where it differs. 11. Most backtracking verbs in assertions have their normal actions. They are not confined to the assertion. 12. There are some differences that are concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE2 it is set to "b". 13. PCRE2's handling of duplicate subpattern numbers and duplicate sub- pattern names is not as general as Perl's. This is a consequence of the fact the PCRE2 works internally just with numbers, using an external table to translate between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b>B), where the two capturing parentheses have the same number but different names, is not supported, and causes an error at compile time. If it were allowed, it would not be possible to distinguish which parentheses matched, because both names map to cap- turing subpattern number 1. To avoid this confusing situation, an error is given at compile time. 14. Perl used to recognize comments in some places that PCRE2 does not, for example, between the ( and ? at the start of a subpattern. If the /x modifier is set, Perl allowed white space between ( and ? though the latest Perls give an error (for a while it was just deprecated). There may still be some cases where Perl behaves differently. 15. Perl, when in warning mode, gives warnings for character classes such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter- als. PCRE2 has no warning features, so it gives an error in these cases because they are almost certainly user mistakes. 16. In PCRE2, the upper/lower case character properties Lu and Ll are not affected when case-independent matching is specified. For example, \p{Lu} always matches an upper case letter. I think Perl has changed in this respect; in the release at the time of writing (5.16), \p{Lu} and \p{Ll} match all letters, regardless of case, when case independence is specified. 17. PCRE2 provides some extensions to the Perl regular expression facilities. Perl 5.10 includes new features that are not in earlier versions of Perl, some of which (such as named parentheses) have been in PCRE2 for some time. This list is with respect to Perl 5.10: (a) Although lookbehind assertions in PCRE2 must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. Perl requires them all to have the same length. (b) From PCRE2 10.23, back references to groups of fixed length are supported in lookbehinds, provided that there is no possibility of ref- erencing a non-unique number or name. Perl does not support backrefer- ences in lookbehinds. (c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $ meta-character matches only at the very end of the string. (d) A backslash followed by a letter with no special meaning is faulted. (Perl can be made to issue a warning.) (e) If PCRE2_UNGREEDY is set, the greediness of the repetition quanti- fiers is inverted, that is, by default they are not greedy, but if fol- lowed by a question mark they are. (f) PCRE2_ANCHORED can be used at matching time to force a pattern to be tried only at the first matching position in the subject string. (g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, and PCRE2_NO_AUTO_CAPTURE options have no Perl equivalents. (h) The \R escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE2_BSR_ANYCRLF option. (i) The callout facility is PCRE2-specific. (j) The partial matching facility is PCRE2-specific. (k) The alternative matching function (pcre2_dfa_match() matches in a different way and is not Perl-compatible. (l) PCRE2 recognizes some special sequences such as (*CR) at the start of a pattern that set overall options that cannot be changed within the pattern. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 18 October 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2JIT(3) Library Functions Manual PCRE2JIT(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) PCRE2 JUST-IN-TIME COMPILER SUPPORT Just-in-time compiling is a heavyweight optimization that can greatly speed up pattern matching. However, it comes at the cost of extra pro- cessing before the match is performed, so it is of most benefit when the same pattern is going to be matched many times. This does not nec- essarily mean many calls of a matching function; if the pattern is not anchored, matching attempts may take place many times at various posi- tions in the subject, even for a single call. Therefore, if the subject string is very long, it may still pay to use JIT even for one-off matches. JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE2 libraries. JIT support applies only to the traditional Perl-compatible matching function. It does not apply when the DFA matching function is being used. The code for this support was written by Zoltan Herczeg. AVAILABILITY OF JIT SUPPORT JIT support is an optional feature of PCRE2. The "configure" option --enable-jit (or equivalent CMake option) must be set when PCRE2 is built if you want to use JIT. The support is limited to the following hardware platforms: ARM 32-bit (v5, v7, and Thumb2) ARM 64-bit Intel x86 32-bit and 64-bit MIPS 32-bit and 64-bit Power PC 32-bit and 64-bit SPARC 32-bit If --enable-jit is set on an unsupported platform, compilation fails. A program can tell if JIT support is available by calling pcre2_con- fig() with the PCRE2_CONFIG_JIT option. The result is 1 when JIT is available, and 0 otherwise. However, a simple program does not need to check this in order to use JIT. The API is implemented in a way that falls back to the interpretive code if JIT is not available. For pro- grams that need the best possible performance, there is also a "fast path" API that is JIT-specific. SIMPLE USE OF JIT To make use of the JIT support in the simplest way, all you have to do is to call pcre2_jit_compile() after successfully compiling a pattern with pcre2_compile(). This function has two arguments: the first is the compiled pattern pointer that was returned by pcre2_compile(), and the second is zero or more of the following option bits: PCRE2_JIT_COM- PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT. If JIT support is not available, a call to pcre2_jit_compile() does nothing and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the compiled pattern is passed to the JIT compiler, which turns it into machine code that executes much faster than the normal interpretive code, but yields exactly the same results. The returned value from pcre2_jit_compile() is zero on success, or a negative error code. There is a limit to the size of pattern that JIT supports, imposed by the size of machine stack that it uses. The exact rules are not docu- mented because they may change at any time, in particular, when new optimizations are introduced. If a pattern is too big, a call to pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY. PCRE2_JIT_COMPLETE requests the JIT compiler to generate code for com- plete matches. If you want to run partial matches using the PCRE2_PAR- TIAL_HARD or PCRE2_PARTIAL_SOFT options of pcre2_match(), you should set one or both of the other options as well as, or instead of PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code for each of the three modes (normal, soft partial, hard partial). When pcre2_match() is called, the appropriate code is run if it is avail- able. Otherwise, the pattern is matched using interpretive code. You can call pcre2_jit_compile() multiple times for the same compiled pattern. It does nothing if it has previously compiled code for any of the option bits. For example, you can call it once with PCRE2_JIT_COM- PLETE and (perhaps later, when you find you need partial matching) again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time it will ignore PCRE2_JIT_COMPLETE and just compile code for partial match- ing. If pcre2_jit_compile() is called with no option bits set, it imme- diately returns zero. This is an alternative way of testing whether JIT is available. At present, it is not possible to free JIT compiled code except when the entire compiled pattern is freed by calling pcre2_code_free(). In some circumstances you may need to call additional functions. These are described in the section entitled "Controlling the JIT stack" below. There are some pcre2_match() options that are not supported by JIT, and there are also some pattern items that JIT cannot handle. Details are given below. In both cases, matching automatically falls back to the interpretive code. If you want to know whether JIT was actually used for a particular match, you should arrange for a JIT callback function to be set up as described in the section entitled "Controlling the JIT stack" below, even if you do not need to supply a non-default JIT stack. Such a callback function is called whenever JIT code is about to be obeyed. If the match-time options are not right for JIT execution, the callback function is not obeyed. If the JIT compiler finds an unsupported item, no JIT data is gener- ated. You can find out if JIT matching is available after compiling a pattern by calling pcre2_pattern_info() with the PCRE2_INFO_JITSIZE option. A non-zero result means that JIT compilation was successful. A result of 0 means that JIT support is not available, or the pattern was not processed by pcre2_jit_compile(), or the JIT compiler was not able to handle the pattern. UNSUPPORTED OPTIONS AND PATTERN ITEMS The pcre2_match() options that are supported for JIT matching are PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. The PCRE2_ANCHORED option is not supported at match time. If the PCRE2_NO_JIT option is passed to pcre2_match() it disables the use of JIT, forcing matching by the interpreter code. The only unsupported pattern items are \C (match a single data unit) when running in a UTF mode, and a callout immediately before an asser- tion condition in a conditional group. RETURN VALUES FROM JIT MATCHING When a pattern is matched using JIT matching, the return values are the same as those given by the interpretive pcre2_match() code, with the addition of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means that the memory used for the JIT stack was insufficient. See "Control- ling the JIT stack" below for a discussion of JIT stack usage. The error code PCRE2_ERROR_MATCHLIMIT is returned by the JIT code if searching a very large pattern tree goes on for too long, as it is in the same circumstance when JIT is not used, but the details of exactly what is counted are not the same. The PCRE2_ERROR_RECURSIONLIMIT error code is never returned when JIT matching is used. CONTROLLING THE JIT STACK When the compiled JIT code runs, it needs a block of memory to use as a stack. By default, it uses 32K on the machine stack. However, some large or complicated patterns need more than this. The error PCRE2_ERROR_JIT_STACKLIMIT is given when there is not enough stack. Three functions are provided for managing blocks of memory for use as JIT stacks. There is further discussion about the use of JIT stacks in the section entitled "JIT stack FAQ" below. The pcre2_jit_stack_create() function creates a JIT stack. Its argu- ments are a starting size, a maximum size, and a general context (for memory allocation functions, or NULL for standard memory allocation). It returns a pointer to an opaque structure of type pcre2_jit_stack, or NULL if there is an error. The pcre2_jit_stack_free() function is used to free a stack that is no longer needed. (For the technically minded: the address space is allocated by mmap or VirtualAlloc.) JIT uses far less memory for recursion than the interpretive code, and a maximum stack size of 512K to 1M should be more than enough for any pattern. The pcre2_jit_stack_assign() function specifies which stack JIT code should use. Its arguments are as follows: pcre2_match_context *mcontext pcre2_jit_callback callback void *data The first argument is a pointer to a match context. When this is subse- quently passed to a matching function, its information determines which JIT stack is used. There are three cases for the values of the other two options: (1) If callback is NULL and data is NULL, an internal 32K block on the machine stack is used. This is the default when a match context is created. (2) If callback is NULL and data is not NULL, data must be a pointer to a valid JIT stack, the result of calling pcre2_jit_stack_create(). (3) If callback is not NULL, it must point to a function that is called with data as an argument at the start of matching, in order to set up a JIT stack. If the return from the callback function is NULL, the internal 32K stack is used; otherwise the return value must be a valid JIT stack, the result of calling pcre2_jit_stack_create(). A callback function is obeyed whenever JIT code is about to be run; it is not obeyed when pcre2_match() is called with options that are incom- patible for JIT matching. A callback function can therefore be used to determine whether a match operation was executed by JIT or by the interpreter. You may safely use the same JIT stack for more than one pattern (either by assigning directly or by callback), as long as the patterns are matched sequentially in the same thread. Currently, the only way to set up non-sequential matches in one thread is to use callouts: if a call- out function starts another match, that match must use a different JIT stack to the one used for currently suspended match(es). In a multithread application, if you do not specify a JIT stack, or if you assign or pass back NULL from a callback, that is thread-safe, because each thread has its own machine stack. However, if you assign or pass back a non-NULL JIT stack, this must be a different stack for each thread so that the application is thread-safe. Strictly speaking, even more is allowed. You can assign the same non- NULL stack to a match context that is used by any number of patterns, as long as they are not used for matching by multiple threads at the same time. For example, you could use the same stack in all compiled patterns, with a global mutex in the callback to wait until the stack is available for use. However, this is an inefficient solution, and not recommended. This is a suggestion for how a multithreaded program that needs to set up non-default JIT stacks might operate: During thread initalization thread_local_var = pcre2_jit_stack_create(...) During thread exit pcre2_jit_stack_free(thread_local_var) Use a one-line callback function return thread_local_var All the functions described in this section do nothing if JIT is not available. JIT STACK FAQ (1) Why do we need JIT stacks? PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack where the local data of the current node is pushed before checking its child nodes. Allocating real machine stack on some platforms is diffi- cult. For example, the stack chain needs to be updated every time if we extend the stack on PowerPC. Although it is possible, its updating time overhead decreases performance. So we do the recursion in memory. (2) Why don't we simply allocate blocks of memory with malloc()? Modern operating systems have a nice feature: they can reserve an address space instead of allocating memory. We can safely allocate mem- ory pages inside this address space, so the stack could grow without moving memory data (this is important because of pointers). Thus we can allocate 1M address space, and use only a single memory page (usually 4K) if that is enough. However, we can still grow up to 1M anytime if needed. (3) Who "owns" a JIT stack? The owner of the stack is the user program, not the JIT studied pattern or anything else. The user program must ensure that if a stack is being used by pcre2_match(), (that is, it is assigned to a match context that is passed to the pattern currently running), that stack must not be used by any other threads (to avoid overwriting the same memory area). The best practice for multithreaded programs is to allocate a stack for each thread, and return this stack through the JIT callback function. (4) When should a JIT stack be freed? You can free a JIT stack at any time, as long as it will not be used by pcre2_match() again. When you assign the stack to a match context, only a pointer is set. There is no reference counting or any other magic. You can free compiled patterns, contexts, and stacks in any order, any- time. Just do not call pcre2_match() with a match context pointing to an already freed stack, as that will cause SEGFAULT. (Also, do not free a stack currently used by pcre2_match() in another thread). You can also replace the stack in a context at any time when it is not in use. You should free the previous stack before assigning a replacement. (5) Should I allocate/free a stack every time before/after calling pcre2_match()? No, because this is too costly in terms of resources. However, you could implement some clever idea which release the stack if it is not used in let's say two minutes. The JIT callback can help to achieve this without keeping a list of patterns. (6) OK, the stack is for long term memory allocation. But what happens if a pattern causes stack overflow with a stack of 1M? Is that 1M kept until the stack is freed? Especially on embedded sytems, it might be a good idea to release mem- ory sometimes without freeing the stack. There is no API for this at the moment. Probably a function call which returns with the currently allocated memory for any stack and another which allows releasing mem- ory (shrinking the stack) would be a good idea if someone needs this. (7) This is too much of a headache. Isn't there any better solution for JIT stack handling? No, thanks to Windows. If POSIX threads were used everywhere, we could throw out this complicated API. FREEING JIT SPECULATIVE MEMORY void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext); The JIT executable allocator does not free all memory when it is possi- ble. It expects new allocations, and keeps some free memory around to improve allocation speed. However, in low memory conditions, it might be better to free all possible memory. You can cause this to happen by calling pcre2_jit_free_unused_memory(). Its argument is a general con- text, for custom memory management, or NULL for standard memory manage- ment. EXAMPLE CODE This is a single-threaded example that specifies a JIT stack without using a callback. A real program should include error checking after all the function calls. int rc; pcre2_code *re; pcre2_match_data *match_data; pcre2_match_context *mcontext; pcre2_jit_stack *jit_stack; re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0, &errornumber, &erroffset, NULL); rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE); mcontext = pcre2_match_context_create(NULL); jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL); pcre2_jit_stack_assign(mcontext, NULL, jit_stack); match_data = pcre2_match_data_create(re, 10); rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext); /* Process result */ pcre2_code_free(re); pcre2_match_data_free(match_data); pcre2_match_context_free(mcontext); pcre2_jit_stack_free(jit_stack); JIT FAST PATH API Because the API described above falls back to interpreted matching when JIT is not available, it is convenient for programs that are written for general use in many environments. However, calling JIT via pcre2_match() does have a performance impact. Programs that are written for use where JIT is known to be available, and which need the best possible performance, can instead use a "fast path" API to call JIT matching directly instead of calling pcre2_match() (obviously only for patterns that have been successfully processed by pcre2_jit_compile()). The fast path function is called pcre2_jit_match(), and it takes exactly the same arguments as pcre2_match(). The return values are also the same, plus PCRE2_ERROR_JIT_BADOPTION if a matching mode (partial or complete) is requested that was not compiled. Unsupported option bits (for example, PCRE2_ANCHORED) are ignored, as is the PCRE2_NO_JIT option. When you call pcre2_match(), as well as testing for invalid options, a number of other sanity checks are performed on the arguments. For exam- ple, if the subject pointer is NULL, an immediate error is given. Also, unless PCRE2_NO_UTF_CHECK is set, a UTF subject string is tested for validity. In the interests of speed, these checks do not happen on the JIT fast path, and if invalid data is passed, the result is undefined. Bypassing the sanity checks and the pcre2_match() wrapping can give speedups of more than 10%. SEE ALSO pcre2api(3) AUTHOR Philip Hazel (FAQ by Zoltan Herczeg) University Computing Service Cambridge, England. REVISION Last updated: 05 June 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2LIMITS(3) Library Functions Manual PCRE2LIMITS(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) SIZE AND OTHER LIMITATIONS There are some size limitations in PCRE2 but it is hoped that they will never in practice be relevant. The maximum size of a compiled pattern is approximately 64K code units for the 8-bit and 16-bit libraries if PCRE2 is compiled with the default internal linkage size, which is 2 bytes for these libraries. If you want to process regular expressions that are truly enormous, you can compile PCRE2 with an internal linkage size of 3 or 4 (when build- ing the 16-bit library, 3 is rounded up to 4). See the README file in the source distribution and the pcre2build documentation for details. In these cases the limit is substantially larger. However, the speed of execution is slower. In the 32-bit library, the internal linkage size is always 4. The maximum length of a source pattern string is essentially unlimited; it is the largest number a PCRE2_SIZE variable can hold. However, the program that calls pcre2_compile() can specify a smaller limit. The maximum length (in code units) of a subject string is one less than the largest number a PCRE2_SIZE variable can hold. PCRE2_SIZE is an unsigned integer type, usually defined as size_t. Its maximum value (that is ~(PCRE2_SIZE)0) is reserved as a special indicator for zero- terminated strings and unset offsets. Note that when using the traditional matching function, PCRE2 uses recursion to handle subpatterns and indefinite repetition. This means that the available stack space may limit the size of a subject string that can be processed by certain patterns. For a discussion of stack issues, see the pcre2stack documentation. All values in repeating quantifiers must be less than 65536. The maximum length of a lookbehind assertion is 65535 characters. There is no limit to the number of parenthesized subpatterns, but there can be no more than 65535 capturing subpatterns. There is, however, a limit to the depth of nesting of parenthesized subpatterns of all kinds. This is imposed in order to limit the amount of system stack used at compile time. The default limit can be specified when PCRE2 is built; the default default is 250. An application can change this limit by calling pcre2_set_parens_nest_limit() to set the limit in a compile context. The maximum length of name for a named subpattern is 32 code units, and the maximum number of named subpatterns is 10000. The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or (*THEN) verb is 255 code units for the 8-bit library and 65535 code units for the 16-bit and 32-bit libraries. The maximum length of a string argument to a callout is the largest number a 32-bit unsigned integer can hold. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 26 October 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2MATCHING(3) Library Functions Manual PCRE2MATCHING(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) PCRE2 MATCHING ALGORITHMS This document describes the two different algorithms that are available in PCRE2 for matching a compiled regular expression against a given subject string. The "standard" algorithm is the one provided by the pcre2_match() function. This works in the same as as Perl's matching function, and provide a Perl-compatible matching operation. The just- in-time (JIT) optimization that is described in the pcre2jit documenta- tion is compatible with this function. An alternative algorithm is provided by the pcre2_dfa_match() function; it operates in a different way, and is not Perl-compatible. This alter- native has advantages and disadvantages compared with the standard algorithm, and these are described below. When there is only one possible way in which a given subject string can match a pattern, the two algorithms give the same answer. A difference arises, however, when there are multiple possibilities. For example, if the pattern ^<.*> is matched against the string <something> <something else> <something further> there are three possible answers. The standard algorithm finds only one of them, whereas the alternative algorithm finds all three. REGULAR EXPRESSIONS AS TREES The set of strings that are matched by a regular expression can be rep- resented as a tree structure. An unlimited repetition in the pattern makes the tree of infinite size, but it is still a tree. Matching the pattern to a given subject string (from a given starting point) can be thought of as a search of the tree. There are two ways to search a tree: depth-first and breadth-first, and these correspond to the two matching algorithms provided by PCRE2. THE STANDARD MATCHING ALGORITHM In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- sions", the standard algorithm is an "NFA algorithm". It conducts a depth-first search of the pattern tree. That is, it proceeds along a single path through the tree, checking that the subject matches what is required. When there is a mismatch, the algorithm tries any alterna- tives at the current point, and if they all fail, it backs up to the previous branch point in the tree, and tries the next alternative branch at that level. This often involves backing up (moving to the left) in the subject string as well. The order in which repetition branches are tried is controlled by the greedy or ungreedy nature of the quantifier. If a leaf node is reached, a matching string has been found, and at that point the algorithm stops. Thus, if there is more than one possi- ble match, this algorithm returns the first one that it finds. Whether this is the shortest, the longest, or some intermediate length depends on the way the greedy and ungreedy repetition quantifiers are specified in the pattern. Because it ends up with a single path through the tree, it is rela- tively straightforward for this algorithm to keep track of the sub- strings that are matched by portions of the pattern in parentheses. This provides support for capturing parentheses and back references. THE ALTERNATIVE MATCHING ALGORITHM This algorithm conducts a breadth-first search of the tree. Starting from the first matching point in the subject, it scans the subject string from left to right, once, character by character, and as it does this, it remembers all the paths through the tree that represent valid matches. In Friedl's terminology, this is a kind of "DFA algorithm", though it is not implemented as a traditional finite state machine (it keeps multiple states active simultaneously). Although the general principle of this matching algorithm is that it scans the subject string only once, without backtracking, there is one exception: when a lookaround assertion is encountered, the characters following or preceding the current point have to be independently inspected. The scan continues until either the end of the subject is reached, or there are no more unterminated paths. At this point, terminated paths represent the different matching possibilities (if there are none, the match has failed). Thus, if there is more than one possible match, this algorithm finds all of them, and in particular, it finds the long- est. The matches are returned in decreasing order of length. There is an option to stop the algorithm after the first match (which is neces- sarily the shortest) is found. Note that all the matches that are found start at the same point in the subject. If the pattern cat(er(pillar)?)? is matched against the string "the caterpillar catchment", the result is the three strings "caterpillar", "cater", and "cat" that start at the fifth character of the subject. The algorithm does not automati- cally move on to find matches that start at later positions. PCRE2's "auto-possessification" optimization usually applies to charac- ter repeats at the end of a pattern (as well as internally). For exam- ple, the pattern "a\d+" is compiled as if it were "a\d++" because there is no point even considering the possibility of backtracking into the repeated digits. For DFA matching, this means that only one possible match is found. If you really do want multiple matches in such cases, either use an ungreedy repeat ("a\d+?") or set the PCRE2_NO_AUTO_POS- SESS option when compiling. There are a number of features of PCRE2 regular expressions that are not supported by the alternative matching algorithm. They are as fol- lows: 1. Because the algorithm finds all possible matches, the greedy or ungreedy nature of repetition quantifiers is not relevant (though it may affect auto-possessification, as just described). During matching, greedy and ungreedy quantifiers are treated in exactly the same way. However, possessive quantifiers can make a difference when what follows could also match what is quantified, for example in a pattern like this: ^a++\w! This pattern matches "aaab!" but not "aaa!", which would be matched by a non-possessive quantifier. Similarly, if an atomic group is present, it is matched as if it were a standalone pattern at the current point, and the longest match is then "locked in" for the rest of the overall pattern. 2. When dealing with multiple paths through the tree simultaneously, it is not straightforward to keep track of captured substrings for the different matching possibilities, and PCRE2's implementation of this algorithm does not attempt to do this. This means that no captured sub- strings are available. 3. Because no substrings are captured, back references within the pat- tern are not supported, and cause errors if encountered. 4. For the same reason, conditional expressions that use a backrefer- ence as the condition or test for a specific group recursion are not supported. 5. Because many paths through the tree may be active, the \K escape sequence, which resets the start of the match when encountered (but may be on some paths and not on others), is not supported. It causes an error if encountered. 6. Callouts are supported, but the value of the capture_top field is always 1, and the value of the capture_last field is always 0. 7. The \C escape sequence, which (in the standard algorithm) always matches a single code unit, even in a UTF mode, is not supported in these modes, because the alternative algorithm moves through the sub- ject string one character (not code unit) at a time, for all active paths through the tree. 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not supported. (*FAIL) is supported, and behaves like a failing negative assertion. ADVANTAGES OF THE ALTERNATIVE ALGORITHM Using the alternative matching algorithm provides the following advan- tages: 1. All possible matches (at a single point in the subject) are automat- ically found, and in particular, the longest match is found. To find more than one match using the standard algorithm, you have to do kludgy things with callouts. 2. Because the alternative algorithm scans the subject string just once, and never needs to backtrack (except for lookbehinds), it is pos- sible to pass very long subject strings to the matching function in several pieces, checking for partial matching each time. Although it is also possible to do multi-segment matching using the standard algo- rithm, by retaining partially matched substrings, it is more compli- cated. The pcre2partial documentation gives details of partial matching and discusses multi-segment matching. DISADVANTAGES OF THE ALTERNATIVE ALGORITHM The alternative algorithm suffers from a number of disadvantages: 1. It is substantially slower than the standard algorithm. This is partly because it has to search for all possible matches, but is also because it is less susceptible to optimization. 2. Capturing parentheses and back references are not supported. 3. Although atomic groups are supported, their use does not provide the performance advantage that it does for the standard algorithm. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 29 September 2014 Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------ PCRE2PARTIAL(3) Library Functions Manual PCRE2PARTIAL(3) NAME PCRE2 - Perl-compatible regular expressions PARTIAL MATCHING IN PCRE2 In normal use of PCRE2, if the subject string that is passed to a matching function matches as far as it goes, but is too short to match the entire pattern, PCRE2_ERROR_NOMATCH is returned. There are circum- stances where it might be helpful to distinguish this case from other cases in which there is no match. Consider, for example, an application where a human is required to type in data for a field with specific formatting requirements. An example might be a date in the form ddmmmyy, defined by this pattern: ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ If the application sees the user's keystrokes one by one, and can check that what has been typed so far is potentially valid, it is able to raise an error as soon as a mistake is made, by beeping and not reflecting the character that has been typed, for example. This immedi- ate feedback is likely to be a better user interface than a check that is delayed until the entire string has been entered. Partial matching can also be useful when the subject string is very long and is not all available at once. PCRE2 supports partial matching by means of the PCRE2_PARTIAL_SOFT and PCRE2_PARTIAL_HARD options, which can be set when calling a matching function. The difference between the two options is whether or not a partial match is preferred to an alternative complete match, though the details differ between the two types of matching function. If both options are set, PCRE2_PARTIAL_HARD takes precedence. If you want to use partial matching with just-in-time optimized code, you must call pcre2_jit_compile() with one or both of these options: PCRE2_JIT_PARTIAL_SOFT PCRE2_JIT_PARTIAL_HARD PCRE2_JIT_COMPLETE should also be set if you are going to run non-par- tial matches on the same pattern. If the appropriate JIT mode has not been compiled, interpretive matching code is used. Setting a partial matching option disables two of PCRE2's standard optimizations. PCRE2 remembers the last literal code unit in a pattern, and abandons matching immediately if it is not present in the subject string. This optimization cannot be used for a subject string that might match only partially. PCRE2 also knows the minimum length of a matching string, and does not bother to run the matching function on shorter strings. This optimization is also disabled for partial match- ing. PARTIAL MATCHING USING pcre2_match() A partial match occurs during a call to pcre2_match() when the end of the subject string is reached successfully, but matching cannot con- tinue because more characters are needed. However, at least one charac- ter in the subject must have been inspected. This character need not form part of the final matched string; lookbehind assertions and the \K escape sequence provide ways of inspecting characters before the start of a matched string. The requirement for inspecting at least one char- acter exists because an empty string can always be matched; without such a restriction there would always be a partial match of an empty string at the end of the subject. When a partial match is returned, the first two elements in the ovector point to the portion of the subject that was matched, but the values in the rest of the ovector are undefined. The appearance of \K in the pat- tern has no effect for a partial match. Consider this pattern: /abc\K123/ If it is matched against "456abc123xyz" the result is a complete match, and the ovector defines the matched string as "123", because \K resets the "start of match" point. However, if a partial match is requested and the subject string is "456abc12", a partial match is found for the string "abc12", because all these characters are needed for a subse- quent re-match with additional characters. What happens when a partial match is identified depends on which of the two partial matching options are set. PCRE2_PARTIAL_SOFT WITH pcre2_match() If PCRE2_PARTIAL_SOFT is set when pcre2_match() identifies a partial match, the partial match is remembered, but matching continues as nor- mal, and other alternatives in the pattern are tried. If no complete match can be found, PCRE2_ERROR_PARTIAL is returned instead of PCRE2_ERROR_NOMATCH. This option is "soft" because it prefers a complete match over a par- tial match. All the various matching items in a pattern behave as if the subject string is potentially complete. For example, \z, \Z, and $ match at the end of the subject, as normal, and for \b and \B the end of the subject is treated as a non-alphanumeric. If there is more than one partial match, the first one that was found provides the data that is returned. Consider this pattern: /123\w+X|dogY/ If this is matched against the subject string "abc123dog", both alter- natives fail to match, but the end of the subject is reached during matching, so PCRE2_ERROR_PARTIAL is returned. The offsets are set to 3 and 9, identifying "123dog" as the first partial match that was found. (In this example, there are two partial matches, because "dog" on its own partially matches the second alternative.) PCRE2_PARTIAL_HARD WITH pcre2_match() If PCRE2_PARTIAL_HARD is set for pcre2_match(), PCRE2_ERROR_PARTIAL is returned as soon as a partial match is found, without continuing to search for possible complete matches. This option is "hard" because it prefers an earlier partial match over a later complete match. For this reason, the assumption is made that the end of the supplied subject string may not be the true end of the available data, and so, if \z, \Z, \b, \B, or $ are encountered at the end of the subject, the result is PCRE2_ERROR_PARTIAL, provided that at least one character in the subject has been inspected. Comparing hard and soft partial matching The difference between the two partial matching options can be illus- trated by a pattern such as: /dog(sbody)?/ This matches either "dog" or "dogsbody", greedily (that is, it prefers the longer string if possible). If it is matched against the string "dog" with PCRE2_PARTIAL_SOFT, it yields a complete match for "dog". However, if PCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PAR- TIAL. On the other hand, if the pattern is made ungreedy the result is different: /dog(sbody)??/ In this case the result is always a complete match because that is found first, and matching never continues after finding a complete match. It might be easier to follow this explanation by thinking of the two patterns like this: /dog(sbody)?/ is the same as /dogsbody|dog/ /dog(sbody)??/ is the same as /dog|dogsbody/ The second pattern will never match "dogsbody", because it will always find the shorter match first. PARTIAL MATCHING USING pcre2_dfa_match() The DFA functions move along the subject string character by character, without backtracking, searching for all possible matches simultane- ously. If the end of the subject is reached before the end of the pat- tern, there is the possibility of a partial match, again provided that at least one character has been inspected. When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if there have been no complete matches. Otherwise, the complete matches are returned. However, if PCRE2_PARTIAL_HARD is set, a partial match takes precedence over any complete matches. The portion of the string that was matched when the longest partial match was found is set as the first matching string. Because the DFA functions always search for all possible matches, and there is no difference between greedy and ungreedy repetition, their behaviour is different from the standard functions when PCRE2_PAR- TIAL_HARD is set. Consider the string "dog" matched against the ungreedy pattern shown above: /dog(sbody)??/ Whereas the standard function stops as soon as it finds the complete match for "dog", the DFA function also finds the partial match for "dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set. PARTIAL MATCHING AND WORD BOUNDARIES If a pattern ends with one of sequences \b or \B, which test for word boundaries, partial matching with PCRE2_PARTIAL_SOFT can give counter- intuitive results. Consider this pattern: /\bcat\b/ This matches "cat", provided there is a word boundary at either end. If the subject string is "the cat", the comparison of the final "t" with a following character cannot take place, so a partial match is found. However, normal matching carries on, and \b matches at the end of the subject when the last character is a letter, so a complete match is found. The result, therefore, is not PCRE2_ERROR_PARTIAL. Using PCRE2_PARTIAL_HARD in this case does yield PCRE2_ERROR_PARTIAL, because then the partial match takes precedence. EXAMPLE OF PARTIAL MATCHING USING PCRE2TEST If the partial_soft (or ps) modifier is present on a pcre2test data line, the PCRE2_PARTIAL_SOFT option is used for the match. Here is a run of pcre2test that uses the date example quoted above: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 25jun04\=ps 0: 25jun04 1: jun data> 25dec3\=ps Partial match: 23dec3 data> 3ju\=ps Partial match: 3ju data> 3juj\=ps No match data> j\=ps No match The first data string is matched completely, so pcre2test shows the matched substrings. The remaining four strings do not match the com- plete pattern, but the first two are partial matches. Similar output is obtained if DFA matching is used. If the partial_hard (or ph) modifier is present on a pcre2test data line, the PCRE2_PARTIAL_HARD option is set for the match. MULTI-SEGMENT MATCHING WITH pcre2_dfa_match() When a partial match has been found using a DFA matching function, it is possible to continue the match by providing additional subject data and calling the function again with the same compiled regular expres- sion, this time setting the PCRE2_DFA_RESTART option. You must pass the same working space as before, because this is where details of the pre- vious partial match are stored. Here is an example using pcre2test: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 23ja\=dfa,ps Partial match: 23ja data> n05\=dfa,dfa_restart 0: n05 The first call has "23ja" as the subject, and requests partial match- ing; the second call has "n05" as the subject for the continued (restarted) match. Notice that when the match is complete, only the last part is shown; PCRE2 does not retain the previously partially- matched string. It is up to the calling program to do that if it needs to. That means that, for an unanchored pattern, if a continued match fails, it is not possible to try again at a new starting point. All this facility is capable of doing is continuing with the previous match attempt. In the previous example, if the second set of data is "ug23" the result is no match, even though there would be a match for "aug23" if the entire string were given at once. Depending on the application, this may or may not be what you want. The only way to allow for start- ing again at the next character is to retain the matched part of the subject and try a new complete match. You can set the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with PCRE2_DFA_RESTART to continue partial matching over multiple segments. This facility can be used to pass very long subject strings to the DFA matching functions. MULTI-SEGMENT MATCHING WITH pcre2_match() Unlike the DFA function, it is not possible to restart the previous match with a new segment of data when using pcre2_match(). Instead, new data must be added to the previous subject string, and the entire match re-run, starting from the point where the partial match occurred. Ear- lier data can be discarded. It is best to use PCRE2_PARTIAL_HARD in this situation, because it does not treat the end of a segment as the end of the subject when matching \z, \Z, \b, \B, and $. Consider an unanchored pattern that matches dates: re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ data> The date is 23ja\=ph Partial match: 23ja At this stage, an application could discard the text preceding "23ja", add on text from the next segment, and call the matching function again. Unlike the DFA matching function, the entire matching string must always be available, and the complete matching process occurs for each call, so more memory and more processing time is needed. ISSUES WITH MULTI-SEGMENT MATCHING Certain types of pattern may give problems with multi-segment matching, whichever matching function is used. 1. If the pattern contains a test for the beginning of a line, you need to pass the PCRE2_NOTBOL option when the subject string for any call does start at the beginning of a line. There is also a PCRE2_NOTEOL option, but in practice when doing multi-segment matching you should be using PCRE2_PARTIAL_HARD, which includes the effect of PCRE2_NOTEOL. 2. If a pattern contains a lookbehind assertion, characters that pre- cede the start of the partial match may have been inspected during the matching process. When using pcre2_match(), sufficient characters must be retained for the next match attempt. You can ensure that enough characters are retained by doing the following: Before doing any matching, find the length of the longest lookbehind in the pattern by calling pcre2_pattern_info() with the PCRE2_INFO_MAXLOOKBEHIND option. Note that the resulting count is in characters, not code units. After a partial match, moving back from the ovector[0] offset in the subject by the number of characters given for the maximum lookbehind gets you to the earliest character that must be retained. In a non-UTF or a 32-bit situation, moving back is just a subtraction, but in UTF-8 or UTF-16 you have to count characters while moving back through the code units. Characters before the point you have now reached can be discarded, and after the next segment has been added to what is retained, you should run the next match with the startoffset argument set so that the match begins at the same point as before. For example, if the pattern "(?<=123)abc" is partially matched against the string "xx123ab", the ovector offsets are 5 and 7 ("ab"). The maxi- mum lookbehind count is 3, so all characters before offset 2 can be discarded. The value of startoffset for the next match should be 3. When pcre2test displays a partial match, it indicates the lookbehind characters with '<' characters: re> "(?<=123)abc" data> xx123ab\=ph Partial match: 123ab <<< 3. Because a partial match must always contain at least one character, what might be considered a partial match of an empty string actually gives a "no match" result. For example: re> /c(?<=abc)x/ data> ab\=ps No match If the next segment begins "cx", a match should be found, but this will only happen if characters from the previous segment are retained. For this reason, a "no match" result should be interpreted as "partial match of an empty string" when the pattern contains lookbehinds. 4. Matching a subject string that is split into multiple segments may not always produce exactly the same result as matching over one single long string, especially when PCRE2_PARTIAL_SOFT is used. The section "Partial Matching and Word Boundaries" above describes an issue that arises if the pattern ends with \b or \B. Another kind of difference may occur when there are multiple matching possibilities, because (for PCRE2_PARTIAL_SOFT) a partial match result is given only when there are no completed matches. This means that as soon as the shortest match has been found, continuation to a new subject segment is no longer possi- ble. Consider this pcre2test example: re> /dog(sbody)?/ data> dogsb\=ps 0: dog data> do\=ps,dfa Partial match: do data> gsb\=ps,dfa,dfa_restart 0: g data> dogsbody\=dfa 0: dogsbody 1: dog The first data line passes the string "dogsb" to a standard matching function, setting the PCRE2_PARTIAL_SOFT option. Although the string is a partial match for "dogsbody", the result is not PCRE2_ERROR_PARTIAL, because the shorter string "dog" is a complete match. Similarly, when the subject is presented to a DFA matching function in several parts ("do" and "gsb" being the first two) the match stops when "dog" has been found, and it is not possible to continue. On the other hand, if "dogsbody" is presented as a single string, a DFA matching function finds both matches. Because of these problems, it is best to use PCRE2_PARTIAL_HARD when matching multi-segment data. The example above then behaves differ- ently: re> /dog(sbody)?/ data> dogsb\=ph Partial match: dogsb data> do\=ps,dfa Partial match: do data> gsb\=ph,dfa,dfa_restart Partial match: gsb 5. Patterns that contain alternatives at the top level which do not all start with the same pattern item may not work as expected when PCRE2_DFA_RESTART is used. For example, consider this pattern: 1234|3789 If the first part of the subject is "ABC123", a partial match of the first alternative is found at offset 3. There is no partial match for the second alternative, because such a match does not start at the same point in the subject string. Attempting to continue with the string "7890" does not yield a match because only those alternatives that match at one point in the subject are remembered. The problem arises because the start of the second alternative matches within the first alternative. There is no problem with anchored patterns or patterns such as: 1234|ABCD where no string can be a partial match for both alternatives. This is not a problem if a standard matching function is used, because the entire match has to be rerun each time: re> /1234|3789/ data> ABC123\=ph Partial match: 123 data> 1237890 0: 3789 Of course, instead of using PCRE2_DFA_RESTART, the same technique of re-running the entire match can also be used with the DFA matching function. Another possibility is to work with two buffers. If a partial match at offset n in the first buffer is followed by "no match" when PCRE2_DFA_RESTART is used on the second buffer, you can then try a new match starting at offset n+1 in the first buffer. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 22 December 2014 Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------ PCRE2PATTERN(3) Library Functions Manual PCRE2PATTERN(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) PCRE2 REGULAR EXPRESSION DETAILS The syntax and semantics of the regular expressions that are supported by PCRE2 are described in detail below. There is a quick-reference syn- tax summary in the pcre2syntax page. PCRE2 tries to match Perl syntax and semantics as closely as it can. PCRE2 also supports some alterna- tive regular expression syntax (which does not conflict with the Perl syntax) in order to provide some compatibility with regular expressions in Python, .NET, and Oniguruma. Perl's regular expressions are described in its own documentation, and regular expressions in general are covered in a number of books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers regular expressions in great detail. This description of PCRE2's regular expressions is intended as reference material. This document discusses the patterns that are supported by PCRE2 when its main matching function, pcre2_match(), is used. PCRE2 also has an alternative matching function, pcre2_dfa_match(), which matches using a different algorithm that is not Perl-compatible. Some of the features discussed below are not available when DFA matching is used. The advan- tages and disadvantages of the alternative function, and how it differs from the normal function, are discussed in the pcre2matching page. SPECIAL START-OF-PATTERN ITEMS A number of options that can be passed to pcre2_compile() can also be set by special items at the start of a pattern. These are not Perl-com- patible, but are provided to make these options accessible to pattern writers who are not able to change the program that processes the pat- tern. Any number of these items may appear, but they must all be together right at the start of the pattern string, and the letters must be in upper case. UTF support In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either as single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 can be specified for the 32-bit library, in which case it constrains the character values to valid Unicode code points. To process UTF strings, PCRE2 must be built to include Unicode support (which is the default). When using UTF strings you must either call the compiling function with the PCRE2_UTF option, or the pattern must start with the special sequence (*UTF), which is equivalent to setting the relevant option. How setting a UTF mode affects pattern matching is mentioned in several places below. There is also a summary of features in the pcre2unicode page. Some applications that allow their users to supply patterns may wish to restrict them to non-UTF data for security reasons. If the PCRE2_NEVER_UTF option is passed to pcre2_compile(), (*UTF) is not allowed, and its appearance in a pattern causes an error. Unicode property support Another special sequence that may appear at the start of a pattern is (*UCP). This has the same effect as setting the PCRE2_UCP option: it causes sequences such as \d and \w to use Unicode properties to deter- mine character types, instead of recognizing only characters with codes less than 256 via a lookup table. Some applications that allow their users to supply patterns may wish to restrict them for security reasons. If the PCRE2_NEVER_UCP option is passed to pcre2_compile(), (*UCP) is not allowed, and its appearance in a pattern causes an error. Locking out empty string matching Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same effect as passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option to whichever matching function is subsequently called to match the pat- tern. These options lock out the matching of empty strings, either entirely, or only at the start of the subject. Disabling auto-possessification If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making quantifiers possessive when what follows cannot match the repeated item. For example, by default a+b is treated as a++b. For more details, see the pcre2api documentation. Disabling start-up optimizations If a pattern starts with (*NO_START_OPT), it has the same effect as setting the PCRE2_NO_START_OPTIMIZE option. This disables several opti- mizations for quickly reaching "no match" results. For more details, see the pcre2api documentation. Disabling automatic anchoring If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect as setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimiza- tions that apply to patterns whose top-level branches all start with .* (match any number of arbitrary characters). For more details, see the pcre2api documentation. Disabling JIT compilation If a pattern that starts with (*NO_JIT) is successfully compiled, an attempt by the application to apply the JIT optimization by calling pcre2_jit_compile() is ignored. Setting match and recursion limits The caller of pcre2_match() can set a limit on the number of times the internal match() function is called and on the maximum depth of recur- sive calls. These facilities are provided to catch runaway matches that are provoked by patterns with huge matching trees (a typical example is a pattern with nested unlimited repeats) and to avoid running out of system stack by too much recursion. When one of these limits is reached, pcre2_match() gives an error return. The limits can also be set by items at the start of the pattern of the form (*LIMIT_MATCH=d) (*LIMIT_RECURSION=d) where d is any number of decimal digits. However, the value of the set- ting must be less than the value set (or defaulted) by the caller of pcre2_match() for it to have any effect. In other words, the pattern writer can lower the limits set by the programmer, but not raise them. If there is more than one setting of one of these limits, the lower value is used. The match limit is used (but in a different way) when JIT is being used, but it is not relevant, and is ignored, when matching with pcre2_dfa_match(). However, the recursion limit is relevant for DFA matching, which does use some function recursion, in particular, for recursions within the pattern. Newline conventions PCRE2 supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (line- feed) character, the two-character sequence CRLF, any of the three pre- ceding, or any Unicode newline sequence. The pcre2api page has further discussion about newlines, and shows how to set the newline convention when calling pcre2_compile(). It is also possible to specify a newline convention by starting a pat- tern string with one of the following five sequences: (*CR) carriage return (*LF) linefeed (*CRLF) carriage return, followed by linefeed (*ANYCRLF) any of the three above (*ANY) all Unicode newline sequences These override the default and the options given to the compiling func- tion. For example, on a Unix system where LF is the default newline sequence, the pattern (*CR)a.b changes the convention to CR. That pattern matches "a\nb" because LF is no longer a newline. If more than one of these settings is present, the last one is used. The newline convention affects where the circumflex and dollar asser- tions are true. It also affects the interpretation of the dot metachar- acter when PCRE2_DOTALL is not set, and the behaviour of \N. However, it does not affect what the \R escape sequence matches. By default, this is any Unicode newline sequence, for Perl compatibility. However, this can be changed; see the description of \R in the section entitled "Newline sequences" below. A change of \R setting can be combined with a change of newline convention. Specifying what \R matches It is possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF at compile time. This effect can also be achieved by starting a pattern with (*BSR_ANYCRLF). For completeness, (*BSR_UNI- CODE) is also recognized, corresponding to PCRE2_BSR_UNICODE. EBCDIC CHARACTER CODES PCRE2 can be compiled to run in an environment that uses EBCDIC as its character code rather than ASCII or Unicode (typically a mainframe sys- tem). In the sections below, character code values are ASCII or Uni- code; in an EBCDIC environment these characters may have different code values, and there are no code points greater than 255. CHARACTERS AND METACHARACTERS A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern The quick brown fox matches a portion of a subject string that is identical to itself. When caseless matching is specified (the PCRE2_CASELESS option), letters are matched independently of case. The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of metacharacters, which do not stand for themselves but instead are interpreted in some special way. There are two different sets of metacharacters: those that are recog- nized anywhere in the pattern except within square brackets, and those that are recognized within square brackets. Outside square brackets, the metacharacters are as follows: \ general escape character with several uses ^ assert start of string (or line, in multiline mode) $ assert end of string (or line, in multiline mode) . match any character except newline (by default) [ start character class definition | start of alternative branch ( start subpattern ) end subpattern ? extends the meaning of ( also 0 or 1 quantifier also quantifier minimizer * 0 or more quantifier + 1 or more quantifier also "possessive quantifier" { start min/max quantifier Part of a pattern that is in square brackets is called a "character class". In a character class the only metacharacters are: \ general escape character ^ negate the class, but only if the first character - indicates character range [ POSIX character class (only if followed by POSIX syntax) ] terminates the character class The following sections describe the use of each of the metacharacters. BACKSLASH The backslash character has several uses. Firstly, if it is followed by a character that is not a number or a letter, it takes away any special meaning that character may have. This use of backslash as an escape character applies both inside and outside character classes. For example, if you want to match a * character, you write \* in the pattern. This escaping action applies whether or not the following character would otherwise be interpreted as a metacharacter, so it is always safe to precede a non-alphanumeric with backslash to specify that it stands for itself. In particular, if you want to match a back- slash, you write \\. In a UTF mode, only ASCII numbers and letters have any special meaning after a backslash. All other characters (in particular, those whose codepoints are greater than 127) are treated as literals. If a pattern is compiled with the PCRE2_EXTENDED option, most white space in the pattern (other than in a character class), and characters between a # outside a character class and the next newline, inclusive, are ignored. An escaping backslash can be used to include a white space or # character as part of the pattern. If you want to remove the special meaning from a sequence of charac- ters, you can do so by putting them between \Q and \E. This is differ- ent from Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE2, whereas in Perl, $ and @ cause variable interpola- tion. Note the following examples: Pattern PCRE2 matches Perl matches \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyz The \Q...\E sequence is recognized both inside and outside character classes. An isolated \E that is not preceded by \Q is ignored. If \Q is not followed by \E later in the pattern, the literal interpretation continues to the end of the pattern (that is, \E is assumed at the end). If the isolated \Q is inside a character class, this causes an error, because the character class is not terminated. Non-printing characters A second use of backslash provides a way of encoding non-printing char- acters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters in a pattern, but when a pattern is being prepared by text editing, it is often easier to use one of the following escape sequences than the binary character it represents. In an ASCII or Unicode environment, these escapes are as follows: \a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is any printable ASCII character \e escape (hex 1B) \f form feed (hex 0C) \n linefeed (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) \0dd character with octal code 0dd \ddd character with octal code ddd, or back reference \o{ddd..} character with octal code ddd.. \xhh character with hex code hh \x{hhh..} character with hex code hhh.. (default mode) \uhhhh character with hex code hhhh (when PCRE2_ALT_BSUX is set) The precise effect of \cx on ASCII characters is as follows: if x is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the code unit following \c has a value less than 32 or greater than 126, a compile-time error occurs. When PCRE2 is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t gen- erate the appropriate EBCDIC code values. The \c escape is processed as specified for Perl in the perlebcdic document. The only characters that are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. Any other character provokes a compile-time error. The sequence \c@ encodes character code 0; after \c the letters (in either case) encode characters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 (hex 1B to hex 1F), and \c? becomes either 255 (hex FF) or 95 (hex 5F). Thus, apart from \c?, these escapes generate the same character code values as they do in an ASCII environment, though the meanings of the values mostly differ. For example, \cG always generates code value 7, which is BEL in ASCII but DEL in EBCDIC. The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but because 127 is not a control character in EBCDIC, Perl makes it generate the APC character. Unfortunately, there are several variants of EBCDIC. In most of them the APC character has the value 255 (hex FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC values, PCRE2 makes \c? generate 95; otherwise it generates 255. After \0 up to two further octal digits are read. If there are fewer than two digits, just those that are present are used. Thus the sequence \0\x\015 specifies two binary zeros followed by a CR character (code value 13). Make sure you supply two digits after the initial zero if the pattern character that follows is itself an octal digit. The escape \o must be followed by a sequence of octal digits, enclosed in braces. An error occurs if this is not the case. This escape is a recent addition to Perl; it provides way of specifying character code points as octal numbers greater than 0777, and it also allows octal numbers and back references to be unambiguously specified. For greater clarity and unambiguity, it is best to avoid following \ by a digit greater than zero. Instead, use \o{} or \x{} to specify charac- ter numbers, and \g{} to specify back references. The following para- graphs describe the old, ambiguous syntax. The handling of a backslash followed by a digit other than 0 is compli- cated, and Perl has changed over time, causing PCRE2 also to change. Outside a character class, PCRE2 reads the digit and any following dig- its as a decimal number. If the number is less than 10, begins with the digit 8 or 9, or if there are at least that many previous capturing left parentheses in the expression, the entire sequence is taken as a back reference. A description of how this works is given later, follow- ing the discussion of parenthesized subpatterns. Otherwise, up to three octal digits are read to form a character code. Inside a character class, PCRE2 handles \8 and \9 as the literal char- acters "8" and "9", and otherwise reads up to three octal digits fol- lowing the backslash, using them to generate a data character. Any sub- sequent digits stand for themselves. For example, outside a character class: \040 is another way of writing an ASCII space \40 is the same, provided there are fewer than 40 previous capturing subpatterns \7 is always a back reference \11 might be a back reference, or another way of writing a tab \011 is always a tab \0113 is a tab followed by the character "3" \113 might be a back reference, otherwise the character with octal code 113 \377 might be a back reference, otherwise the value 255 (decimal) \81 is always a back reference Note that octal values of 100 or greater that are specified using this syntax must not be introduced by a leading zero, because no more than three octal digits are ever read. By default, after \x that is not followed by {, from zero to two hexa- decimal digits are read (letters can be in upper or lower case). Any number of hexadecimal digits may appear between \x{ and }. If a charac- ter other than a hexadecimal digit appears between \x{ and }, or if there is no terminating }, an error occurs. If the PCRE2_ALT_BSUX option is set, the interpretation of \x is as just described only when it is followed by two hexadecimal digits. Oth- erwise, it matches a literal "x" character. In this mode mode, support for code points greater than 256 is provided by \u, which must be fol- lowed by four hexadecimal digits; otherwise it matches a literal "u" character. Characters whose value is less than 256 can be defined by either of the two syntaxes for \x (or by \u in PCRE2_ALT_BSUX mode). There is no dif- ference in the way they are handled. For example, \xdc is exactly the same as \x{dc} (or \u00dc in PCRE2_ALT_BSUX mode). Constraints on character values Characters that are specified using octal or hexadecimal numbers are limited to certain values, as follows: 8-bit non-UTF mode less than 0x100 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint 16-bit non-UTF mode less than 0x10000 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint 32-bit non-UTF mode less than 0x100000000 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so- called "surrogate" codepoints), and 0xffef. Escape sequences in character classes All the sequences that define a single character value can be used both inside and outside character classes. In addition, inside a character class, \b is interpreted as the backspace character (hex 08). \N is not allowed in a character class. \B, \R, and \X are not special inside a character class. Like other unrecognized alphabetic escape sequences, they cause an error. Outside a character class, these sequences have different meanings. Unsupported escape sequences In Perl, the sequences \l, \L, \u, and \U are recognized by its string handler and used to modify the case of following characters. By default, PCRE2 does not support these escape sequences. However, if the PCRE2_ALT_BSUX option is set, \U matches a "U" character, and \u can be used to define a character by code point, as described in the previous section. Absolute and relative back references The sequence \g followed by a signed or unsigned number, optionally enclosed in braces, is an absolute or relative back reference. A named back reference can be coded as \g{name}. Back references are discussed later, following the discussion of parenthesized subpatterns. Absolute and relative subroutine calls For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a "subroutine". Details are discussed later. Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call. Generic character types Another use of backslash is for specifying generic character types: \d any decimal digit \D any character that is not a decimal digit \h any horizontal white space character \H any character that is not a horizontal white space character \s any white space character \S any character that is not a white space character \v any vertical white space character \V any character that is not a vertical white space character \w any "word" character \W any "non-word" character There is also the single sequence \N, which matches a non-newline char- acter. This is the same as the "." metacharacter when PCRE2_DOTALL is not set. Perl also uses \N to match characters by name; PCRE2 does not support this. Each pair of lower and upper case escape sequences partitions the com- plete set of characters into two disjoint sets. Any given character matches one, and only one, of each pair. The sequences can appear both inside and outside character classes. They each match one character of the appropriate type. If the current matching point is at the end of the subject string, all of them fail, because there is no character to match. The default \s characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32), which are defined as white space in the "C" locale. This list may vary if locale-specific matching is taking place. For example, in some locales the "non-breaking space" character (\xA0) is recognized as white space, and in others the VT character is not. A "word" character is an underscore or any character that is a letter or digit. By default, the definition of letters and digits is con- trolled by PCRE2's low-valued character tables, and may vary if locale- specific matching is taking place (see "Locale support" in the pcre2api page). For example, in a French locale such as "fr_FR" in Unix-like systems, or "french" in Windows, some character codes greater than 127 are used for accented letters, and these are then matched by \w. The use of locales with Unicode is discouraged. By default, characters whose code points are greater than 127 never match \d, \s, or \w, and always match \D, \S, and \W, although this may be different for characters in the range 128-255 when locale-specific matching is happening. These escape sequences retain their original meanings from before Unicode support was available, mainly for effi- ciency reasons. If the PCRE2_UCP option is set, the behaviour is changed so that Unicode properties are used to determine character types, as follows: \d any character that matches \p{Nd} (decimal digit) \s any character that matches \p{Z} or \h or \v \w any character that matches \p{L} or \p{N}, plus underscore The upper case escapes match the inverse sets of characters. Note that \d matches only decimal digits, whereas \w matches any Unicode digit, as well as any Unicode letter, and underscore. Note also that PCRE2_UCP affects \b, and \B because they are defined in terms of \w and \W. Matching these sequences is noticeably slower when PCRE2_UCP is set. The sequences \h, \H, \v, and \V, in contrast to the other sequences, which match only ASCII characters by default, always match a specific list of code points, whether or not PCRE2_UCP is set. The horizontal space characters are: U+0009 Horizontal tab (HT) U+0020 Space U+00A0 Non-break space U+1680 Ogham space mark U+180E Mongolian vowel separator U+2000 En quad U+2001 Em quad U+2002 En space U+2003 Em space U+2004 Three-per-em space U+2005 Four-per-em space U+2006 Six-per-em space U+2007 Figure space U+2008 Punctuation space U+2009 Thin space U+200A Hair space U+202F Narrow no-break space U+205F Medium mathematical space U+3000 Ideographic space The vertical space characters are: U+000A Linefeed (LF) U+000B Vertical tab (VT) U+000C Form feed (FF) U+000D Carriage return (CR) U+0085 Next line (NEL) U+2028 Line separator U+2029 Paragraph separator In 8-bit, non-UTF-8 mode, only the characters with code points less than 256 are relevant. Newline sequences Outside a character class, by default, the escape sequence \R matches any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the following: (?>\r\n|\n|\x0b|\f|\r|\x85) This is an example of an "atomic group", details of which are given below. This particular group matches either the two-character sequence CR followed by LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car- riage return, U+000D), or NEL (next line, U+0085). Because this is an atomic group, the two-character sequence is treated as a single unit that cannot be split. In other modes, two additional characters whose codepoints are greater than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- rator, U+2029). Unicode support is not needed for these characters to be recognized. It is possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF at compile time. (BSR is an abbrevation for "back- slash R".) This can be made the default when PCRE2 is built; if this is the case, the other behaviour can be requested via the PCRE2_BSR_UNI- CODE option. It is also possible to specify these settings by starting a pattern string with one of the following sequences: (*BSR_ANYCRLF) CR, LF, or CRLF only (*BSR_UNICODE) any Unicode newline sequence These override the default and the options given to the compiling func- tion. Note that these special settings, which are not Perl-compatible, are recognized only at the very start of a pattern, and that they must be in upper case. If more than one of them is present, the last one is used. They can be combined with a change of newline convention; for example, a pattern can start with: (*ANY)(*BSR_ANYCRLF) They can also be combined with the (*UTF) or (*UCP) special sequences. Inside a character class, \R is treated as an unrecognized escape sequence, and causes an error. Unicode character properties When PCRE2 is built with Unicode support (the default), three addi- tional escape sequences that match characters with specific properties are available. In 8-bit non-UTF-8 mode, these sequences are of course limited to testing characters whose codepoints are less than 256, but they do work in this mode. The extra escape sequences are: \p{xx} a character with the xx property \P{xx} a character without the xx property \X a Unicode extended grapheme cluster The property names represented by xx above are limited to the Unicode script names, the general category properties, "Any", which matches any character (including newline), and some special PCRE2 properties (described in the next section). Other Perl properties such as "InMu- sicalSymbols" are not supported by PCRE2. Note that \P{Any} does not match any characters, so always causes a match failure. Sets of Unicode characters are defined as belonging to certain scripts. A character from one of these sets can be matched using a script name. For example: \p{Greek} \P{Han} Those that are not part of an identified script are lumped together as "Common". The current list of scripts is: Ahom, Anatolian_Hieroglyphs, Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hieroglyphs, Elbasan, Ethiopic, Geor- gian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hatran, Hebrew, Hiragana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese, Kaithi, Kan- nada, Katakana, Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian, Maha- jani, Malayalam, Mandaic, Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Multani, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Hungarian, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Shavian, Siddham, SignWriting, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi. Each character has exactly one Unicode general category property, spec- ified by a two-letter abbreviation. For compatibility with Perl, nega- tion can be specified by including a circumflex between the opening brace and the property name. For example, \p{^Lu} is the same as \P{Lu}. If only one letter is specified with \p or \P, it includes all the gen- eral category properties that start with that letter. In this case, in the absence of negation, the curly brackets in the escape sequence are optional; these two examples have the same effect: \p{L} \pL The following general category property codes are supported: C Other Cc Control Cf Format Cn Unassigned Co Private use Cs Surrogate L Letter Ll Lower case letter Lm Modifier letter Lo Other letter Lt Title case letter Lu Upper case letter M Mark Mc Spacing mark Me Enclosing mark Mn Non-spacing mark N Number Nd Decimal number Nl Letter number No Other number P Punctuation Pc Connector punctuation Pd Dash punctuation Pe Close punctuation Pf Final punctuation Pi Initial punctuation Po Other punctuation Ps Open punctuation S Symbol Sc Currency symbol Sk Modifier symbol Sm Mathematical symbol So Other symbol Z Separator Zl Line separator Zp Paragraph separator Zs Space separator The special property L& is also supported: it matches a character that has the Lu, Ll, or Lt property, in other words, a letter that is not classified as a modifier or "other". The Cs (Surrogate) property applies only to characters in the range U+D800 to U+DFFF. Such characters are not valid in Unicode strings and so cannot be tested by PCRE2, unless UTF validity checking has been turned off (see the discussion of PCRE2_NO_UTF_CHECK in the pcre2api page). Perl does not support the Cs property. The long synonyms for property names that Perl supports (such as \p{Letter}) are not supported by PCRE2, nor is it permitted to prefix any of these properties with "Is". No character that is in the Unicode table has the Cn (unassigned) prop- erty. Instead, this property is assumed for any code point that is not in the Unicode table. Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} always matches only upper case letters. This is different from the behaviour of current versions of Perl. Matching characters by Unicode property is not fast, because PCRE2 has to do a multistage table lookup in order to find a character's prop- erty. That is why the traditional escape sequences such as \d and \w do not use Unicode properties in PCRE2 by default, though you can make them do so by setting the PCRE2_UCP option or by starting the pattern with (*UCP). Extended grapheme clusters The \X escape matches any number of Unicode characters that form an "extended grapheme cluster", and treats the sequence as an atomic group (see below). Unicode supports various kinds of composite character by giving each character a grapheme breaking property, and having rules that use these properties to define the boundaries of extended grapheme clusters. \X always matches at least one character. Then it decides whether to add additional characters according to the following rules for ending a cluster: 1. End at the end of the subject string. 2. Do not end between CR and LF; otherwise end after any control char- acter. 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters are of five types: L, V, T, LV, and LVT. An L character may be followed by an L, V, LV, or LVT character; an LV or V character may be followed by a V or T character; an LVT or T character may be follwed only by a T character. 4. Do not end before extending characters or spacing marks. Characters with the "mark" property always have the "extend" grapheme breaking property. 5. Do not end after prepend characters. 6. Otherwise, end the cluster. PCRE2's additional properties As well as the standard Unicode properties described above, PCRE2 sup- ports four more that make it possible to convert traditional escape sequences such as \w and \s to use Unicode properties. PCRE2 uses these non-standard, non-Perl properties internally when PCRE2_UCP is set. However, they may also be used explicitly. These properties are: Xan Any alphanumeric character Xps Any POSIX space character Xsp Any Perl space character Xwd Any Perl "word" character Xan matches characters that have either the L (letter) or the N (num- ber) property. Xps matches the characters tab, linefeed, vertical tab, form feed, or carriage return, and any other character that has the Z (separator) property. Xsp is the same as Xps; in PCRE1 it used to exclude vertical tab, for Perl compatibility, but Perl changed. Xwd matches the same characters as Xan, plus underscore. There is another non-standard property, Xuc, which matches any charac- ter that can be represented by a Universal Character Name in C++ and other programming languages. These are the characters $, @, ` (grave accent), and all characters with Unicode code points greater than or equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit. Note that the Xuc property does not match these sequences but the char- acters that they represent.) Resetting the match start The escape sequence \K causes any previously matched characters not to be included in the final matched sequence. For example, the pattern: foo\Kbar matches "foobar", but reports that it has matched "bar". This feature is similar to a lookbehind assertion (described below). However, in this case, the part of the subject before the real match does not have to be of fixed length, as lookbehind assertions do. The use of \K does not interfere with the setting of captured substrings. For example, when the pattern (foo)\Kbar matches "foobar", the first substring is still set to "foo". Perl documents that the use of \K within assertions is "not well defined". In PCRE2, \K is acted upon when it occurs inside positive assertions, but is ignored in negative assertions. Note that when a pattern such as (?=ab\K) matches, the reported start of the match can be greater than the end of the match. Simple assertions The final use of backslash is for certain simple assertions. An asser- tion specifies a condition that has to be met at a particular point in a match, without consuming any characters from the subject string. The use of subpatterns for more complicated assertions is described below. The backslashed assertions are: \b matches at a word boundary \B matches when not at a word boundary \A matches at the start of the subject \Z matches at the end of the subject also matches before a newline at the end of the subject \z matches only at the end of the subject \G matches at the first matching position in the subject Inside a character class, \b has a different meaning; it matches the backspace character. If any other of these assertions appears in a character class, an "invalid escape sequence" error is generated. A word boundary is a position in the subject string where the current character and the previous character do not both match \w or \W (i.e. one matches \w and the other matches \W), or the start or end of the string if the first or last character matches \w, respectively. In a UTF mode, the meanings of \w and \W can be changed by setting the PCRE2_UCP option. When this is done, it also affects \b and \B. Neither PCRE2 nor Perl has a separate "start of word" or "end of word" metase- quence. However, whatever follows \b normally determines which it is. For example, the fragment \ba matches "a" at the start of a word. The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described in the next section) in that they only ever match at the very start and end of the subject string, whatever options are set. Thus, they are independent of multiline mode. These three asser- tions are not affected by the PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the behaviour of the circumflex and dollar metachar- acters. However, if the startoffset argument of pcre2_match() is non- zero, indicating that matching is to start at a point other than the beginning of the subject, \A can never match. The difference between \Z and \z is that \Z matches before a newline at the end of the string as well as at the very end, whereas \z matches only at the end. The \G assertion is true only when the current matching position is at the start point of the match, as specified by the startoffset argument of pcre2_match(). It differs from \A when the value of startoffset is non-zero. By calling pcre2_match() multiple times with appropriate arguments, you can mimic Perl's /g option, and it is in this kind of implementation where \G can be useful. Note, however, that PCRE2's interpretation of \G, as the start of the current match, is subtly different from Perl's, which defines it as the end of the previous match. In Perl, these can be different when the previously matched string was empty. Because PCRE2 does just one match at a time, it cannot reproduce this behaviour. If all the alternatives of a pattern begin with \G, the expression is anchored to the starting match position, and the "anchored" flag is set in the compiled regular expression. CIRCUMFLEX AND DOLLAR The circumflex and dollar metacharacters are zero-width assertions. That is, they test for a particular condition being true without con- suming any characters from the subject string. These two metacharacters are concerned with matching the starts and ends of lines. If the new- line convention is set so that only the two-character sequence CRLF is recognized as a newline, isolated CR and LF characters are treated as ordinary data characters, and are not recognized as newlines. Outside a character class, in the default matching mode, the circumflex character is an assertion that is true only if the current matching point is at the start of the subject string. If the startoffset argu- ment of pcre2_match() is non-zero, or if PCRE2_NOTBOL is set, circum- flex can never match if the PCRE2_MULTILINE option is unset. Inside a character class, circumflex has an entirely different meaning (see below). Circumflex need not be the first character of the pattern if a number of alternatives are involved, but it should be the first thing in each alternative in which it appears if the pattern is ever to match that branch. If all possible alternatives start with a circumflex, that is, if the pattern is constrained to match only at the start of the sub- ject, it is said to be an "anchored" pattern. (There are also other constructs that can cause a pattern to be anchored.) The dollar character is an assertion that is true only if the current matching point is at the end of the subject string, or immediately before a newline at the end of the string (by default), unless PCRE2_NOTEOL is set. Note, however, that it does not actually match the newline. Dollar need not be the last character of the pattern if a num- ber of alternatives are involved, but it should be the last item in any branch in which it appears. Dollar has no special meaning in a charac- ter class. The meaning of dollar can be changed so that it matches only at the very end of the string, by setting the PCRE2_DOLLAR_ENDONLY option at compile time. This does not affect the \Z assertion. The meanings of the circumflex and dollar metacharacters are changed if the PCRE2_MULTILINE option is set. When this is the case, a dollar character matches before any newlines in the string, as well as at the very end, and a circumflex matches immediately after internal newlines as well as at the start of the subject string. It does not match after a newline that ends the string, for compatibility with Perl. However, this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option. For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n represents a newline) in multiline mode, but not otherwise. Consequently, patterns that are anchored in single line mode because all branches start with ^ are not anchored in multiline mode, and a match for circumflex is possible when the startoffset argument of pcre2_match() is non-zero. The PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set. When the newline convention (see "Newline conventions" below) recog- nizes the two-character sequence CRLF as a newline, this is preferred, even if the single characters CR and LF are also recognized as new- lines. For example, if the newline convention is "any", a multiline mode circumflex matches before "xyz" in the string "abc\r\nxyz" rather than after CR, even though CR on its own is a valid newline. (It also matches at the very start of the string, of course.) Note that the sequences \A, \Z, and \z can be used to match the start and end of the subject in both modes, and if all branches of a pattern start with \A it is always anchored, whether or not PCRE2_MULTILINE is set. FULL STOP (PERIOD, DOT) AND \N Outside a character class, a dot in the pattern matches any one charac- ter in the subject string except (by default) a character that signi- fies the end of a line. When a line ending is defined as a single character, dot never matches that character; when the two-character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, but otherwise it matches all characters (including isolated CRs and LFs). When any Uni- code line endings are being recognized, dot does not match CR or LF or any of the other line ending characters. The behaviour of dot with regard to newlines can be changed. If the PCRE2_DOTALL option is set, a dot matches any one character, without exception. If the two-character sequence CRLF is present in the sub- ject string, it takes two dots to match it. The handling of dot is entirely independent of the handling of circum- flex and dollar, the only relationship being that they both involve newlines. Dot has no special meaning in a character class. The escape sequence \N behaves like a dot, except that it is not affected by the PCRE2_DOTALL option. In other words, it matches any character except one that signifies the end of a line. Perl also uses \N to match characters by name; PCRE2 does not support this. MATCHING A SINGLE CODE UNIT Outside a character class, the escape sequence \C matches any one code unit, whether or not a UTF mode is set. In the 8-bit library, one code unit is one byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches line-ending characters. The feature is provided in Perl in order to match individual bytes in UTF-8 mode, but it is unclear how it can use- fully be used. Because \C breaks up characters into individual code units, matching one unit with \C in UTF-8 or UTF-16 mode means that the rest of the string may start with a malformed UTF character. This has undefined results, because PCRE2 assumes that it is matching character by charac- ter in a valid UTF string (by default it checks the subject string's validity at the start of processing unless the PCRE2_NO_UTF_CHECK option is used). An application can lock out the use of \C by setting the PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also possible to build PCRE2 with the use of \C permanently disabled. PCRE2 does not allow \C to appear in lookbehind assertions (described below) in UTF-8 or UTF-16 modes, because this would make it impossible to calculate the length of the lookbehind. Neither the alternative matching function pcre2_dfa_match() nor the JIT optimizer support \C in these UTF modes. The former gives a match-time error; the latter fails to optimize and so the match is always run using the interpreter. In the 32-bit library, however, \C is always supported (when not explicitly locked out) because it always matches a single code unit, whether or not UTF-32 is specified. In general, the \C escape sequence is best avoided. However, one way of using it that avoids the problem of malformed UTF-8 or UTF-16 charac- ters is to use a lookahead to check the length of the next character, as in this pattern, which could be used with a UTF-8 string (ignore white space and line breaks): (?| (?=[\x00-\x7f])(\C) | (?=[\x80-\x{7ff}])(\C)(\C) | (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) In this example, a group that starts with (?| resets the capturing parentheses numbers in each alternative (see "Duplicate Subpattern Num- bers" below). The assertions at the start of each branch check the next UTF-8 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The character's individual bytes are then captured by the appropriate number of \C groups. SQUARE BRACKETS AND CHARACTER CLASSES An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not spe- cial by default. If a closing square bracket is required as a member of the class, it should be the first data character in the class (after an initial circumflex, if present) or escaped with a backslash. This means that, by default, an empty class cannot be defined. However, if the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at the start does end the (empty) class. A character class matches a single character in the subject. A matched character must be in the set of characters defined by the class, unless the first character in the class definition is a circumflex, in which case the subject character must not be in the set defined by the class. If a circumflex is actually required as a member of the class, ensure it is not the first character, or escape it with a backslash. For example, the character class [aeiou] matches any lower case vowel, while [^aeiou] matches any character that is not a lower case vowel. Note that a circumflex is just a convenient notation for specifying the characters that are in the class by enumerating those that are not. A class that starts with a circumflex is not an assertion; it still con- sumes a character from the subject string, and therefore it fails if the current pointer is at the end of the string. When caseless matching is set, any letters in a class represent both their upper case and lower case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a caseful version would. Characters that might indicate line breaks are never treated in any special way when matching character classes, whatever line-ending sequence is in use, and whatever setting of the PCRE2_DOTALL and PCRE2_MULTILINE options is used. A class such as [^a] always matches one of these characters. The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W may appear in a character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any hexadeci- mal digit. In UTF modes, the PCRE2_UCP option affects the meanings of \d, \s, \w and their upper case partners, just as it does when they appear outside a character class, as described in the section entitled "Generic character types" above. The escape sequence \b has a different meaning inside a character class; it matches the backspace character. The sequences \B, \N, \R, and \X are not special inside a character class. Like any other unrecognized escape sequences, they cause an error. The minus (hyphen) character can be used to specify a range of charac- ters in a character class. For example, [d-m] matches any letter between d and m, inclusive. If a minus character is required in a class, it must be escaped with a backslash or appear in a position where it cannot be interpreted as indicating a range, typically as the first or last character in the class, or immediately after a range. For example, [b-d-z] matches letters in the range b to d, a hyphen charac- ter, or z. Perl treats a hyphen as a literal if it appears before or after a POSIX class (see below) or a character type escape such as as \d, but gives a warning in its warning mode, as this is most likely a user error. As PCRE2 has no facility for warning, an error is given in these cases. It is not possible to have the literal character "]" as the end charac- ter of a range. A pattern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed by a literal string "46]", so it would match "W46]" or "-46]". However, if the "]" is escaped with a backslash it is interpreted as the end of range, so [W-\]46] is inter- preted as a class containing a range followed by two other characters. The octal or hexadecimal representation of "]" can also be used to end a range. Ranges normally include all code points between the start and end char- acters, inclusive. They can also be used for code points specified numerically, for example [\000-\037]. Ranges can include any characters that are valid for the current mode. There is a special case in EBCDIC environments for ranges whose end points are both specified as literal letters in the same case. For com- patibility with Perl, EBCDIC code points within the range that are not letters are omitted. For example, [h-k] matches only four characters, even though the codes for h and k are 0x88 and 0x92, a range of 11 code points. However, if the range is specified numerically, for example, [\x88-\x92] or [h-\x92], all code points are included. If a range that includes letters is used when caseless matching is set, it matches the letters in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character tables for a French locale are in use, [\xc8-\xcb] matches accented E characters in both cases. A circumflex can conveniently be used with the upper case character types to specify a more restricted set of characters than the matching lower case type. For example, the class [^\W_] matches any letter or digit, but not underscore, whereas [\w] includes underscore. A positive character class should be read as "something OR something OR ..." and a negative class as "NOT something AND NOT something AND NOT ...". The only metacharacters that are recognized in character classes are backslash, hyphen (only where it can be interpreted as specifying a range), circumflex (only at the start), opening square bracket (only when it can be interpreted as introducing a POSIX class name, or for a special compatibility feature - see the next two sections), and the terminating closing square bracket. However, escaping other non- alphanumeric characters does no harm. POSIX CHARACTER CLASSES Perl supports the POSIX notation for character classes. This uses names enclosed by [: and :] within the enclosing square brackets. PCRE2 also supports this notation. For example, [01[:alpha:]%] matches "0", "1", any alphabetic character, or "%". The supported class names are: alnum letters and digits alpha letters ascii character codes 0 - 127 blank space or tab only cntrl control characters digit decimal digits (same as \d) graph printing characters, excluding space lower lower case letters print printing characters, including space punct printing characters, excluding letters and digits and space space white space (the same as \s from PCRE2 8.34) upper upper case letters word "word" characters (same as \w) xdigit hexadecimal digits The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32). If locale-specific matching is taking place, the list of space characters may be different; there may be fewer or more of them. "Space" and \s match the same set of characters. The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 5.8. Another Perl extension is negation, which is indicated by a ^ character after the colon. For example, [12[:^digit:]] matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not supported, and an error is given if they are encountered. By default, characters with values greater than 127 do not match any of the POSIX character classes, although this may be different for charac- ters in the range 128-255 when locale-specific matching is happening. However, if the PCRE2_UCP option is passed to pcre2_compile(), some of the classes are changed so that Unicode character properties are used. This is achieved by replacing certain POSIX classes with other sequences, as follows: [:alnum:] becomes \p{Xan} [:alpha:] becomes \p{L} [:blank:] becomes \h [:cntrl:] becomes \p{Cc} [:digit:] becomes \p{Nd} [:lower:] becomes \p{Ll} [:space:] becomes \p{Xps} [:upper:] becomes \p{Lu} [:word:] becomes \p{Xwd} Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX classes are handled specially in UCP mode: [:graph:] This matches characters that have glyphs that mark the page when printed. In Unicode property terms, it matches all char- acters with the L, M, N, P, S, or Cf properties, except for: U+061C Arabic Letter Mark U+180E Mongolian Vowel Separator U+2066 - U+2069 Various "isolate"s [:print:] This matches the same characters as [:graph:] plus space characters that are not controls, that is, characters with the Zs property. [:punct:] This matches all characters that have the Unicode P (punctua- tion) property, plus those characters with code points less than 256 that have the S (Symbol) property. The other POSIX classes are unchanged, and match only characters with code points less than 256. COMPATIBILITY FEATURE FOR WORD BOUNDARIES In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of word". PCRE2 treats these items as follows: [[:<:]] is converted to \b(?=\w) [[:>:]] is converted to \b(?<=\w) Only these exact character sequences are recognized. A sequence such as [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is not compatible with Perl. It is provided to help migrations from other environments, and is best not used in any new patterns. Note that \b matches at the start and the end of a word (see "Simple asser- tions" above), and in a Perl-style pattern the preceding or following character normally shows which is wanted, without the need for the assertions that are used above in order to give exactly the POSIX be- haviour. VERTICAL BAR Vertical bar characters are used to separate alternative patterns. For example, the pattern gilbert|sullivan matches either "gilbert" or "sullivan". Any number of alternatives may appear, and an empty alternative is permitted (matching the empty string). The matching process tries each alternative in turn, from left to right, and the first one that succeeds is used. If the alternatives are within a subpattern (defined below), "succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern. INTERNAL OPTION SETTING The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, and PCRE2_EXTENDED options (which are Perl-compatible) can be changed from within the pattern by a sequence of Perl option letters enclosed between "(?" and ")". The option letters are i for PCRE2_CASELESS m for PCRE2_MULTILINE s for PCRE2_DOTALL x for PCRE2_EXTENDED For example, (?im) sets caseless, multiline matching. It is also possi- ble to unset these options by preceding the letter with a hyphen, and a combined setting and unsetting such as (?im-sx), which sets PCRE2_CASE- LESS and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and PCRE2_EXTENDED, is also permitted. If a letter appears both before and after the hyphen, the option is unset. An empty options setting "(?)" is allowed. Needless to say, it has no effect. The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be changed in the same way as the Perl-compatible options by using the characters J and U respectively. When one of these option changes occurs at top level (that is, not inside subpattern parentheses), the change applies to the remainder of the pattern that follows. An option change within a subpattern (see below for a description of subpatterns) affects only that part of the subpattern that follows it, so (a(?i)b)c matches abc and aBc and no other strings (assuming PCRE2_CASELESS is not used). By this means, options can be made to have different set- tings in different parts of the pattern. Any changes made in one alter- native do carry on into subsequent branches within the same subpattern. For example, (a(?i)b|c) matches "ab", "aB", "c", and "C", even though when matching "C" the first branch is abandoned before the option setting. This is because the effects of option settings happen at compile time. There would be some very weird behaviour otherwise. As a convenient shorthand, if any option settings are required at the start of a non-capturing subpattern (see the next section), the option letters may appear between the "?" and the ":". Thus the two patterns (?i:saturday|sunday) (?:(?i)saturday|sunday) match exactly the same set of strings. Note: There are other PCRE2-specific options that can be set by the application when the compiling function is called. The pattern can con- tain special leading sequences such as (*CRLF) to override what the application has set or what has been defaulted. Details are given in the section entitled "Newline sequences" above. There are also the (*UTF) and (*UCP) leading sequences that can be used to set UTF and Unicode property modes; they are equivalent to setting the PCRE2_UTF and PCRE2_UCP options, respectively. However, the application can set the PCRE2_NEVER_UTF and PCRE2_NEVER_UCP options, which lock out the use of the (*UTF) and (*UCP) sequences. SUBPATTERNS Subpatterns are delimited by parentheses (round brackets), which can be nested. Turning part of a pattern into a subpattern does two things: 1. It localizes a set of alternatives. For example, the pattern cat(aract|erpillar|) matches "cataract", "caterpillar", or "cat". Without the parentheses, it would match "cataract", "erpillar" or an empty string. 2. It sets up the subpattern as a capturing subpattern. This means that, when the whole pattern matches, the portion of the subject string that matched the subpattern is passed back to the caller, separately from the portion that matched the whole pattern. (This applies only to the traditional matching function; the DFA matching function does not support capturing.) Opening parentheses are counted from left to right (starting from 1) to obtain numbers for the capturing subpatterns. For example, if the string "the red king" is matched against the pattern the ((red|white) (king|queen)) the captured substrings are "red king", "red", and "king", and are num- bered 1, 2, and 3, respectively. The fact that plain parentheses fulfil two functions is not always helpful. There are often times when a grouping subpattern is required without a capturing requirement. If an opening parenthesis is followed by a question mark and a colon, the subpattern does not do any captur- ing, and is not counted when computing the number of any subsequent capturing subpatterns. For example, if the string "the white queen" is matched against the pattern the ((?:red|white) (king|queen)) the captured substrings are "white queen" and "queen", and are numbered 1 and 2. The maximum number of capturing subpatterns is 65535. As a convenient shorthand, if any option settings are required at the start of a non-capturing subpattern, the option letters may appear between the "?" and the ":". Thus the two patterns (?i:saturday|sunday) (?:(?i)saturday|sunday) match exactly the same set of strings. Because alternative branches are tried from left to right, and options are not reset until the end of the subpattern is reached, an option setting in one branch does affect subsequent branches, so the above patterns match "SUNDAY" as well as "Saturday". DUPLICATE SUBPATTERN NUMBERS Perl 5.10 introduced a feature whereby each alternative in a subpattern uses the same numbers for its capturing parentheses. Such a subpattern starts with (?| and is itself a non-capturing subpattern. For example, consider this pattern: (?|(Sat)ur|(Sun))day Because the two alternatives are inside a (?| group, both sets of cap- turing parentheses are numbered one. Thus, when the pattern matches, you can look at captured substring number one, whichever alternative matched. This construct is useful when you want to capture part, but not all, of one of a number of alternatives. Inside a (?| group, paren- theses are numbered as usual, but the number is reset at the start of each branch. The numbers of any capturing parentheses that follow the subpattern start after the highest number used in any branch. The fol- lowing example is taken from the Perl documentation. The numbers under- neath show in which buffer the captured content will be stored. # before ---------------branch-reset----------- after / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x # 1 2 2 3 2 3 4 A back reference to a numbered subpattern uses the most recent value that is set for that number by any subpattern. The following pattern matches "abcabc" or "defdef": /(?|(abc)|(def))\1/ In contrast, a subroutine call to a numbered subpattern always refers to the first one in the pattern with the given number. The following pattern matches "abcabc" or "defabc": /(?|(abc)|(def))(?1)/ A relative reference such as (?-1) is no different: it is just a conve- nient way of computing an absolute group number. If a condition test for a subpattern's having matched refers to a non- unique number, the test is true if any of the subpatterns of that num- ber have matched. An alternative approach to using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section. NAMED SUBPATTERNS Identifying capturing parentheses by number is simple, but it can be very hard to keep track of the numbers in complicated regular expres- sions. Furthermore, if an expression is modified, the numbers may change. To help with this difficulty, PCRE2 supports the naming of sub- patterns. This feature was not added to Perl until release 5.10. Python had the feature earlier, and PCRE1 introduced it at release 4.0, using the Python syntax. PCRE2 supports both the Perl and the Python syntax. Perl allows identically numbered subpatterns to have different names, but PCRE2 does not. In PCRE2, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing parentheses from other parts of the pattern, such as back references, recursion, and conditions, can be made by name as well as by number. Names consist of up to 32 alphanumeric characters and underscores, but must start with a non-digit. Named capturing parentheses are still allocated numbers as well as names, exactly as if the names were not present. The PCRE2 API provides function calls for extracting the name- to-number translation table from a compiled pattern. There are also convenience functions for extracting a captured substring by name. By default, a name must be unique within a pattern, but it is possible to relax this constraint by setting the PCRE2_DUPNAMES option at com- pile time. (Duplicate names are also always permitted for subpatterns with the same number, set up as described in the previous section.) Duplicate names can be useful for patterns where only one instance of the named parentheses can match. Suppose you want to match the name of a weekday, either as a 3-letter abbreviation or as the full name, and in both cases you want to extract the abbreviation. This pattern (ignoring the line breaks) does the job: (?<DN>Mon|Fri|Sun)(?:day)?| (?<DN>Tue)(?:sday)?| (?<DN>Wed)(?:nesday)?| (?<DN>Thu)(?:rsday)?| (?<DN>Sat)(?:urday)? There are five capturing substrings, but only one is ever set after a match. (An alternative way of solving this problem is to use a "branch reset" subpattern, as described in the previous section.) The convenience functions for extracting the data by name returns the substring for the first (and in this example, the only) subpattern of that name that matched. This saves searching to find which numbered subpattern it was. If you make a back reference to a non-unique named subpattern from elsewhere in the pattern, the subpatterns to which the name refers are checked in the order in which they appear in the overall pattern. The first one that is set is used for the reference. For example, this pat- tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo": (?:(?<n>foo)|(?<n>bar))\k<n> If you make a subroutine call to a non-unique named subpattern, the one that corresponds to the first occurrence of the name is used. In the absence of duplicate numbers (see the previous section) this is the one with the lowest number. If you use a named reference in a condition test (see the section about conditions below), either to check whether a subpattern has matched, or to check for recursion, all subpatterns with the same name are tested. If the condition is true for any one of them, the overall condition is true. This is the same behaviour as testing by number. For further details of the interfaces for handling named subpatterns, see the pcre2api documentation. Warning: You cannot use different names to distinguish between two sub- patterns with the same number because PCRE2 uses only the numbers when matching. For this reason, an error is given at compile time if differ- ent names are given to subpatterns with the same number. However, you can always give the same name to subpatterns with the same number, even when PCRE2_DUPNAMES is not set. REPETITION Repetition is specified by quantifiers, which can follow any of the following items: a literal data character the dot metacharacter the \C escape sequence the \X escape sequence the \R escape sequence an escape such as \d or \pL that matches a single character a character class a back reference a parenthesized subpattern (including most assertions) a subroutine call to a subpattern (recursive or otherwise) The general repetition quantifier specifies a minimum and maximum num- ber of permitted matches, by giving the two numbers in curly brackets (braces), separated by a comma. The numbers must be less than 65536, and the first must be less than or equal to the second. For example: z{2,4} matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special character. If the second number is omitted, but the comma is present, there is no upper limit; if the second number and the comma are both omitted, the quantifier specifies an exact number of required matches. Thus [aeiou]{3,} matches at least 3 successive vowels, but may match many more, whereas \d{8} matches exactly 8 digits. An opening curly bracket that appears in a position where a quantifier is not allowed, or one that does not match the syntax of a quantifier, is taken as a literal character. For exam- ple, {,6} is not a quantifier, but a literal string of four characters. In UTF modes, quantifiers apply to characters rather than to individual code units. Thus, for example, \x{100}{2} matches two characters, each of which is represented by a two-byte sequence in a UTF-8 string. Simi- larly, \X{3} matches three Unicode extended grapheme clusters, each of which may be several code units long (and they may be of different lengths). The quantifier {0} is permitted, causing the expression to behave as if the previous item and the quantifier were not present. This may be use- ful for subpatterns that are referenced as subroutines from elsewhere in the pattern (but see also the section entitled "Defining subpatterns for use by reference only" below). Items other than subpatterns that have a {0} quantifier are omitted from the compiled pattern. For convenience, the three most common quantifiers have single-charac- ter abbreviations: * is equivalent to {0,} + is equivalent to {1,} ? is equivalent to {0,1} It is possible to construct infinite loops by following a subpattern that can match no characters with a quantifier that has no upper limit, for example: (a?)* Earlier versions of Perl and PCRE1 used to give an error at compile time for such patterns. However, because there are cases where this can be useful, such patterns are now accepted, but if any repetition of the subpattern does in fact match no characters, the loop is forcibly bro- ken. By default, the quantifiers are "greedy", that is, they match as much as possible (up to the maximum number of permitted times), without causing the rest of the pattern to fail. The classic example of where this gives problems is in trying to match comments in C programs. These appear between /* and */ and within the comment, individual * and / characters may appear. An attempt to match C comments by applying the pattern /\*.*\*/ to the string /* first comment */ not comment /* second comment */ fails, because it matches the entire string owing to the greediness of the .* item. If a quantifier is followed by a question mark, it ceases to be greedy, and instead matches the minimum number of times possible, so the pat- tern /\*.*?\*/ does the right thing with the C comments. The meaning of the various quantifiers is not otherwise changed, just the preferred number of matches. Do not confuse this use of question mark with its use as a quantifier in its own right. Because it has two uses, it can sometimes appear doubled, as in \d??\d which matches one digit by preference, but can match two if that is the only way the rest of the pattern matches. If the PCRE2_UNGREEDY option is set (an option that is not available in Perl), the quantifiers are not greedy by default, but individual ones can be made greedy by following them with a question mark. In other words, it inverts the default behaviour. When a parenthesized subpattern is quantified with a minimum repeat count that is greater than 1 or with a limited maximum, more memory is required for the compiled pattern, in proportion to the size of the minimum or maximum. If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option (equivalent to Perl's /s) is set, thus allowing the dot to match new- lines, the pattern is implicitly anchored, because whatever follows will be tried against every character position in the subject string, so there is no point in retrying the overall match at any position after the first. PCRE2 normally treats such a pattern as though it were preceded by \A. In cases where it is known that the subject string contains no new- lines, it is worth setting PCRE2_DOTALL in order to obtain this opti- mization, or alternatively, using ^ to indicate anchoring explicitly. However, there are some cases where the optimization cannot be used. When .* is inside capturing parentheses that are the subject of a back reference elsewhere in the pattern, a match at the start may fail where a later one succeeds. Consider, for example: (.*)abc\1 If the subject is "xyz123abc123" the match point is the fourth charac- ter. For this reason, such a pattern is not implicitly anchored. Another case where implicit anchoring is not applied is when the lead- ing .* is inside an atomic group. Once again, a match at the start may fail where a later one succeeds. Consider this pattern: (?>.*?a)b It matches "ab" in the subject "aab". The use of the backtracking con- trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly. When a capturing subpattern is repeated, the value captured is the sub- string that matched the final iteration. For example, after (tweedle[dume]{3}\s*)+ has matched "tweedledum tweedledee" the value of the captured substring is "tweedledee". However, if there are nested capturing subpatterns, the corresponding captured values may have been set in previous itera- tions. For example, after (a|(b))+ matches "aba" the value of the second captured substring is "b". ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of what follows normally causes the repeated item to be re-evaluated to see if a different number of repeats allows the rest of the pattern to match. Sometimes it is useful to prevent this, either to change the nature of the match, or to cause it fail earlier than it otherwise might, when the author of the pattern knows there is no point in carrying on. Consider, for example, the pattern \d+foo when applied to the subject line 123456bar After matching all 6 digits and then failing to match "foo", the normal action of the matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and so on, before ultimately failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides the means for specifying that once a subpattern has matched, it is not to be re-evaluated in this way. If we use atomic grouping for the previous example, the matcher gives up immediately on failing to match "foo" the first time. The notation is a kind of special parenthesis, starting with (?> as in this example: (?>\d+)foo This kind of parenthesis "locks up" the part of the pattern it con- tains once it has matched, and a failure further into the pattern is prevented from backtracking into it. Backtracking past it to previous items, however, works as normal. An alternative description is that a subpattern of this type matches exactly the string of characters that an identical standalone pattern would match, if anchored at the current point in the subject string. Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as the above example can be thought of as a maximizing repeat that must swallow everything it can. So, while both \d+ and \d+? are pre- pared to adjust the number of digits they match in order to make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits. Atomic groups in general can of course contain arbitrarily complicated subpatterns, and can be nested. However, when the subpattern for an atomic group is just a single repeated item, as in the example above, a simpler notation, called a "possessive quantifier" can be used. This consists of an additional + character following a quantifier. Using this notation, the previous example can be rewritten as \d++foo Note that a possessive quantifier can be used with an entire group, for example: (abc|xyz){2,3}+ Possessive quantifiers are always greedy; the setting of the PCRE2_UNGREEDY option is ignored. They are a convenient notation for the simpler forms of atomic group. However, there is no difference in the meaning of a possessive quantifier and the equivalent atomic group, though there may be a performance difference; possessive quantifiers should be slightly faster. The possessive quantifier syntax is an extension to the Perl 5.8 syn- tax. Jeffrey Friedl originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he built Sun's Java package, and PCRE1 copied it from there. It ultimately found its way into Perl at release 5.10. PCRE2 has an optimization that automatically "possessifies" certain simple pattern constructs. For example, the sequence A+B is treated as A++B because there is no point in backtracking into a sequence of A's when B must follow. This feature can be disabled by the PCRE2_NO_AUTO- POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS). When a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited number of times, the use of an atomic group is the only way to avoid some failing matches taking a very long time indeed. The pattern (\D+|<\d+>)*[!?] matches an unlimited number of substrings that either consist of non- digits, or digits enclosed in <>, followed by either ! or ?. When it matches, it runs quickly. However, if it is applied to aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa it takes a long time before reporting failure. This is because the string can be divided between the internal \D+ repeat and the external * repeat in a large number of ways, and all have to be tried. (The example uses [!?] rather than a single character at the end, because both PCRE2 and Perl have an optimization that allows for fast failure when a single character is used. They remember the last single charac- ter that is required for a match, and fail early if it is not present in the string.) If the pattern is changed so that it uses an atomic group, like this: ((?>\D+)|<\d+>)*[!?] sequences of non-digits cannot be broken, and failure happens quickly. BACK REFERENCES Outside a character class, a backslash followed by a digit greater than 0 (and possibly further digits) is a back reference to a capturing sub- pattern earlier (that is, to its left) in the pattern, provided there have been that many previous capturing left parentheses. However, if the decimal number following the backslash is less than 8, it is always taken as a back reference, and causes an error only if there are not that many capturing left parentheses in the entire pat- tern. In other words, the parentheses that are referenced need not be to the left of the reference for numbers less than 8. A "forward back reference" of this type can make sense when a repetition is involved and the subpattern to the right has participated in an earlier itera- tion. It is not possible to have a numerical "forward back reference" to a subpattern whose number is 8 or more using this syntax because a sequence such as \50 is interpreted as a character defined in octal. See the subsection entitled "Non-printing characters" above for further details of the handling of digits following a backslash. There is no such problem when named parentheses are used. A back reference to any subpattern is possible using named parentheses (see below). Another way of avoiding the ambiguity inherent in the use of digits following a backslash is to use the \g escape sequence. This escape must be followed by a signed or unsigned number, optionally enclosed in braces. These examples are all identical: (ring), \1 (ring), \g1 (ring), \g{1} An unsigned number specifies an absolute reference without the ambigu- ity that is present in the older syntax. It is also useful when literal digits follow the reference. A signed number is a relative reference. Consider this example: (abc(def)ghi)\g{-1} The sequence \g{-1} is a reference to the most recently started captur- ing subpattern before \g, that is, is it equivalent to \2 in this exam- ple. Similarly, \g{-2} would be equivalent to \1. The use of relative references can be helpful in long patterns, and also in patterns that are created by joining together fragments that contain references within themselves. The sequence \g{+1} is a reference to the next capturing subpattern. This kind of forward reference can be useful it patterns that repeat. Perl does not support the use of + in this way. A back reference matches whatever actually matched the capturing sub- pattern in the current subject string, rather than anything matching the subpattern itself (see "Subpatterns as subroutines" below for a way of doing that). So the pattern (sens|respons)e and \1ibility matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If caseful matching is in force at the time of the back reference, the case of letters is relevant. For exam- ple, ((?i)rah)\s+\1 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original capturing subpattern is matched caselessly. There are several different ways of writing back references to named subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified back reference syntax, in which \g can be used for both numeric and named references, is also supported. We could rewrite the above example in any of the following ways: (?<p1>(?i)rah)\s+\k<p1> (?'p1'(?i)rah)\s+\k{p1} (?P<p1>(?i)rah)\s+(?P=p1) (?<p1>(?i)rah)\s+\g{p1} A subpattern that is referenced by name may appear in the pattern before or after the reference. There may be more than one back reference to the same subpattern. If a subpattern has not actually been used in a particular match, any back references to it always fail by default. For example, the pattern (a|(bc))\2 always fails if it starts to match "a" rather than "bc". However, if the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a back reference to an unset value matches an empty string. Because there may be many capturing parentheses in a pattern, all dig- its following a backslash are taken as part of a potential back refer- ence number. If the pattern continues with a digit character, some delimiter must be used to terminate the back reference. If the PCRE2_EXTENDED option is set, this can be white space. Otherwise, the \g{ syntax or an empty comment (see "Comments" below) can be used. Recursive back references A back reference that occurs inside the parentheses to which it refers fails when the subpattern is first used, so, for example, (a\1) never matches. However, such references can be useful inside repeated sub- patterns. For example, the pattern (a|b\1)+ matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- ation of the subpattern, the back reference matches the character string corresponding to the previous iteration. In order for this to work, the pattern must be such that the first iteration does not need to match the back reference. This can be done using alternation, as in the example above, or by a quantifier with a minimum of zero. Back references of this type cause the group that they reference to be treated as an atomic group. Once the whole group has been matched, a subsequent matching failure cannot cause backtracking into the middle of the group. ASSERTIONS An assertion is a test on the characters following or preceding the current matching point that does not consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above. More complicated assertions are coded as subpatterns. There are two kinds: those that look ahead of the current position in the subject string, and those that look behind it. An assertion subpattern is matched in the normal way, except that it does not cause the current matching position to be changed. Assertion subpatterns are not capturing subpatterns. If such an asser- tion contains capturing subpatterns within it, these are counted for the purposes of numbering the capturing subpatterns in the whole pat- tern. However, substring capturing is carried out only for positive assertions. (Perl sometimes, but not always, does do capturing in nega- tive assertions.) WARNING: If a positive assertion containing one or more capturing sub- patterns succeeds, but failure to match later in the pattern causes backtracking over this assertion, the captures within the assertion are reset only if no higher numbered captures are already set. This is, unfortunately, a fundamental limitation of the current implementation; it may get removed in a future reworking. For compatibility with Perl, most assertion subpatterns may be repeated; though it makes no sense to assert the same thing several times, the side effect of capturing parentheses may occasionally be useful. However, an assertion that forms the condition for a condi- tional subpattern may not be quantified. In practice, for other asser- tions, there only three cases: (1) If the quantifier is {0}, the assertion is never obeyed during matching. However, it may contain internal capturing parenthesized groups that are called from elsewhere via the subroutine mechanism. (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it were {0,1}. At run time, the rest of the pattern match is tried with and without the assertion, the order depending on the greed- iness of the quantifier. (3) If the minimum repetition is greater than zero, the quantifier is ignored. The assertion is obeyed just once when encountered during matching. Lookahead assertions Lookahead assertions start with (?= for positive assertions and (?! for negative assertions. For example, \w+(?=;) matches a word followed by a semicolon, but does not include the semi- colon in the match, and foo(?!bar) matches any occurrence of "foo" that is not followed by "bar". Note that the apparently similar pattern (?!foo)bar does not find an occurrence of "bar" that is preceded by something other than "foo"; it finds any occurrence of "bar" whatsoever, because the assertion (?!foo) is always true when the next three characters are "bar". A lookbehind assertion is needed to achieve the other effect. If you want to force a matching failure at some point in a pattern, the most convenient way to do it is with (?!) because an empty string always matches, so an assertion that requires there not to be an empty string must always fail. The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). Lookbehind assertions Lookbehind assertions start with (?<= for positive assertions and (?<! for negative assertions. For example, (?<!foo)bar does find an occurrence of "bar" that is not preceded by "foo". The contents of a lookbehind assertion are restricted such that all the strings it matches must have a fixed length. However, if there are sev- eral top-level alternatives, they do not all have to have the same fixed length. Thus (?<=bullock|donkey) is permitted, but (?<!dogs?|cats?) causes an error at compile time. Branches that match different length strings are permitted only at the top level of a lookbehind assertion. This is an extension compared with Perl, which requires all branches to match the same length of string. An assertion such as (?<=ab(c|de)) is not permitted, because its single top-level branch can match two different lengths, but it is acceptable to PCRE2 if rewritten to use two top-level branches: (?<=abc|abde) In some cases, the escape sequence \K (see above) can be used instead of a lookbehind assertion to get round the fixed-length restriction. The implementation of lookbehind assertions is, for each alternative, to temporarily move the current position back by the fixed length and then try to match. If there are insufficient characters before the cur- rent position, the assertion fails. In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which matches a single code unit even in a UTF mode) to appear in lookbehind assertions, because it makes it impossible to calculate the length of the lookbehind. The \X and \R escapes, which can match different num- bers of code units, are never permitted in lookbehinds. "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long as the subpattern matches a fixed-length string. However, recursion, that is, a "subroutine" call into a group that is already active, is not supported. Perl does not support back references in lookbehinds. PCRE2 does sup- port them, but only if certain conditions are met. The PCRE2_MATCH_UNSET_BACKREF option must not be set, there must be no use of (?| in the pattern (it creates duplicate subpattern numbers), and if the back reference is by name, the name must be unique. Of course, the referenced subpattern must itself be of fixed length. The following pattern matches words containing at least two characters that begin and end with the same character: \b(\w)\w++(?<=\1) Possessive quantifiers can be used in conjunction with lookbehind assertions to specify efficient matching of fixed-length strings at the end of subject strings. Consider a simple pattern such as abcd$ when applied to a long string that does not match. Because matching proceeds from left to right, PCRE2 will look for each "a" in the sub- ject and then see if what follows matches the rest of the pattern. If the pattern is specified as ^.*abcd$ the initial .* matches the entire string at first, but when this fails (because there is no following "a"), it backtracks to match all but the last character, then all but the last two characters, and so on. Once again the search for "a" covers the entire string, from right to left, so we are no better off. However, if the pattern is written as ^.*+(?<=abcd) there can be no backtracking for the .*+ item because of the possessive quantifier; it can match only the entire string. The subsequent lookbe- hind assertion does a single test on the last four characters. If it fails, the match fails immediately. For long strings, this approach makes a significant difference to the processing time. Using multiple assertions Several assertions (of any sort) may occur in succession. For example, (?<=\d{3})(?<!999)foo matches "foo" preceded by three digits that are not "999". Notice that each of the assertions is applied independently at the same point in the subject string. First there is a check that the previous three characters are all digits, and then there is a check that the same three characters are not "999". This pattern does not match "foo" pre- ceded by six characters, the first of which are digits and the last three of which are not "999". For example, it doesn't match "123abc- foo". A pattern to do that is (?<=\d{3}...)(?<!999)foo This time the first assertion looks at the preceding six characters, checking that the first three are digits, and then the second assertion checks that the preceding three characters are not "999". Assertions can be nested in any combination. For example, (?<=(?<!foo)bar)baz matches an occurrence of "baz" that is preceded by "bar" which in turn is not preceded by "foo", while (?<=\d{3}(?!999)...)foo is another pattern that matches "foo" preceded by three digits and any three characters that are not "999". CONDITIONAL SUBPATTERNS It is possible to cause the matching process to obey a subpattern con- ditionally or to choose between two alternative subpatterns, depending on the result of an assertion, or whether a specific capturing subpat- tern has already been matched. The two possible forms of conditional subpattern are: (?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern) If the condition is satisfied, the yes-pattern is used; otherwise the no-pattern (if present) is used. If there are more than two alterna- tives in the subpattern, a compile-time error occurs. Each of the two alternatives may itself contain nested subpatterns of any form, includ- ing conditional subpatterns; the restriction to two alternatives applies only at the level of the condition. This pattern fragment is an example where the alternatives are complex: (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) There are five kinds of condition: references to subpatterns, refer- ences to recursion, two pseudo-conditions called DEFINE and VERSION, and assertions. Checking for a used subpattern by number If the text between the parentheses consists of a sequence of digits, the condition is true if a capturing subpattern of that number has pre- viously matched. If there is more than one capturing subpattern with the same number (see the earlier section about duplicate subpattern numbers), the condition is true if any of them have matched. An alter- native notation is to precede the digits with a plus or minus sign. In this case, the subpattern number is relative rather than absolute. The most recently opened parentheses can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside loops it can also make sense to refer to subsequent groups. The next parentheses to be opened can be referenced as (?(+1), and so on. (The value zero in any of these forms is not used; it provokes a compile-time error.) Consider the following pattern, which contains non-significant white space to make it more readable (assume the PCRE2_EXTENDED option) and to divide it into three parts for ease of discussion: ( \( )? [^()]+ (?(1) \) ) The first part matches an optional opening parenthesis, and if that character is present, sets it as the first captured substring. The sec- ond part matches one or more characters that are not parentheses. The third part is a conditional subpattern that tests whether or not the first set of parentheses matched. If they did, that is, if subject started with an opening parenthesis, the condition is true, and so the yes-pattern is executed and a closing parenthesis is required. Other- wise, since no-pattern is not present, the subpattern matches nothing. In other words, this pattern matches a sequence of non-parentheses, optionally enclosed in parentheses. If you were embedding this pattern in a larger one, you could use a relative reference: ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... This makes the fragment independent of the parentheses in the larger pattern. Checking for a used subpattern by name Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used subpattern by name. For compatibility with earlier versions of PCRE1, which had this facility before Perl, the syntax (?(name)...) is also recognized. Note, however, that undelimited names consisting of the letter R followed by digits are ambiguous (see the following sec- tion). Rewriting the above example to use a named subpattern gives this: (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them has matched. Checking for pattern recursion "Recursion" in this sense refers to any subroutine-like call from one part of the pattern to another, whether or not it is actually recur- sive. See the sections entitled "Recursive patterns" and "Subpatterns as subroutines" below for details of recursion and subpattern calls. If a condition is the string (R), and there is no subpattern with the name R, the condition is true if matching is currently in a recursion or subroutine call to the whole pattern or any subpattern. If digits follow the letter R, and there is no subpattern with that name, the condition is true if the most recent call is into a subpattern with the given number, which must exist somewhere in the overall pattern. This is a contrived example that is equivalent to a+b: ((?(R1)a+|(?1)b)) However, in both cases, if there is a subpattern with a matching name, the condition tests for its being set, as described in the section above, instead of testing for recursion. For example, creating a group with the name R1 by adding (?<R1>) to the above pattern completely changes its meaning. If a name preceded by ampersand follows the letter R, for example: (?(R&name)...) the condition is true if the most recent recursion is into a subpattern of that name (which must exist within the pattern). This condition does not check the entire recursion stack. It tests only the current level. If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them is the most recent recursion. At "top level", all these recursion test conditions are false. Defining subpatterns for use by reference only If the condition is the string (DEFINE), the condition is always false, even if there is a group with the name DEFINE. In this case, there may be only one alternative in the subpattern. It is always skipped if con- trol reaches this point in the pattern; the idea of DEFINE is that it can be used to define subroutines that can be referenced from else- where. (The use of subroutines is described below.) For example, a pat- tern to match an IPv4 address such as "192.168.23.245" could be written like this (ignore white space and line breaks): (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b (?&byte) (\.(?&byte)){3} \b The first part of the pattern is a DEFINE group inside which a another group named "byte" is defined. This matches an individual component of an IPv4 address (a number less than 256). When matching takes place, this part of the pattern is skipped because DEFINE acts like a false condition. The rest of the pattern uses references to the named group to match the four dot-separated components of an IPv4 address, insist- ing on a word boundary at each end. Checking the PCRE2 version Programs that link with a PCRE2 library can check the version by call- ing pcre2_config() with appropriate arguments. Users of applications that do not have access to the underlying code cannot do this. A spe- cial "condition" called VERSION exists to allow such users to discover which version of PCRE2 they are dealing with by using this condition to match a string such as "yesno". VERSION must be followed either by "=" or ">=" and a version number. For example: (?(VERSION>=10.4)yes|no) This pattern matches "yes" if the PCRE2 version is greater or equal to 10.4, or "no" otherwise. The fractional part of the version number may not contain more than two digits. Assertion conditions If the condition is not in any of the above formats, it must be an assertion. This may be a positive or negative lookahead or lookbehind assertion. Consider this pattern, again containing non-significant white space, and with the two alternatives on the second line: (?(?=[^a-z]*[a-z]) \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed by a letter. In other words, it tests for the presence of at least one letter in the subject. If a letter is found, the subject is matched against the first alternative; otherwise it is matched against the second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. COMMENTS There are two ways of including comments in patterns that are processed by PCRE2. In both cases, the start of the comment must not be in a character class, nor in the middle of any other sequence of related characters such as (?: or a subpattern name or number. The characters that make up a comment play no part in the pattern matching. The sequence (?# marks the start of a comment that continues up to the next closing parenthesis. Nested parentheses are not permitted. If the PCRE2_EXTENDED option is set, an unescaped # character also introduces a comment, which in this case continues to immediately after the next newline character or character sequence in the pattern. Which charac- ters are interpreted as newlines is controlled by an option passed to the compiling function or by a special sequence at the start of the pattern, as described in the section entitled "Newline conventions" above. Note that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count. For example, consider this pattern when PCRE2_EXTENDED is set, and the default newline convention (a single linefeed character) is in force: abc #comment \n still comment On encountering the # character, pcre2_compile() skips along, looking for a newline in the pattern. The sequence \n is still literal at this stage, so it does not terminate the comment. Only an actual character with the code value 0x0a (the default newline) does so. RECURSIVE PATTERNS Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses. Without the use of recursion, the best that can be done is to use a pattern that matches up to some fixed depth of nesting. It is not possible to handle an arbitrary nesting depth. For some time, Perl has provided a facility that allows regular expres- sions to recurse (amongst other things). It does this by interpolating Perl code in the expression at run time, and the code can refer to the expression itself. A Perl pattern using code interpolation to solve the parentheses problem can be created like this: $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively to the pattern in which it appears. Obviously, PCRE2 cannot support the interpolation of Perl code. Instead, it supports special syntax for recursion of the entire pat- tern, and also for individual subpattern recursion. After its introduc- tion in PCRE1 and Python, this kind of recursion was subsequently introduced into Perl at release 5.10. A special item that consists of (? followed by a number greater than zero and a closing parenthesis is a recursive subroutine call of the subpattern of the given number, provided that it occurs inside that subpattern. (If not, it is a non-recursive subroutine call, which is described in the next section.) The special item (?R) or (?0) is a recursive call of the entire regular expression. This PCRE2 pattern solves the nested parentheses problem (assume the PCRE2_EXTENDED option is set so that white space is ignored): \( ( [^()]++ | (?R) )* \) First it matches an opening parenthesis. Then it matches any number of substrings which can either be a sequence of non-parentheses, or a recursive match of the pattern itself (that is, a correctly parenthe- sized substring). Finally there is a closing parenthesis. Note the use of a possessive quantifier to avoid backtracking into sequences of non- parentheses. If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you could use this: ( \( ( [^()]++ | (?1) )* \) ) We have put the pattern into parentheses, and caused the recursion to refer to them instead of the whole pattern. In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made easier by the use of relative references. Instead of (?1) in the pattern above you can write (?-2) to refer to the second most recently opened parentheses preceding the recursion. In other words, a negative number counts capturing parentheses leftwards from the point at which it is encountered. Be aware however, that if duplicate subpattern numbers are in use, rel- ative references refer to the earliest subpattern with the appropriate number. Consider, for example: (?|(a)|(b)) (c) (?-2) The first two capturing groups (a) and (b) are both numbered 1, and group (c) is number 2. When the reference (?-2) is encountered, the second most recently opened parentheses has the number 1, but it is the first such group (the (a) group) to which the recursion refers. This would be the same if an absolute reference (?1) was used. In other words, relative references are just a shorthand for computing a group number. It is also possible to refer to subsequently opened parentheses, by writing references such as (?+2). However, these cannot be recursive because the reference is not inside the parentheses that are refer- enced. They are always non-recursive subroutine calls, as described in the next section. An alternative approach is to use named parentheses. The Perl syntax for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup- ported. We could rewrite the above example as follows: (?<pn> \( ( [^()]++ | (?&pn) )* \) ) If there is more than one subpattern with the same name, the earliest one is used. The example pattern that we have been looking at contains nested unlim- ited repeats, and so the use of a possessive quantifier for matching strings of non-parentheses is important when applying the pattern to strings that do not match. For example, when this pattern is applied to (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() it yields "no match" quickly. However, if a possessive quantifier is not used, the match runs for a very long time indeed because there are so many different ways the + and * repeats can carve up the subject, and all have to be tested before failure can be reported. At the end of a match, the values of capturing parentheses are those from the outermost level. If you want to obtain intermediate values, a callout function can be used (see below and the pcre2callout documenta- tion). If the pattern above is matched against (ab(cd)ef) the value for the inner capturing parentheses (numbered 2) is "ef", which is the last value taken on at the top level. If a capturing sub- pattern is not matched at the top level, its final captured value is unset, even if it was (temporarily) set at a deeper level during the matching process. If there are more than 15 capturing parentheses in a pattern, PCRE2 has to obtain extra memory from the heap to store data during a recursion. If no memory can be obtained, the match fails with the PCRE2_ERROR_NOMEMORY error. Do not confuse the (?R) item with the condition (R), which tests for recursion. Consider this pattern, which matches text in angle brack- ets, allowing for arbitrary nesting. Only digits are allowed in nested brackets (that is, when recursing), whereas any characters are permit- ted at the outer level. < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > In this pattern, (?(R) is the start of a conditional subpattern, with two different alternatives for the recursive and non-recursive cases. The (?R) item is the actual recursive call. Differences in recursion processing between PCRE2 and Perl Recursion processing in PCRE2 differs from Perl in two important ways. In PCRE2 (like Python, but unlike Perl), a recursive subpattern call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure. This can be illustrated by the following pattern, which purports to match a palin- dromic string that contains an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"): ^(.|(.)(?1)\2)$ The idea is that it either matches a single character, or two identical characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE2 it does not if the pattern is longer than three characters. Consider the subject string "abcba": At the top level, the first character is matched, but as it is not at the end of the string, the first alternative fails; the second alterna- tive is taken and the recursion kicks in. The recursive call to subpat- tern 1 successfully matches the next character ("b"). (Note that the beginning and end of line tests are not part of the recursion). Back at the top level, the next character ("c") is compared with what subpattern 2 matched, which was "a". This fails. Because the recursion is treated as an atomic group, there are now no backtracking points, and so the entire match fails. (Perl is able, at this point, to re- enter the recursion and try the second alternative.) However, if the pattern is written with the alternatives in the other order, things are different: ^((.)(?1)\2|.)$ This time, the recursing alternative is tried first, and continues to recurse until it runs out of characters, at which point the recursion fails. But this time we do have another alternative to try at the higher level. That is the big difference: in the previous case the remaining alternative is at a deeper recursion level, which PCRE2 can- not use. To change the pattern so that it matches all palindromic strings, not just those with an odd number of characters, it is tempting to change the pattern to this: ^((.)(?1)\2|.?)$ Again, this works in Perl, but not in PCRE2, and for the same reason. When a deeper recursion has matched a single character, it cannot be entered again in order to match an empty string. The solution is to separate the two cases, and write out the odd and even cases as alter- natives at the higher level: ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) If you want to match typical palindromic phrases, the pattern has to ignore all non-word characters, which can be done like this: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ If run with the PCRE2_CASELESS option, this pattern matches phrases such as "A man, a plan, a canal: Panama!" and it works in both PCRE2 and Perl. Note the use of the possessive quantifier *+ to avoid back- tracking into sequences of non-word characters. Without this, PCRE2 takes a great deal longer (ten times or more) to match typical phrases, and Perl takes so long that you think it has gone into a loop. WARNING: The palindrome-matching patterns above work only if the sub- ject string does not start with a palindrome that is shorter than the entire string. For example, although "abcba" is correctly matched, if the subject is "ababa", PCRE2 finds the palindrome "aba" at the start, then fails at top level because the end of the string does not follow. Once again, it cannot jump back into the recursion to try other alter- natives, so the entire match fails. The second way in which PCRE2 and Perl differ in their recursion pro- cessing is in the handling of captured values. In Perl, when a subpat- tern is called recursively or as a subpattern (see the next section), it has no access to any values that were captured outside the recur- sion, whereas in PCRE2 these values can be referenced. Consider this pattern: ^(.)(\1|a(?2)) In PCRE2, this pattern matches "bab". The first capturing parentheses match "b", then in the second group, when the back reference \1 fails to match "b", the second alternative matches "a" and then recurses. In the recursion, \1 does now match "b" and so the whole match succeeds. In Perl, the pattern fails to match because inside the recursive call \1 cannot access the externally set value. SUBPATTERNS AS SUBROUTINES If the syntax for a recursive subpattern call (either by number or by name) is used outside the parentheses to which it refers, it operates like a subroutine in a programming language. The called subpattern may be defined before or after the reference. A numbered reference can be absolute or relative, as in these examples: (...(absolute)...)...(?2)... (...(relative)...)...(?-1)... (...(?+1)...(relative)... An earlier example pointed out that the pattern (sens|respons)e and \1ibility matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If instead the pattern (sens|respons)e and (?1)ibility is used, it does match "sense and responsibility" as well as the other two strings. Another example is given in the discussion of DEFINE above. All subroutine calls, whether recursive or not, are always treated as atomic groups. That is, once a subroutine has matched some of the sub- ject string, it is never re-entered, even if it contains untried alter- natives and there is a subsequent matching failure. Any capturing parentheses that are set during the subroutine call revert to their previous values afterwards. Processing options such as case-independence are fixed when a subpat- tern is defined, so if it is used as a subroutine, such options cannot be changed for different calls. For example, consider this pattern: (abc)(?i:(?-1)) It matches "abcabc". It does not match "abcABC" because the change of processing option does not affect the called subpattern. ONIGURUMA SUBROUTINE SYNTAX For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a subroutine, possibly recursively. Here are two of the examples used above, rewrit- ten using this syntax: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) (sens|respons)e and \g'1'ibility PCRE2 supports an extension to Oniguruma: if a number is preceded by a plus or a minus sign it is taken as a relative reference. For example: (abc)(?i:\g<-1>) Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call. CALLOUTS Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl code to be obeyed in the middle of matching a regular expression. This makes it possible, amongst other things, to extract different sub- strings that match the same pair of parentheses when there is a repeti- tion. PCRE2 provides a similar feature, but of course it cannot obey arbi- trary Perl code. The feature is called "callout". The caller of PCRE2 provides an external function by putting its entry point in a match context using the function pcre2_set_callout(), and then passing that context to pcre2_match() or pcre2_dfa_match(). If no match context is passed, or if the callout entry point is set to NULL, callouts are dis- abled. Within a regular expression, (?C<arg>) indicates a point at which the external function is to be called. There are two kinds of callout: those with a numerical argument and those with a string argument. (?C) on its own with no argument is treated as (?C0). A numerical argument allows the application to distinguish between different callouts. String arguments were added for release 10.20 to make it possible for script languages that use PCRE2 to embed short scripts within patterns in a similar way to Perl. During matching, when PCRE2 reaches a callout point, the external func- tion is called. It is provided with the number or string argument of the callout, the position in the pattern, and one item of data that is also set in the match block. The callout function may cause matching to proceed, to backtrack, or to fail. By default, PCRE2 implements a number of optimizations at matching time, and one side-effect is that sometimes callouts are skipped. If you need all possible callouts to happen, you need to set options that disable the relevant optimizations. More details, including a complete description of the programming interface to the callout function, are given in the pcre2callout documentation. Callouts with numerical arguments If you just want to have a means of identifying different callout points, put a number less than 256 after the letter C. For example, this pattern has two callout points: (?C1)abc(?C2)def If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical callouts are automatically installed before each item in the pattern. They are all numbered 255. If there is a conditional group in the pat- tern whose condition is an assertion, an additional callout is inserted just before the condition. An explicit callout may also be set at this position, as in this example: (?(?C9)(?=a)abc|def) Note that this applies only to assertion conditions, not to other types of condition. Callouts with string arguments A delimited string may be used instead of a number as a callout argu- ment. The starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is the same as the start, except for {, where the end- ing delimiter is }. If the ending delimiter is needed within the string, it must be doubled. For example: (?C'ab ''c'' d')xyz(?C{any text})pqr The doubling is removed before the string is passed to the callout function. BACKTRACKING CONTROL Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are still described in the Perl documentation as "experimental and subject to change or removal in a future version of Perl". It goes on to say: "Their usage in production code should be noted to avoid problems during upgrades." The same remarks apply to the PCRE2 features described in this section. The new verbs make use of what was previously invalid syntax: an open- ing parenthesis followed by an asterisk. They are generally of the form (*VERB) or (*VERB:NAME). Some verbs take either form, possibly behaving differently depending on whether or not a name is present. By default, for compatibility with Perl, a name is any sequence of characters that does not include a closing parenthesis. The name is not processed in any way, and it is not possible to include a closing parenthesis in the name. This can be changed by setting the PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati- ble. When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to verb names and only an unescaped closing parenthesis terminates the name. However, the only backslash items that are permitted are \Q, \E, and sequences such as \x{100} that define character code points. Char- acter type escapes such as \d are faulted. A closing parenthesis can be included in a name either as \) or between \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED option is also set, unescaped whitespace in verb names is skipped, and #-comments are recognized, exactly as in the rest of the pattern. PCRE2_EXTENDED does not affect verb names unless PCRE2_ALT_VERBNAMES is also set. The maximum length of a name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit libraries. If the name is empty, that is, if the closing parenthesis immediately follows the colon, the effect is as if the colon were not there. Any number of these verbs may occur in a pat- tern. Since these verbs are specifically related to backtracking, most of them can be used only when the pattern is to be matched using the tra- ditional matching function, because these use a backtracking algorithm. With the exception of (*FAIL), which behaves like a failing negative assertion, the backtracking control verbs cause an error if encountered by the DFA matching function. The behaviour of these verbs in repeated groups, assertions, and in subpatterns called as subroutines (whether or not recursively) is docu- mented below. Optimizations that affect backtracking verbs PCRE2 contains some optimizations that are used to speed up matching by running some checks at the start of each match attempt. For example, it may know the minimum length of matching subject, or that a particular character must be present. When one of these optimizations bypasses the running of a match, any included backtracking verbs will not, of course, be processed. You can suppress the start-of-match optimizations by setting the PCRE2_NO_START_OPTIMIZE option when calling pcre2_com- pile(), or by starting the pattern with (*NO_START_OPT). There is more discussion of this option in the section entitled "Compiling a pattern" in the pcre2api documentation. Experiments with Perl suggest that it too has similar optimizations, sometimes leading to anomalous results. Verbs that act immediately The following verbs act as soon as they are encountered. They may not be followed by a name. (*ACCEPT) This verb causes the match to end successfully, skipping the remainder of the pattern. However, when it is inside a subpattern that is called as a subroutine, only that subpattern is ended successfully. Matching then continues at the outer level. If (*ACCEPT) in triggered in a posi- tive assertion, the assertion succeeds; in a negative assertion, the assertion fails. If (*ACCEPT) is inside capturing parentheses, the data so far is cap- tured. For example: A((?:A|B(*ACCEPT)|C)D) This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- tured by the outer parentheses. (*FAIL) or (*F) This verb causes a matching failure, forcing backtracking to occur. It is equivalent to (?!) but easier to read. The Perl documentation notes that it is probably useful only when combined with (?{}) or (??{}). Those are, of course, Perl features that are not present in PCRE2. The nearest equivalent is the callout feature, as for example in this pat- tern: a+(?C)(*FAIL) A match with the string "aaaa" always fails, but the callout is taken before each backtrack happens (in this example, 10 times). Recording which path was taken There is one verb whose main purpose is to track how a match was arrived at, though it also has a secondary use in conjunction with advancing the match starting point (see (*SKIP) below). (*MARK:NAME) or (*:NAME) A name is always required with this verb. There may be as many instances of (*MARK) as you like in a pattern, and their names do not have to be unique. When a match succeeds, the name of the last-encountered (*MARK:NAME), (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the caller as described in the section entitled "Other information about the match" in the pcre2api documentation. Here is an example of pcre2test output, where the "mark" modifier requests the retrieval and outputting of (*MARK) data: re> /X(*MARK:A)Y|X(*MARK:B)Z/mark data> XY 0: XY MK: A XZ 0: XZ MK: B The (*MARK) name is tagged with "MK:" in this output, and in this exam- ple it indicates which of the two alternatives matched. This is a more efficient way of obtaining this information than putting each alterna- tive in its own capturing parentheses. If a verb with a name is encountered in a positive assertion that is true, the name is recorded and passed back if it is the last-encoun- tered. This does not happen for negative assertions or failing positive assertions. After a partial match or a failed match, the last encountered name in the entire match process is returned. For example: re> /X(*MARK:A)Y|X(*MARK:B)Z/mark data> XP No match, mark = B Note that in this unanchored example the mark is retained from the match attempt that started at the letter "X" in the subject. Subsequent match attempts starting at "P" and then with an empty string do not get as far as the (*MARK) item, but nevertheless do not reset it. If you are interested in (*MARK) values after failed matches, you should probably set the PCRE2_NO_START_OPTIMIZE option (see above) to ensure that the match is always attempted. Verbs that act after backtracking The following verbs do nothing when they are encountered. Matching con- tinues with what follows, but if there is no subsequent match, causing a backtrack to the verb, a failure is forced. That is, backtracking cannot pass to the left of the verb. However, when one of these verbs appears inside an atomic group (which includes any group that is called as a subroutine) or in an assertion that is true, its effect is con- fined to that group, because once the group has been matched, there is never any backtracking into it. In this situation, backtracking has to jump to the left of the entire atomic group or assertion. These verbs differ in exactly what kind of failure occurs when back- tracking reaches them. The behaviour described below is what happens when the verb is not in a subroutine or an assertion. Subsequent sec- tions cover these special cases. (*COMMIT) This verb, which may not be followed by a name, causes the whole match to fail outright if there is a later matching failure that causes back- tracking to reach it. Even if the pattern is unanchored, no further attempts to find a match by advancing the starting point take place. If (*COMMIT) is the only backtracking verb that is encountered, once it has been passed pcre2_match() is committed to finding a match at the current starting point, or not at all. For example: a+(*COMMIT)b This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor, or "I've started, so I must finish." The name of the most recently passed (*MARK) in the path is passed back when (*COMMIT) forces a match failure. If there is more than one backtracking verb in a pattern, a different one that follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a match does not always guarantee that a match must be at this starting point. Note that (*COMMIT) at the start of a pattern is not the same as an anchor, unless PCRE2's start-of-match optimizations are turned off, as shown in this output from pcre2test: re> /(*COMMIT)abc/ data> xyzabc 0: abc data> re> /(*COMMIT)abc/no_start_optimize data> xyzabc No match For the first pattern, PCRE2 knows that any match must start with "a", so the optimization skips along the subject to "a" before applying the pattern to the first set of data. The match attempt then succeeds. The second pattern disables the optimization that skips along to the first character. The pattern is now applied starting at "x", and so the (*COMMIT) causes the match to fail without trying any other starting points. (*PRUNE) or (*PRUNE:NAME) This verb causes the match to fail at the current starting position in the subject if there is a later matching failure that causes backtrack- ing to reach it. If the pattern is unanchored, the normal "bumpalong" advance to the next starting character then happens. Backtracking can occur as usual to the left of (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but if there is no match to the right, backtracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative to an atomic group or possessive quan- tifier, but there are some uses of (*PRUNE) that cannot be expressed in any other way. In an anchored pattern (*PRUNE) has the same effect as (*COMMIT). The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is remembered for passing back to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK), ignoring those set by (*PRUNE) or (*THEN). (*SKIP) This verb, when given without a name, is like (*PRUNE), except that if the pattern is unanchored, the "bumpalong" advance is not to the next character, but to the position in the subject where (*SKIP) was encoun- tered. (*SKIP) signifies that whatever text was matched leading up to it cannot be part of a successful match. Consider: a+(*SKIP)b If the subject is "aaaac...", after the first match attempt fails (starting at the first character in the string), the starting point skips on to start the next attempt at "c". Note that a possessive quan- tifer does not have the same effect as this example; although it would suppress backtracking during the first match attempt, the second attempt would start at the second character instead of skipping on to "c". (*SKIP:NAME) When (*SKIP) has an associated name, its behaviour is modified. When it is triggered, the previous path through the pattern is searched for the most recent (*MARK) that has the same name. If one is found, the "bumpalong" advance is to the subject position that corresponds to that (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a matching name is found, the (*SKIP) is ignored. Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME). (*THEN) or (*THEN:NAME) This verb causes a skip to the next innermost alternative when back- tracking reaches it. That is, it cancels any further backtracking within the current alternative. Its name comes from the observation that it can be used for a pattern-based if-then-else block: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... If the COND1 pattern matches, FOO is tried (and possibly further items after the end of the group if FOO succeeds); on failure, the matcher skips to the second alternative and tries COND2, without backtracking into COND1. If that succeeds and BAR fails, COND3 is tried. If subse- quently BAZ fails, there are no more alternatives, so there is a back- track to whatever came before the entire group. If (*THEN) is not inside an alternation, it acts like (*PRUNE). The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is remembered for passing back to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK), ignoring those set by (*PRUNE) and (*THEN). A subpattern that does not contain a | character is just a part of the enclosing alternative; it is not a nested alternation with only one alternative. The effect of (*THEN) extends beyond such a subpattern to the enclosing alternative. Consider this pattern, where A, B, etc. are complex pattern fragments that do not contain any | characters at this level: A (B(*THEN)C) | D If A and B are matched, but there is a failure in C, matching does not backtrack into A; instead it moves to the next alternative, that is, D. However, if the subpattern containing (*THEN) is given an alternative, it behaves differently: A (B(*THEN)C | (*FAIL)) | D The effect of (*THEN) is now confined to the inner subpattern. After a failure in C, matching moves to (*FAIL), which causes the whole subpat- tern to fail because there are no more alternatives to try. In this case, matching does now backtrack into A. Note that a conditional subpattern is not considered as having two alternatives, because only one is ever used. In other words, the | character in a conditional subpattern has a different meaning. Ignoring white space, consider: ^.*? (?(?=a) a | b(*THEN)c ) If the subject is "ba", this pattern does not match. Because .*? is ungreedy, it initially matches zero characters. The condition (?=a) then fails, the character "b" is matched, but "c" is not. At this point, matching does not backtrack to .*? as might perhaps be expected from the presence of the | character. The conditional subpattern is part of the single alternative that comprises the whole pattern, and so the match fails. (If there was a backtrack into .*?, allowing it to match "b", the match would succeed.) The verbs just described provide four different "strengths" of control when subsequent matching fails. (*THEN) is the weakest, carrying on the match at the next alternative. (*PRUNE) comes next, failing the match at the current starting position, but allowing an advance to the next character (for an unanchored pattern). (*SKIP) is similar, except that the advance may be more than one character. (*COMMIT) is the strongest, causing the entire match to fail. More than one backtracking verb If more than one backtracking verb is present in a pattern, the one that is backtracked onto first acts. For example, consider this pat- tern, where A, B, etc. are complex pattern fragments: (A(*COMMIT)B(*THEN)C|ABD) If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes the next alternative (ABD) to be tried. This behaviour is consistent, but is not always the same as Perl's. It means that if two or more backtracking verbs appear in succession, all the the last of them has no effect. Consider this example: ...(*COMMIT)(*PRUNE)... If there is a matching failure to the right, backtracking onto (*PRUNE) causes it to be triggered, and its action is taken. There can never be a backtrack onto (*COMMIT). Backtracking verbs in repeated groups PCRE2 differs from Perl in its handling of backtracking verbs in repeated groups. For example, consider: /(a(*COMMIT)b)+ac/ If the subject is "abac", Perl matches, but PCRE2 fails because the (*COMMIT) in the second repeat of the group acts. Backtracking verbs in assertions (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack. (*ACCEPT) in a positive assertion causes the assertion to succeed with- out any further processing. In a negative assertion, (*ACCEPT) causes the assertion to fail without any further processing. The other backtracking verbs are not treated specially if they appear in a positive assertion. In particular, (*THEN) skips to the next alternative in the innermost enclosing group that has alternations, whether or not this is within the assertion. Negative assertions are, however, different, in order to ensure that changing a positive assertion into a negative assertion changes its result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a neg- ative assertion to be true, without considering any further alternative branches in the assertion. Backtracking into (*THEN) causes it to skip to the next enclosing alternative within the assertion (the normal be- haviour), but if the assertion does not have such an alternative, (*THEN) behaves like (*PRUNE). Backtracking verbs in subroutines These behaviours occur whether or not the subpattern is called recur- sively. Perl's treatment of subroutines is different in some cases. (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces an immediate backtrack. (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to succeed without any further processing. Matching then contin- ues after the subroutine call. (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause the subroutine match to fail. (*THEN) skips to the next alternative in the innermost enclosing group within the subpattern that has alternatives. If there is no such group within the subpattern, (*THEN) causes the subroutine match to fail. SEE ALSO pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3), pcre2(3). AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 27 December 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2PERFORM(3) Library Functions Manual PCRE2PERFORM(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) PCRE2 PERFORMANCE Two aspects of performance are discussed below: memory usage and pro- cessing time. The way you express your pattern as a regular expression can affect both of them. COMPILED PATTERN MEMORY USAGE Patterns are compiled by PCRE2 into a reasonably efficient interpretive code, so that most simple patterns do not use much memory. However, there is one case where the memory usage of a compiled pattern can be unexpectedly large. If a parenthesized subpattern has a quantifier with a minimum greater than 1 and/or a limited maximum, the whole subpattern is repeated in the compiled code. For example, the pattern (abc|def){2,4} is compiled as if it were (abc|def)(abc|def)((abc|def)(abc|def)?)? (Technical aside: It is done this way so that backtrack points within each of the repetitions can be independently maintained.) For regular expressions whose quantifiers use only small numbers, this is not usually a problem. However, if the numbers are large, and par- ticularly if such repetitions are nested, the memory usage can become an embarrassment. For example, the very simple pattern ((ab){1,1000}c){1,3} uses 51K bytes when compiled using the 8-bit library. When PCRE2 is compiled with its default internal pointer size of two bytes, the size limit on a compiled pattern is 64K code units in the 8-bit and 16-bit libraries, and this is reached with the above pattern if the outer rep- etition is increased from 3 to 4. PCRE2 can be compiled to use larger internal pointers and thus handle larger compiled patterns, but it is better to try to rewrite your pattern to use less memory if you can. One way of reducing the memory usage for such patterns is to make use of PCRE2's "subroutine" facility. Re-writing the above pattern as ((ab)(?2){0,999}c)(?1){0,2} reduces the memory requirements to 18K, and indeed it remains under 20K even with the outer repetition increased to 100. However, this pattern is not exactly equivalent, because the "subroutine" calls are treated as atomic groups into which there can be no backtracking if there is a subsequent matching failure. Therefore, PCRE2 cannot do this kind of rewriting automatically. Furthermore, there is a noticeable loss of speed when executing the modified pattern. Nevertheless, if the atomic grouping is not a problem and the loss of speed is acceptable, this kind of rewriting will allow you to process patterns that PCRE2 cannot otherwise handle. STACK USAGE AT RUN TIME When pcre2_match() is used for matching, certain kinds of pattern can cause it to use large amounts of the process stack. In some environ- ments the default process stack is quite small, and if it runs out the result is often SIGSEGV. Rewriting your pattern can often help. The pcre2stack documentation discusses this issue in detail. PROCESSING TIME Certain items in regular expression patterns are processed more effi- ciently than others. It is more efficient to use a character class like [aeiou] than a set of single-character alternatives such as (a|e|i|o|u). In general, the simplest construction that provides the required behaviour is usually the most efficient. Jeffrey Friedl's book contains a lot of useful general discussion about optimizing regular expressions for efficient performance. This document contains a few observations about PCRE2. Using Unicode character properties (the \p, \P, and \X escapes) is slow, because PCRE2 has to use a multi-stage table lookup whenever it needs a character's property. If you can find an alternative pattern that does not use character properties, it will probably be faster. By default, the escape sequences \b, \d, \s, and \w, and the POSIX character classes such as [:alpha:] do not use Unicode properties, partly for backwards compatibility, and partly for performance reasons. However, you can set the PCRE2_UCP option or start the pattern with (*UCP) if you want Unicode character properties to be used. This can double the matching time for items such as \d, when matched with pcre2_match(); the performance loss is less with a DFA matching func- tion, and in both cases there is not much difference for \b. When a pattern begins with .* not in atomic parentheses, nor in paren- theses that are the subject of a backreference, and the PCRE2_DOTALL option is set, the pattern is implicitly anchored by PCRE2, since it can match only at the start of a subject string. If the pattern has multiple top-level branches, they must all be anchorable. The optimiza- tion can be disabled by the PCRE2_NO_DOTSTAR_ANCHOR option, and is automatically disabled if the pattern contains (*PRUNE) or (*SKIP). If PCRE2_DOTALL is not set, PCRE2 cannot make this optimization, because the dot metacharacter does not then match a newline, and if the subject string contains newlines, the pattern may match from the char- acter immediately following one of them instead of from the very start. For example, the pattern .*second matches the subject "first\nand second" (where \n stands for a newline character), with the match starting at the seventh character. In order to do this, PCRE2 has to retry the match starting after every newline in the subject. If you are using such a pattern with subject strings that do not con- tain newlines, the best performance is obtained by setting PCRE2_DOTALL, or starting the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE2 from having to scan along the sub- ject looking for a newline to restart at. Beware of patterns that contain nested indefinite repeats. These can take a long time to run when applied to a string that does not match. Consider the pattern fragment ^(a+)* This can match "aaaa" in 16 different ways, and this number increases very rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + repeats can match different numbers of times.) When the remainder of the pattern is such that the entire match is going to fail, PCRE2 has in principle to try every possible variation, and this can take an extremely long time, even for relatively short strings. An optimization catches some of the more simple cases such as (a+)*b where a literal character follows. Before embarking on the standard matching procedure, PCRE2 checks that there is a "b" later in the sub- ject string, and if there is not, it fails the match immediately. How- ever, when there is no following literal this optimization cannot be used. You can see the difference by comparing the behaviour of (a+)*\d with the pattern above. The former gives a failure almost instantly when applied to a whole line of "a" characters, whereas the latter takes an appreciable time with strings longer than about 20 characters. In many cases, the solution to this kind of performance issue is to use an atomic group or a possessive quantifier. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 02 January 2015 Copyright (c) 1997-2015 University of Cambridge. ------------------------------------------------------------------------------ PCRE2POSIX(3) Library Functions Manual PCRE2POSIX(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) SYNOPSIS #include <pcre2posix.h> int regcomp(regex_t *preg, const char *pattern, int cflags); int regexec(const regex_t *preg, const char *string, size_t nmatch, regmatch_t pmatch[], int eflags); size_t regerror(int errcode, const regex_t *preg, char *errbuf, size_t errbuf_size); void regfree(regex_t *preg); DESCRIPTION This set of functions provides a POSIX-style API for the PCRE2 regular expression 8-bit library. See the pcre2api documentation for a descrip- tion of PCRE2's native API, which contains much additional functional- ity. There are no POSIX-style wrappers for PCRE2's 16-bit and 32-bit libraries. The functions described here are just wrapper functions that ultimately call the PCRE2 native API. Their prototypes are defined in the pcre2posix.h header file, and on Unix systems the library itself is called libpcre2-posix.a, so can be accessed by adding -lpcre2-posix to the command for linking an application that uses them. Because the POSIX functions call the native ones, it is also necessary to add -lpcre2-8. Those POSIX option bits that can reasonably be mapped to PCRE2 native options have been implemented. In addition, the option REG_EXTENDED is defined with the value zero. This has no effect, but since programs that are written to the POSIX interface often use it, this makes it easier to slot in PCRE2 as a replacement library. Other POSIX options are not even defined. There are also some options that are not defined by POSIX. These have been added at the request of users who want to make use of certain PCRE2-specific features via the POSIX calling interface. When PCRE2 is called via these functions, it is only the API that is POSIX-like in style. The syntax and semantics of the regular expres- sions themselves are still those of Perl, subject to the setting of various PCRE2 options, as described below. "POSIX-like in style" means that the API approximates to the POSIX definition; it is not fully POSIX-compatible, and in multi-unit encoding domains it is probably even less compatible. The header for these functions is supplied as pcre2posix.h to avoid any potential clash with other POSIX libraries. It can, of course, be renamed or aliased as regex.h, which is the "correct" name. It provides two structure types, regex_t for compiled internal forms, and reg- match_t for returning captured substrings. It also defines some con- stants whose names start with "REG_"; these are used for setting options and identifying error codes. COMPILING A PATTERN The function regcomp() is called to compile a pattern into an internal form. The pattern is a C string terminated by a binary zero, and is passed in the argument pattern. The preg argument is a pointer to a regex_t structure that is used as a base for storing information about the compiled regular expression. The argument cflags is either zero, or contains one or more of the bits defined by the following macros: REG_DOTALL The PCRE2_DOTALL option is set when the regular expression is passed for compilation to the native function. Note that REG_DOTALL is not part of the POSIX standard. REG_ICASE The PCRE2_CASELESS option is set when the regular expression is passed for compilation to the native function. REG_NEWLINE The PCRE2_MULTILINE option is set when the regular expression is passed for compilation to the native function. Note that this does not mimic the defined POSIX behaviour for REG_NEWLINE (see the following sec- tion). REG_NOSUB When a pattern that is compiled with this flag is passed to regexec() for matching, the nmatch and pmatch arguments are ignored, and no cap- tured strings are returned. Versions of the PCRE library prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile option, but this no longer happens because it disables the use of back references. REG_UCP The PCRE2_UCP option is set when the regular expression is passed for compilation to the native function. This causes PCRE2 to use Unicode properties when matchine \d, \w, etc., instead of just recognizing ASCII values. Note that REG_UCP is not part of the POSIX standard. REG_UNGREEDY The PCRE2_UNGREEDY option is set when the regular expression is passed for compilation to the native function. Note that REG_UNGREEDY is not part of the POSIX standard. REG_UTF The PCRE2_UTF option is set when the regular expression is passed for compilation to the native function. This causes the pattern itself and all data strings used for matching it to be treated as UTF-8 strings. Note that REG_UTF is not part of the POSIX standard. In the absence of these flags, no options are passed to the native function. This means the the regex is compiled with PCRE2 default semantics. In particular, the way it handles newline characters in the subject string is the Perl way, not the POSIX way. Note that setting PCRE2_MULTILINE has only some of the effects specified for REG_NEWLINE. It does not affect the way newlines are matched by the dot metacharac- ter (they are not) or by a negative class such as [^a] (they are). The yield of regcomp() is zero on success, and non-zero otherwise. The preg structure is filled in on success, and one member of the structure is public: re_nsub contains the number of capturing subpatterns in the regular expression. Various error codes are defined in the header file. NOTE: If the yield of regcomp() is non-zero, you must not attempt to use the contents of the preg structure. If, for example, you pass it to regexec(), the result is undefined and your program is likely to crash. MATCHING NEWLINE CHARACTERS This area is not simple, because POSIX and Perl take different views of things. It is not possible to get PCRE2 to obey POSIX semantics, but then PCRE2 was never intended to be a POSIX engine. The following table lists the different possibilities for matching newline characters in Perl and PCRE2: Default Change with . matches newline no PCRE2_DOTALL newline matches [^a] yes not changeable $ matches \n at end yes PCRE2_DOLLAR_ENDONLY $ matches \n in middle no PCRE2_MULTILINE ^ matches \n in middle no PCRE2_MULTILINE This is the equivalent table for a POSIX-compatible pattern matcher: Default Change with . matches newline yes REG_NEWLINE newline matches [^a] yes REG_NEWLINE $ matches \n at end no REG_NEWLINE $ matches \n in middle no REG_NEWLINE ^ matches \n in middle no REG_NEWLINE This behaviour is not what happens when PCRE2 is called via its POSIX API. By default, PCRE2's behaviour is the same as Perl's, except that there is no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2 and Perl, there is no way to stop newline from matching [^a]. Default POSIX newline handling can be obtained by setting PCRE2_DOTALL and PCRE2_DOLLAR_ENDONLY when calling pcre2_compile() directly, but there is no way to make PCRE2 behave exactly as for the REG_NEWLINE action. When using the POSIX API, passing REG_NEWLINE to PCRE2's reg- comp() function causes PCRE2_MULTILINE to be passed to pcre2_compile(), and REG_DOTALL passes PCRE2_DOTALL. There is no way to pass PCRE2_DOL- LAR_ENDONLY. MATCHING A PATTERN The function regexec() is called to match a compiled pattern preg against a given string, which is by default terminated by a zero byte (but see REG_STARTEND below), subject to the options in eflags. These can be: REG_NOTBOL The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match- ing function. REG_NOTEMPTY The PCRE2_NOTEMPTY option is set when calling the underlying PCRE2 matching function. Note that REG_NOTEMPTY is not part of the POSIX standard. However, setting this option can give more POSIX-like behav- iour in some situations. REG_NOTEOL The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match- ing function. REG_STARTEND The string is considered to start at string + pmatch[0].rm_so and to have a terminating NUL located at string + pmatch[0].rm_eo (there need not actually be a NUL at that location), regardless of the value of nmatch. This is a BSD extension, compatible with but not specified by IEEE Standard 1003.2 (POSIX.2), and should be used with caution in software intended to be portable to other systems. Note that a non-zero rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location of the string, not how it is matched. Setting REG_STARTEND and passing pmatch as NULL are mutually exclusive; the error REG_INVARG is returned. If the pattern was compiled with the REG_NOSUB flag, no data about any matched strings is returned. The nmatch and pmatch arguments of regexec() are ignored (except possibly as input for REG_STARTEND). The value of nmatch may be zero, and the value pmatch may be NULL (unless REG_STARTEND is set); in both these cases no data about any matched strings is returned. Otherwise, the portion of the string that was matched, and also any captured substrings, are returned via the pmatch argument, which points to an array of nmatch structures of type regmatch_t, containing the members rm_so and rm_eo. These contain the byte offset to the first character of each substring and the offset to the first character after the end of each substring, respectively. The 0th element of the vector relates to the entire portion of string that was matched; subsequent elements relate to the capturing subpatterns of the regular expression. Unused entries in the array have both structure members set to -1. A successful match yields a zero return; various error codes are defined in the header file, of which REG_NOMATCH is the "expected" failure code. ERROR MESSAGES The regerror() function maps a non-zero errorcode from either regcomp() or regexec() to a printable message. If preg is not NULL, the error should have arisen from the use of that structure. A message terminated by a binary zero is placed in errbuf. If the buffer is too short, only the first errbuf_size - 1 characters of the error message are used. The yield of the function is the size of buffer needed to hold the whole message, including the terminating zero. This value is greater than errbuf_size if the message was truncated. MEMORY USAGE Compiling a regular expression causes memory to be allocated and asso- ciated with the preg structure. The function regfree() frees all such memory, after which preg may no longer be used as a compiled expres- sion. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 31 January 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2SAMPLE(3) Library Functions Manual PCRE2SAMPLE(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) PCRE2 SAMPLE PROGRAM A simple, complete demonstration program to get you started with using PCRE2 is supplied in the file pcre2demo.c in the src directory in the PCRE2 distribution. A listing of this program is given in the pcre2demo documentation. If you do not have a copy of the PCRE2 distribution, you can save this listing to re-create the contents of pcre2demo.c. The demonstration program compiles the regular expression that is its first argument, and matches it against the subject string in its second argument. No PCRE2 options are set, and default character tables are used. If matching succeeds, the program outputs the portion of the sub- ject that matched, together with the contents of any captured sub- strings. If the -g option is given on the command line, the program then goes on to check for further matches of the same regular expression in the same subject string. The logic is a little bit tricky because of the possi- bility of matching an empty string. Comments in the code explain what is going on. The code in pcre2demo.c is an 8-bit program that uses the PCRE2 8-bit library. It handles strings and characters that are stored in 8-bit code units. By default, one character corresponds to one code unit, but if the pattern starts with "(*UTF)", both it and the subject are treated as UTF-8 strings, where characters may occupy multiple code units. If PCRE2 is installed in the standard include and library directories for your operating system, you should be able to compile the demonstra- tion program using a command like this: cc -o pcre2demo pcre2demo.c -lpcre2-8 If PCRE2 is installed elsewhere, you may need to add additional options to the command line. For example, on a Unix-like system that has PCRE2 installed in /usr/local, you can compile the demonstration program using a command like this: cc -o pcre2demo -I/usr/local/include pcre2demo.c \ -L/usr/local/lib -lpcre2-8 Once you have built the demonstration program, you can run simple tests like this: ./pcre2demo 'cat|dog' 'the cat sat on the mat' ./pcre2demo -g 'cat|dog' 'the dog sat on the cat' Note that there is a much more comprehensive test program, called pcre2test, which supports many more facilities for testing regular expressions using all three PCRE2 libraries (8-bit, 16-bit, and 32-bit, though not all three need be installed). The pcre2demo program is pro- vided as a relatively simple coding example. If you try to run pcre2demo when PCRE2 is not installed in the standard library directory, you may get an error like this on some operating systems (e.g. Solaris): ld.so.1: pcre2demo: fatal: libpcre2-8.so.0: open failed: No such file or directory This is caused by the way shared library support works on those sys- tems. You need to add -R/usr/local/lib (for example) to the compile command to get round this problem. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 02 February 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2SERIALIZE(3) Library Functions Manual PCRE2SERIALIZE(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) SAVING AND RE-USING PRECOMPILED PCRE2 PATTERNS int32_t pcre2_serialize_decode(pcre2_code **codes, int32_t number_of_codes, const uint32_t *bytes, pcre2_general_context *gcontext); int32_t pcre2_serialize_encode(pcre2_code **codes, int32_t number_of_codes, uint32_t **serialized_bytes, PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext); void pcre2_serialize_free(uint8_t *bytes); int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes); If you are running an application that uses a large number of regular expression patterns, it may be useful to store them in a precompiled form instead of having to compile them every time the application is run. However, if you are using the just-in-time optimization feature, it is not possible to save and reload the JIT data, because it is posi- tion-dependent. The host on which the patterns are reloaded must be running the same version of PCRE2, with the same code unit width, and must also have the same endianness, pointer width and PCRE2_SIZE type. For example, patterns compiled on a 32-bit system using PCRE2's 16-bit library cannot be reloaded on a 64-bit system, nor can they be reloaded using the 8-bit library. SECURITY CONCERNS The facility for saving and restoring compiled patterns is intended for use within individual applications. As such, the data supplied to pcre2_serialize_decode() is expected to be trusted data, not data from arbitrary external sources. There is only some simple consistency checking, not complete validation of what is being re-loaded. SAVING COMPILED PATTERNS Before compiled patterns can be saved they must be serialized, that is, converted to a stream of bytes. A single byte stream may contain any number of compiled patterns, but they must all use the same character tables. A single copy of the tables is included in the byte stream (its size is 1088 bytes). For more details of character tables, see the sec- tion on locale support in the pcre2api documentation. The function pcre2_serialize_encode() creates a serialized byte stream from a list of compiled patterns. Its first two arguments specify the list, being a pointer to a vector of pointers to compiled patterns, and the length of the vector. The third and fourth arguments point to vari- ables which are set to point to the created byte stream and its length, respectively. The final argument is a pointer to a general context, which can be used to specify custom memory mangagement functions. If this argument is NULL, malloc() is used to obtain memory for the byte stream. The yield of the function is the number of serialized patterns, or one of the following negative error codes: PCRE2_ERROR_BADDATA the number of patterns is zero or less PCRE2_ERROR_BADMAGIC mismatch of id bytes in one of the patterns PCRE2_ERROR_MEMORY memory allocation failed PCRE2_ERROR_MIXEDTABLES the patterns do not all use the same tables PCRE2_ERROR_NULL the 1st, 3rd, or 4th argument is NULL PCRE2_ERROR_BADMAGIC means either that a pattern's code has been cor- rupted, or that a slot in the vector does not point to a compiled pat- tern. Once a set of patterns has been serialized you can save the data in any appropriate manner. Here is sample code that compiles two patterns and writes them to a file. It assumes that the variable fd refers to a file that is open for output. The error checking that should be present in a real application has been omitted for simplicity. int errorcode; uint8_t *bytes; PCRE2_SIZE erroroffset; PCRE2_SIZE bytescount; pcre2_code *list_of_codes[2]; list_of_codes[0] = pcre2_compile("first pattern", PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL); list_of_codes[1] = pcre2_compile("second pattern", PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL); errorcode = pcre2_serialize_encode(list_of_codes, 2, &bytes, &bytescount, NULL); errorcode = fwrite(bytes, 1, bytescount, fd); Note that the serialized data is binary data that may contain any of the 256 possible byte values. On systems that make a distinction between binary and non-binary data, be sure that the file is opened for binary output. Serializing a set of patterns leaves the original data untouched, so they can still be used for matching. Their memory must eventually be freed in the usual way by calling pcre2_code_free(). When you have fin- ished with the byte stream, it too must be freed by calling pcre2_seri- alize_free(). RE-USING PRECOMPILED PATTERNS In order to re-use a set of saved patterns you must first make the serialized byte stream available in main memory (for example, by read- ing from a file). The management of this memory block is up to the application. You can use the pcre2_serialize_get_number_of_codes() function to find out how many compiled patterns are in the serialized data without actually decoding the patterns: uint8_t *bytes = <serialized data>; int32_t number_of_codes = pcre2_serialize_get_number_of_codes(bytes); The pcre2_serialize_decode() function reads a byte stream and recreates the compiled patterns in new memory blocks, setting pointers to them in a vector. The first two arguments are a pointer to a suitable vector and its length, and the third argument points to a byte stream. The final argument is a pointer to a general context, which can be used to specify custom memory mangagement functions for the decoded patterns. If this argument is NULL, malloc() and free() are used. After deserial- ization, the byte stream is no longer needed and can be discarded. int32_t number_of_codes; pcre2_code *list_of_codes[2]; uint8_t *bytes = <serialized data>; int32_t number_of_codes = pcre2_serialize_decode(list_of_codes, 2, bytes, NULL); If the vector is not large enough for all the patterns in the byte stream, it is filled with those that fit, and the remainder are ignored. The yield of the function is the number of decoded patterns, or one of the following negative error codes: PCRE2_ERROR_BADDATA second argument is zero or less PCRE2_ERROR_BADMAGIC mismatch of id bytes in the data PCRE2_ERROR_BADMODE mismatch of code unit size or PCRE2 version PCRE2_ERROR_BADSERIALIZEDDATA other sanity check failure PCRE2_ERROR_MEMORY memory allocation failed PCRE2_ERROR_NULL first or third argument is NULL PCRE2_ERROR_BADMAGIC may mean that the data is corrupt, or that it was compiled on a system with different endianness. Decoded patterns can be used for matching in the usual way, and must be freed by calling pcre2_code_free(). However, be aware that there is a potential race issue if you are using multiple patterns that were decoded from a single byte stream in a multithreaded application. A single copy of the character tables is used by all the decoded patterns and a reference count is used to arrange for its memory to be automati- cally freed when the last pattern is freed, but there is no locking on this reference count. Therefore, if you want to call pcre2_code_free() for these patterns in different threads, you must arrange your own locking, and ensure that pcre2_code_free() cannot be called by two threads at the same time. If a pattern was processed by pcre2_jit_compile() before being serial- ized, the JIT data is discarded and so is no longer available after a save/restore cycle. You can, however, process a restored pattern with pcre2_jit_compile() if you wish. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 24 May 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2STACK(3) Library Functions Manual PCRE2STACK(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) PCRE2 DISCUSSION OF STACK USAGE When you call pcre2_match(), it makes use of an internal function called match(). This calls itself recursively at branch points in the pattern, in order to remember the state of the match so that it can back up and try a different alternative after a failure. As matching proceeds deeper and deeper into the tree of possibilities, the recur- sion depth increases. The match() function is also called in other cir- cumstances, for example, whenever a parenthesized sub-pattern is entered, and in certain cases of repetition. Not all calls of match() increase the recursion depth; for an item such as a* it may be called several times at the same level, after matching different numbers of a's. Furthermore, in a number of cases where the result of the recursive call would immediately be passed back as the result of the current call (a "tail recursion"), the function is just restarted instead. Each time the internal match() function is called recursively, it uses memory from the process stack. For certain kinds of pattern and data, very large amounts of stack may be needed, despite the recognition of "tail recursion". Note that if PCRE2 is compiled with the -fsani- tize=address option of the GCC compiler, the stack requirements are greatly increased. The above comments apply when pcre2_match() is run in its normal inter- pretive manner. If the compiled pattern was processed by pcre2_jit_com- pile(), and just-in-time compiling was successful, and the options passed to pcre2_match() were not incompatible, the matching process uses the JIT-compiled code instead of the match() function. In this case, the memory requirements are handled entirely differently. See the pcre2jit documentation for details. The pcre2_dfa_match() function operates in a different way to pcre2_match(), and uses recursion only when there is a regular expres- sion recursion or subroutine call in the pattern. This includes the processing of assertion and "once-only" subpatterns, which are handled like subroutine calls. Normally, these are never very deep, and the limit on the complexity of pcre2_dfa_match() is controlled by the amount of workspace it is given. However, it is possible to write pat- terns with runaway infinite recursions; such patterns will cause pcre2_dfa_match() to run out of stack unless a limit is applied (see below). The comments in the next three sections do not apply to pcre2_dfa_match(); they are relevant only for pcre2_match() without the JIT optimization. Reducing pcre2_match()'s stack usage You can often reduce the amount of recursion, and therefore the amount of stack used, by modifying the pattern that is being matched. Con- sider, for example, this pattern: ([^<]|<(?!inet))+ It matches from wherever it starts until it encounters "<inet" or the end of the data, and is the kind of pattern that might be used when processing an XML file. Each iteration of the outer parentheses matches either one character that is not "<" or a "<" that is not followed by "inet". However, each time a parenthesis is processed, a recursion occurs, so this formulation uses a stack frame for each matched charac- ter. For a long string, a lot of stack is required. Consider now this rewritten pattern, which matches exactly the same strings: ([^<]++|<(?!inet))+ This uses very much less stack, because runs of characters that do not contain "<" are "swallowed" in one item inside the parentheses. Recur- sion happens only when a "<" character that is not followed by "inet" is encountered (and we assume this is relatively rare). A possessive quantifier is used to stop any backtracking into the runs of non-"<" characters, but that is not related to stack usage. This example shows that one way of avoiding stack problems when match- ing long subject strings is to write repeated parenthesized subpatterns to match more than one character whenever possible. Compiling PCRE2 to use heap instead of stack for pcre2_match() In environments where stack memory is constrained, you might want to compile PCRE2 to use heap memory instead of stack for remembering back- up points when pcre2_match() is running. This makes it run more slowly, however. Details of how to do this are given in the pcre2build documen- tation. When built in this way, instead of using the stack, PCRE2 gets memory for remembering backup points from the heap. By default, the memory is obtained by calling the system malloc() function, but you can arrange to supply your own memory management function. For details, see the section entitled "The match context" in the pcre2api documentation. Since the block sizes are always the same, it may be possible to imple- ment a customized memory handler that is more efficient than the stan- dard function. The memory blocks obtained for this purpose are retained and re-used if possible while pcre2_match() is running. They are all freed just before it exits. Limiting pcre2_match()'s stack usage You can set limits on the number of times the internal match() function is called, both in total and recursively. If a limit is exceeded, pcre2_match() returns an error code. Setting suitable limits should prevent it from running out of stack. The default values of the limits are very large, and unlikely ever to operate. They can be changed when PCRE2 is built, and they can also be set when pcre2_match() is called. For details of these interfaces, see the pcre2build documentation and the section entitled "The match context" in the pcre2api documentation. As a very rough rule of thumb, you should reckon on about 500 bytes per recursion. Thus, if you want to limit your stack usage to 8Mb, you should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can support around 128000 recursions. The pcre2test test program has a modifier called "find_limits" which, if applied to a subject line, causes it to find the smallest limits that allow a a pattern to match. This is done by calling pcre2_match() repeatedly with different limits. Limiting pcre2_dfa_match()'s stack usage The recursion limit, as described above for pcre2_match(), also applies to pcre2_dfa_match(), whose use of recursive function calls for recur- sions in the pattern can lead to runaway stack usage. The non-recursive match limit is not relevant for DFA matching, and is ignored. Changing stack size in Unix-like systems In Unix-like environments, there is not often a problem with the stack unless very long strings are involved, though the default limit on stack size varies from system to system. Values from 8Mb to 64Mb are common. You can find your default limit by running the command: ulimit -s Unfortunately, the effect of running out of stack is often SIGSEGV, though sometimes a more explicit error message is given. You can nor- mally increase the limit on stack size by code such as this: struct rlimit rlim; getrlimit(RLIMIT_STACK, &rlim); rlim.rlim_cur = 100*1024*1024; setrlimit(RLIMIT_STACK, &rlim); This reads the current limits (soft and hard) using getrlimit(), then attempts to increase the soft limit to 100Mb using setrlimit(). You must do this before calling pcre2_match(). Changing stack size in Mac OS X Using setrlimit(), as described above, should also work on Mac OS X. It is also possible to set a stack size when linking a program. There is a discussion about stack sizes in Mac OS X at this web site: http://developer.apple.com/qa/qa2005/qa1419.html. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 23 December 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2SYNTAX(3) Library Functions Manual PCRE2SYNTAX(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY The full syntax and semantics of the regular expressions that are sup- ported by PCRE2 are described in the pcre2pattern documentation. This document contains a quick-reference summary of the syntax. QUOTING \x where x is non-alphanumeric is a literal x \Q...\E treat enclosed characters as literal ESCAPED CHARACTERS This table applies to ASCII and Unicode environments. \a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is any ASCII printing character \e escape (hex 1B) \f form feed (hex 0C) \n newline (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) \0dd character with octal code 0dd \ddd character with octal code ddd, or backreference \o{ddd..} character with octal code ddd.. \U "U" if PCRE2_ALT_BSUX is set (otherwise is an error) \uhhhh character with hex code hhhh (if PCRE2_ALT_BSUX is set) \xhh character with hex code hh \x{hhh..} character with hex code hhh.. Note that \0dd is always an octal code. The treatment of backslash fol- lowed by a non-zero digit is complicated; for details see the section "Non-printing characters" in the pcre2pattern documentation, where details of escape processing in EBCDIC environments are also given. When \x is not followed by {, from zero to two hexadecimal digits are read, but if PCRE2_ALT_BSUX is set, \x must be followed by two hexadec- imal digits to be recognized as a hexadecimal escape; otherwise it matches a literal "x". Likewise, if \u (in ALT_BSUX mode) is not fol- lowed by four hexadecimal digits, it matches a literal "u". CHARACTER TYPES . any character except newline; in dotall mode, any character whatsoever \C one code unit, even in UTF mode (best avoided) \d a decimal digit \D a character that is not a decimal digit \h a horizontal white space character \H a character that is not a horizontal white space character \N a character that is not a newline \p{xx} a character with the xx property \P{xx} a character without the xx property \R a newline sequence \s a white space character \S a character that is not a white space character \v a vertical white space character \V a character that is not a vertical white space character \w a "word" character \W a "non-word" character \X a Unicode extended grapheme cluster \C is dangerous because it may leave the current matching point in the middle of a UTF-8 or UTF-16 character. The application can lock out the use of \C by setting the PCRE2_NEVER_BACKSLASH_C option. It is also possible to build PCRE2 with the use of \C permanently disabled. By default, \d, \s, and \w match only ASCII characters, even in UTF-8 mode or in the 16-bit and 32-bit libraries. However, if locale-specific matching is happening, \s and \w may also match characters with code points in the range 128-255. If the PCRE2_UCP option is set, the behav- iour of these escape sequences is changed to use Unicode properties and they match many more characters. GENERAL CATEGORY PROPERTIES FOR \p and \P C Other Cc Control Cf Format Cn Unassigned Co Private use Cs Surrogate L Letter Ll Lower case letter Lm Modifier letter Lo Other letter Lt Title case letter Lu Upper case letter L& Ll, Lu, or Lt M Mark Mc Spacing mark Me Enclosing mark Mn Non-spacing mark N Number Nd Decimal number Nl Letter number No Other number P Punctuation Pc Connector punctuation Pd Dash punctuation Pe Close punctuation Pf Final punctuation Pi Initial punctuation Po Other punctuation Ps Open punctuation S Symbol Sc Currency symbol Sk Modifier symbol Sm Mathematical symbol So Other symbol Z Separator Zl Line separator Zp Paragraph separator Zs Space separator PCRE2 SPECIAL CATEGORY PROPERTIES FOR \p and \P Xan Alphanumeric: union of properties L and N Xps POSIX space: property Z or tab, NL, VT, FF, CR Xsp Perl space: property Z or tab, NL, VT, FF, CR Xuc Univerally-named character: one that can be represented by a Universal Character Name Xwd Perl word: property Xan or underscore Perl and POSIX space are now the same. Perl added VT to its space char- acter set at release 5.18. SCRIPT NAMES FOR \p AND \P Ahom, Anatolian_Hieroglyphs, Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hieroglyphs, Elbasan, Ethiopic, Geor- gian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hatran, Hebrew, Hiragana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese, Kaithi, Kan- nada, Katakana, Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian, Maha- jani, Malayalam, Mandaic, Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Multani, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Hungarian, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Shavian, Siddham, SignWriting, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi. CHARACTER CLASSES [...] positive character class [^...] negative character class [x-y] range (can be used for hex characters) [[:xxx:]] positive POSIX named set [[:^xxx:]] negative POSIX named set alnum alphanumeric alpha alphabetic ascii 0-127 blank space or tab cntrl control character digit decimal digit graph printing, excluding space lower lower case letter print printing, including space punct printing, excluding alphanumeric space white space upper upper case letter word same as \w xdigit hexadecimal digit In PCRE2, POSIX character set names recognize only ASCII characters by default, but some of them use Unicode properties if PCRE2_UCP is set. You can use \Q...\E inside a character class. QUANTIFIERS ? 0 or 1, greedy ?+ 0 or 1, possessive ?? 0 or 1, lazy * 0 or more, greedy *+ 0 or more, possessive *? 0 or more, lazy + 1 or more, greedy ++ 1 or more, possessive +? 1 or more, lazy {n} exactly n {n,m} at least n, no more than m, greedy {n,m}+ at least n, no more than m, possessive {n,m}? at least n, no more than m, lazy {n,} n or more, greedy {n,}+ n or more, possessive {n,}? n or more, lazy ANCHORS AND SIMPLE ASSERTIONS \b word boundary \B not a word boundary ^ start of subject also after an internal newline in multiline mode (after any newline if PCRE2_ALT_CIRCUMFLEX is set) \A start of subject $ end of subject also before newline at end of subject also before internal newline in multiline mode \Z end of subject also before newline at end of subject \z end of subject \G first matching position in subject MATCH POINT RESET \K reset start of match \K is honoured in positive assertions, but ignored in negative ones. ALTERNATION expr|expr|expr... CAPTURING (...) capturing group (?<name>...) named capturing group (Perl) (?'name'...) named capturing group (Perl) (?P<name>...) named capturing group (Python) (?:...) non-capturing group (?|...) non-capturing group; reset group numbers for capturing groups in each alternative ATOMIC GROUPS (?>...) atomic, non-capturing group COMMENT (?#....) comment (not nestable) OPTION SETTING (?i) caseless (?J) allow duplicate names (?m) multiline (?s) single line (dotall) (?U) default ungreedy (lazy) (?x) extended (ignore white space) (?-...) unset option(s) The following are recognized only at the very start of a pattern or after one of the newline or \R options with similar syntax. More than one of them may appear. (*LIMIT_MATCH=d) set the match limit to d (decimal number) (*LIMIT_RECURSION=d) set the recursion limit to d (decimal number) (*NOTEMPTY) set PCRE2_NOTEMPTY when matching (*NOTEMPTY_ATSTART) set PCRE2_NOTEMPTY_ATSTART when matching (*NO_AUTO_POSSESS) no auto-possessification (PCRE2_NO_AUTO_POSSESS) (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR) (*NO_JIT) disable JIT optimization (*NO_START_OPT) no start-match optimization (PCRE2_NO_START_OPTIMIZE) (*UTF) set appropriate UTF mode for the library in use (*UCP) set PCRE2_UCP (use Unicode properties for \d etc) Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value of the limits set by the caller of pcre2_match() or pcre2_dfa_match(), not increase them. The application can lock out the use of (*UTF) and (*UCP) by setting the PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options, respectively, at compile time. NEWLINE CONVENTION These are recognized only at the very start of the pattern or after option settings with a similar syntax. (*CR) carriage return only (*LF) linefeed only (*CRLF) carriage return followed by linefeed (*ANYCRLF) all three of the above (*ANY) any Unicode newline sequence WHAT \R MATCHES These are recognized only at the very start of the pattern or after option setting with a similar syntax. (*BSR_ANYCRLF) CR, LF, or CRLF (*BSR_UNICODE) any Unicode newline sequence LOOKAHEAD AND LOOKBEHIND ASSERTIONS (?=...) positive look ahead (?!...) negative look ahead (?<=...) positive look behind (?<!...) negative look behind Each top-level branch of a look behind must be of a fixed length. BACKREFERENCES \n reference by number (can be ambiguous) \gn reference by number \g{n} reference by number \g+n relative reference by number (PCRE2 extension) \g-n relative reference by number \g{+n} relative reference by number (PCRE2 extension) \g{-n} relative reference by number \k<name> reference by name (Perl) \k'name' reference by name (Perl) \g{name} reference by name (Perl) \k{name} reference by name (.NET) (?P=name) reference by name (Python) SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) (?R) recurse whole pattern (?n) call subpattern by absolute number (?+n) call subpattern by relative number (?-n) call subpattern by relative number (?&name) call subpattern by name (Perl) (?P>name) call subpattern by name (Python) \g<name> call subpattern by name (Oniguruma) \g'name' call subpattern by name (Oniguruma) \g<n> call subpattern by absolute number (Oniguruma) \g'n' call subpattern by absolute number (Oniguruma) \g<+n> call subpattern by relative number (PCRE2 extension) \g'+n' call subpattern by relative number (PCRE2 extension) \g<-n> call subpattern by relative number (PCRE2 extension) \g'-n' call subpattern by relative number (PCRE2 extension) CONDITIONAL PATTERNS (?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern) (?(n) absolute reference condition (?(+n) relative reference condition (?(-n) relative reference condition (?(<name>) named reference condition (Perl) (?('name') named reference condition (Perl) (?(name) named reference condition (PCRE2, deprecated) (?(R) overall recursion condition (?(Rn) specific numbered group recursion condition (?(R&name) specific named group recursion condition (?(DEFINE) define subpattern for reference (?(VERSION[>]=n.m) test PCRE2 version (?(assert) assertion condition Note the ambiguity of (?(R) and (?(Rn) which might be named reference conditions or recursion tests. Such a condition is interpreted as a reference condition if the relevant named group exists. BACKTRACKING CONTROL The following act immediately they are reached: (*ACCEPT) force successful match (*FAIL) force backtrack; synonym (*F) (*MARK:NAME) set name to be passed back; synonym (*:NAME) The following act only when a subsequent match failure causes a back- track to reach them. They all force a match failure, but they differ in what happens afterwards. Those that advance the start-of-match point do so only if the pattern is not anchored. (*COMMIT) overall failure, no advance of starting point (*PRUNE) advance to next starting character (*PRUNE:NAME) equivalent to (*MARK:NAME)(*PRUNE) (*SKIP) advance to current matching position (*SKIP:NAME) advance to position corresponding to an earlier (*MARK:NAME); if not found, the (*SKIP) is ignored (*THEN) local failure, backtrack to next alternation (*THEN:NAME) equivalent to (*MARK:NAME)(*THEN) CALLOUTS (?C) callout (assumed number 0) (?Cn) callout with numerical data n (?C"text") callout with string data The allowed string delimiters are ` ' " ^ % # $ (which are the same for the start and the end), and the starting delimiter { matched with the ending delimiter }. To encode the ending delimiter within the string, double it. SEE ALSO pcre2pattern(3), pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2(3). AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 23 December 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRE2UNICODE(3) Library Functions Manual PCRE2UNICODE(3) NAME PCRE - Perl-compatible regular expressions (revised API) UNICODE AND UTF SUPPORT When PCRE2 is built with Unicode support (which is the default), it has knowledge of Unicode character properties and can process text strings in UTF-8, UTF-16, or UTF-32 format (depending on the code unit width). However, by default, PCRE2 assumes that one code unit is one character. To process a pattern as a UTF string, where a character may require more than one code unit, you must call pcre2_compile() with the PCRE2_UTF option flag, or the pattern must start with the sequence (*UTF). When either of these is the case, both the pattern and any sub- ject strings that are matched against it are treated as UTF strings instead of strings of individual one-code-unit characters. If you do not need Unicode support you can build PCRE2 without it, in which case the library will be smaller. UNICODE PROPERTY SUPPORT When PCRE2 is built with Unicode support, the escape sequences \p{..}, \P{..}, and \X can be used. The Unicode properties that can be tested are limited to the general category properties such as Lu for an upper case letter or Nd for a decimal number, the Unicode script names such as Arabic or Han, and the derived properties Any and L&. Full lists are given in the pcre2pattern and pcre2syntax documentation. Only the short names for properties are supported. For example, \p{L} matches a let- ter. Its Perl synonym, \p{Letter}, is not supported. Furthermore, in Perl, many properties may optionally be prefixed by "Is", for compati- bility with Perl 5.6. PCRE does not support this. WIDE CHARACTERS AND UTF MODES Codepoints less than 256 can be specified in patterns by either braced or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3). Larger values have to use braced sequences. Unbraced octal code points up to \777 are also recognized; larger ones can be coded using \o{...}. In UTF modes, repeat quantifiers apply to complete UTF characters, not to individual code units. In UTF modes, the dot metacharacter matches one UTF character instead of a single code unit. The escape sequence \C can be used to match a single code unit in a UTF mode, but its use can lead to some strange effects because it breaks up multi-unit characters (see the description of \C in the pcre2pattern documentation). The use of \C is not supported by the alternative matching function pcre2_dfa_match() when in UTF-8 or UTF-16 mode, that is, when a charac- ter may consist of more than one code unit. The use of \C in these modes provokes a match-time error. Also, the JIT optimization does not support \C in these modes. If JIT optimization is requested for a UTF-8 or UTF-16 pattern that contains \C, it will not succeed, and so when pcre2_match() is called, the matching will be carried out by the normal interpretive function. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test characters of any code value, but, by default, the characters that PCRE2 recognizes as digits, spaces, or word characters remain the same set as in non-UTF mode, all with code points less than 256. This remains true even when PCRE2 is built to include Unicode support, because to do otherwise would slow down matching in many common cases. Note that this also applies to \b and \B, because they are defined in terms of \w and \W. If you want to test for a wider sense of, say, "digit", you can use explicit Unicode property tests such as \p{Nd}. Alternatively, if you set the PCRE2_UCP option, the way that the char- acter escapes work is changed so that Unicode properties are used to determine which characters match. There are more details in the section on generic character types in the pcre2pattern documentation. Similarly, characters that match the POSIX named character classes are all low-valued characters, unless the PCRE2_UCP option is set. However, the special horizontal and vertical white space matching escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char- acters, whether or not PCRE2_UCP is set. Case-insensitive matching in UTF mode makes use of Unicode properties. A few Unicode characters such as Greek sigma have more than two code- points that are case-equivalent, and these are treated as such. VALIDITY OF UTF STRINGS When the PCRE2_UTF option is set, the strings passed as patterns and subjects are (by default) checked for validity on entry to the relevant functions. If an invalid UTF string is passed, an negative error code is returned. The code unit offset to the offending character can be extracted from the match data block by calling pcre2_get_startchar(), which is used for this purpose after a UTF error. UTF-16 and UTF-32 strings can indicate their endianness by special code knows as a byte-order mark (BOM). The PCRE2 functions do not handle this, expecting strings to be in host byte order. A UTF string is checked before any other processing takes place. In the case of pcre2_match() and pcre2_dfa_match() calls with a non-zero starting offset, the check is applied only to that part of the subject that could be inspected during matching, and there is a check that the starting offset points to the first code unit of a character or to the end of the subject. If there are no lookbehind assertions in the pat- tern, the check starts at the starting offset. Otherwise, it starts at the length of the longest lookbehind before the starting offset, or at the start of the subject if there are not that many characters before the starting offset. Note that the sequences \b and \B are one-charac- ter lookbehinds. In addition to checking the format of the string, there is a check to ensure that all code points lie in the range U+0 to U+10FFFF, excluding the surrogate area. The so-called "non-character" code points are not excluded because Unicode corrigendum #9 makes it clear that they should not be. Characters in the "Surrogate Area" of Unicode are reserved for use by UTF-16, where they are used in pairs to encode code points with values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs are available independently in the UTF-8 and UTF-32 encodings. (In other words, the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8 and UTF-32.) In some situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve perfor- mance, for example in the case of a long subject string that is being scanned repeatedly. If you set the PCRE2_NO_UTF_CHECK option at com- pile time or at match time, PCRE2 assumes that the pattern or subject it is given (respectively) contains only valid UTF code unit sequences. Passing PCRE2_NO_UTF_CHECK to pcre2_compile() just disables the check for the pattern; it does not also apply to subject strings. If you want to disable the check for a subject string you must pass this option to pcre2_match() or pcre2_dfa_match(). If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the result is undefined and your program may crash or loop indefinitely. Errors in UTF-8 strings The following negative error codes are given for invalid UTF-8 strings: PCRE2_ERROR_UTF8_ERR1 PCRE2_ERROR_UTF8_ERR2 PCRE2_ERROR_UTF8_ERR3 PCRE2_ERROR_UTF8_ERR4 PCRE2_ERROR_UTF8_ERR5 The string ends with a truncated UTF-8 character; the code specifies how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 characters to be no longer than 4 bytes, the encoding scheme (origi- nally defined by RFC 2279) allows for up to 6 bytes, and this is checked first; hence the possibility of 4 or 5 missing bytes. PCRE2_ERROR_UTF8_ERR6 PCRE2_ERROR_UTF8_ERR7 PCRE2_ERROR_UTF8_ERR8 PCRE2_ERROR_UTF8_ERR9 PCRE2_ERROR_UTF8_ERR10 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of the character do not have the binary value 0b10 (that is, either the most significant bit is 0, or the next bit is 1). PCRE2_ERROR_UTF8_ERR11 PCRE2_ERROR_UTF8_ERR12 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes long; these code points are excluded by RFC 3629. PCRE2_ERROR_UTF8_ERR13 A 4-byte character has a value greater than 0x10fff; these code points are excluded by RFC 3629. PCRE2_ERROR_UTF8_ERR14 A 3-byte character has a value in the range 0xd800 to 0xdfff; this range of code points are reserved by RFC 3629 for use with UTF-16, and so are excluded from UTF-8. PCRE2_ERROR_UTF8_ERR15 PCRE2_ERROR_UTF8_ERR16 PCRE2_ERROR_UTF8_ERR17 PCRE2_ERROR_UTF8_ERR18 PCRE2_ERROR_UTF8_ERR19 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes for a value that can be represented by fewer bytes, which is invalid. For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor- rect coding uses just one byte. PCRE2_ERROR_UTF8_ERR20 The two most significant bits of the first byte of a character have the binary value 0b10 (that is, the most significant bit is 1 and the sec- ond is 0). Such a byte can only validly occur as the second or subse- quent byte of a multi-byte character. PCRE2_ERROR_UTF8_ERR21 The first byte of a character has the value 0xfe or 0xff. These values can never occur in a valid UTF-8 string. Errors in UTF-16 strings The following negative error codes are given for invalid UTF-16 strings: PCRE2_ERROR_UTF16_ERR1 Missing low surrogate at end of string PCRE2_ERROR_UTF16_ERR2 Invalid low surrogate follows high surrogate PCRE2_ERROR_UTF16_ERR3 Isolated low surrogate Errors in UTF-32 strings The following negative error codes are given for invalid UTF-32 strings: PCRE2_ERROR_UTF32_ERR1 Surrogate character (0xd800 to 0xdfff) PCRE2_ERROR_UTF32_ERR2 Code point is greater than 0x10ffff AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 03 July 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------