pcre2api

PCRE2 is a new API for PCRE, starting at release 10.0. This document contains a description of all its native functions. See the pcre2 document for an overview of all the PCRE2 documentation.

These functions became obsolete at release 10.30 and are retained only for backward compatibility. They should not be used in new code. The first is replaced by pcre2_set_depth_limit(); the second is no longer needed and has no effect (it always returns zero).

These functions provide a way of converting non-PCRE2 patterns into patterns that can be processed by pcre2_compile(). This facility is experimental and may be changed in future releases. At present, "globs" and POSIX basic and extended patterns can be converted. Details are given in the pcre2convert documentation.

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 simultaneously. On Unix-like systems the libraries are called libpcre2-8, libpcre2-16, and libpcre2-32, and they can also co-exist with the original PCRE libraries. Every PCRE2 function comes in three different forms, one for each library, for example:

There are also three different sets of data types:

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 pointers to constants of the equivalent UCHAR types, that is, they are pointers to vectors of unsigned code units.

Character strings are passed to a PCRE2 library as sequences of unsigned integers in code units of the appropriate width. The length of a string may be given as a number of code units, or the string may be specified as zero-terminated.

Many applications use only one code unit width. For their convenience, macros are defined whose names are the generic forms such as pcre2_compile() and PCRE2_SPTR. These macros use the value of the macro PCRE2_CODE_UNIT_WIDTH to generate the appropriate width-specific function 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 pattern that was compiled with pcre2_compile_16(), you must do so with pcre2_match_16(), not pcre2_match_8() or pcre2_match_32().

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 has its own native API, which is described in this document. There are also some wrapper functions for the 8-bit library that correspond to the POSIX regular expression API, but they do not give access to all the functionality of PCRE2 and they are not thread-safe. They are described in the pcre2posix documentation. 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 also contains definitions 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.

The compiling and matching functions recognize various options that are passed as bits in an options argument. There are also some more complicated parameters such as custom memory management functions and resource limits that are passed in "contexts" (which are just memory blocks, described below). Simple applications do not need to make use of contexts.

Just-in-time (JIT) 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 at the expense of less sanity checking. The JIT-specific functions are discussed in the pcre2jit documentation.

A second matching function, pcre2_dfa_match(), which is not Perl-compatible, 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 lookaround assertions). However, this algorithm does not return captured substrings. A description of the two matching algorithms 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_free() and pcre2_substring_list_free() are also provided, to free memory used for extracted strings. If either of these functions is called with a NULL argument, the function returns immediately without doing anything.

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 compiled pattern (pcre2_pattern_info()) and about the configuration with which PCRE2 was built (pcre2_config()) and that it is using.

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.

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. Note that string lengths are always given in code units. Only in the 8-bit library is such a length the same as the number of bytes in the string.

PCRE2 supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (linefeed) character, the two-character sequence CRLF, any of the three preceding, 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. If it is not, the default is set to LF, which is the Unix standard. 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 advancement for a non-anchored pattern. There is more detail about this in the section Option bits for pcre2_match() 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.

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 applications can use it.

There are several different blocks of data that are used to pass information between the application and the PCRE2 libraries.

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 function pcre2_config() makes it possible for a PCRE2 client to find the value of certain configuration parameters and to discover which optional features have been compiled into the PCRE2 library. The pcre2build documentation has more details about these 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_BADOPTION if the value in the first argument is not recognized. The following information is available:

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 in code units. If the pattern is zero-terminated, the length can be specified as PCRE2_ZERO_TERMINATED. A NULL pattern pointer with a length of zero is treated as an empty string (NULL with a non-zero length causes an error return). 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 compiled 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. If pcre2_code_free() is called with a NULL argument, it returns immediately, without doing anything.

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 matching, though it can be passed to pcre2_jit_compile() if required. If pcre2_code_copy() is called with a NULL argument, it returns NULL.

The pcre2_code_copy() function provides a way for individual threads in a multithreaded application to acquire a private copy of shared compiled 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 automatically freed when pcre2_code_free() is called for the new copy of the compiled code. If pcre2_code_copy_with_tables() is called with a NULL argument, it returns NULL.

The options argument for pcre2_compile() contains various bit settings that affect the compilation. It should be zero if none of them 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 pattern, the contents of the options argument specifies their settings at the start of compilation. The PCRE2_ANCHORED, PCRE2_ENDANCHORED, and PCRE2_NO_UTF_CHECK options can be set at the time of matching as well as at compile time.

Some additional options and 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 immediately. 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.

There are over 100 positive error codes that pcre2_compile() may return if it finds an error in the pattern. There are also some negative error codes that are used for invalid UTF strings when validity checking is in force. These are the same as given by pcre2_match() and pcre2_dfa_match(), and are described in the pcre2unicode documentation. There is no separate documentation for the positive error codes, because the textual error messages that are obtained by calling the pcre2_get_error_message() function (see "Obtaining a textual error message" below) should be self-explanatory. Macro names starting with PCRE2_ERROR_ are defined for both positive and negative error codes in pcre2.h. When compilation is successful *errorptr is set to a value that returns the message no error if passed to pcre2_get_error_message().

The value returned in *erroroffset is an indication of where in the pattern an error occurred. When there is no error, zero is returned. A non-zero value 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. 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 character.

This code fragment shows a typical straightforward call to pcre2_compile():

These functions provide support for JIT compilation, which, if the just-in-time compiler is available, further processes a compiled pattern 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 patterns 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.

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. However, 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.

When PCRE2 is built with Unicode support (the default), certain Unicode character properties can be tested with \p and \P, or, alternatively, 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. PCRE2_UCP also causes upper/lower casing operations on characters with code points greater than 127 to use Unicode properties. These effects apply even when PCRE2_UTF is not set. There are, however, some PCRE2_EXTRA options (see above) that can be used to modify or suppress them.

The use of locales with Unicode is discouraged. If you are handling characters with code points greater than 127, you should either use Unicode support, or use locales, but not try to mix the two.

PCRE2 contains a built-in 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 different.

The built-in tables can be overridden by tables supplied by the application that calls PCRE2. These may be created in a different locale from the default. As more and more applications change to using Unicode, 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 only argument to this function is a general context, which can be used to pass a custom memory allocator. If the argument is NULL, the system malloc() is used. 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 127 are treated as letters), the following code could be used:

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".

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 the matching functions. Thus, for any single pattern, compilation and matching both happen in the same locale, but different patterns can be processed in different locales.

It is the caller’s responsibility to ensure that the memory containing the tables remains available while they are still in use. When they are no longer needed, you can discard them using pcre2_maketables_free(), which should pass as its first parameter the same global context that was used to create the tables.

The pcre2_pattern_info() function returns general information about a compiled pattern. For information about callouts, see the section Information about a pattern’s callouts. The first argument for pcre2_pattern_info() is a pointer to the compiled 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 numbers:

The "magic number" is placed at the start of each compiled pattern as a simple check against passing an arbitrary memory pointer. Here is a typical call of pcre2_pattern_info(), to obtain the length of the compiled pattern:

The possible values for the second argument are defined in pcre2.h, and are as follows:

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 enumeration block, and its second argument is the user_data value that was passed to pcre2_callout_enumerate(). The contents of the callout enumeration block are described in the pcre2callout documentation, which also gives further details about callouts.

It is possible to save compiled patterns on disc or elsewhere, and reload them later, subject to a number of restrictions. 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. Before compiled patterns can be saved, they must be converted to a "serialized" form, which in the case of PCRE2 is really just a bytecode dump. The functions whose names begin with pcre2_serialize_ are used for converting to and from the serialized form. They are described in the pcre2serialize documentation. Note that PCRE2 serialization does not convert compiled patterns to an abstract format like Java or .NET serialization.

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 parts of the subject. 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 functions above. For pcre2_match_data_create(), the first argument is the number of pairs of offsets in the ovector.

When using pcre2_match(), one pair of offsets is required to identify the string that matched the whole pattern, with an additional 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.

When using pcre2_dfa_match() there may be multiple matched substrings of different lengths at the same point in the subject. The ovector should be made large enough to hold as many as are expected.

A minimum of at least 1 pair is imposed by pcre2_match_data_create(), so it is always possible to return the overall matched string in the case of pcre2_match() or the longest match in the case of pcre2_dfa_match(). The maximum number of pairs is 65535; if the first argument of pcre2_match_data_create() is greater than this, 65535 is used.

The second argument of pcre2_match_data_create() is a pointer to a general 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 when matched using pcre2_match(). You should not use this call when matching with pcre2_dfa_match(). The second 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 a successful match. 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, unless, in the case of the subject string, you have used the PCRE2_COPY_MATCHED_SUBJECT option, which is described in the section Option bits for pcre2_match() below.

When a match data block itself is no longer needed, it should be freed by calling pcre2_match_data_free(). If this function is called with a NULL argument, it returns immediately, without doing anything.

The size of a match data block depends on the size of the ovector that it contains. The function pcre2_get_match_data_size() returns the size, in bytes, of the block that is its argument.

When pcre2_match() runs interpretively (that is, without using JIT), it makes use of a vector of data frames for remembering backtracking positions. The size of each individual frame depends on the number of capturing parentheses in the pattern and can be obtained by calling pcre2_pattern_info() with the PCRE2_INFO_FRAMESIZE option (see the section Information about a compiled pattern above).

Heap memory is used for the frames vector; if the initial memory block turns out to be too small during matching, it is automatically expanded. When pcre2_match() returns, the memory is not freed, but remains attached to the match data block, for use by any subsequent matches that use the same block. It is automatically freed when the match data block itself is freed.

You can find the current size of the frames vector that a match data block owns by calling pcre2_get_match_data_heapframes_size(). For a newly created match data block the size will be zero. Some types of match may require a lot of frames and thus a large vector; applications that run in environments where memory is constrained can check this and free the match data block if the heap frames vector has become too big.

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 different 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():

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 section The match context above.

When PCRE2 is built, a default newline convention is set; this is usually 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 Newline conventions in the pcre2pattern page. During matching, the newline choice affects the behaviour 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 contains an explicit CR or LF reference, and so advances only by one character 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 or equivalent octal or hexadecimal 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.

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 "capture group" (Perl terminology) is used for a fragment of a pattern that picks out a substring. PCRE2 supports several other kinds of parenthesized group that do not cause substrings to be captured. The pcre2_pattern_info() function can be used to find out how many capture groups 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 values, 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 contains.

Within the ovector, the first in each pair of values is set to the offset 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 values 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 fully successful match, the first pair of offsets identifies the portion of the subject string that was matched by the entire pattern. The next pair is used for the first captured substring, 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 captured, the returned value is 3. If there are no captured substrings, 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 lookahead 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 capture 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).

It is possible for capture group number n+1 to match some part of the subject when group 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 groups 1 and 3 are matched, but 2 is not. When this happens, both values in the offset pairs corresponding to unused groups are set to PCRE2_UNSET.

Offset values that correspond to unused groups 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)?)? groups 2 and 3 are not matched. The return from the function is 2, because the highest used capture group number is 1. The offsets for the second and third capture groups (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 capturing parentheses, no more than ovector[0] to ovector[2n+1] are set by pcre2_match(). The other elements retain whatever values they previously had. After a failed match attempt, the contents of the ovector are unchanged.

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), pcre2_get_subject() returns the subject pointer that was passed to the matching function. For a successful match with PCRE2_COPY_MATCHED_SUBJECT, it returns a pointer to a copy of the subject that is owned by the match data block instead (except for pcre2_jit_match() which ignores PCRE2_COPY_MATCHED_SUBJECT). The copy is exactly the same length as the subject that was passed to the matching function and is additionally zero-terminated. The copy is freed when the match data block is reused for another match or is freed. If there is any match error other than PCRE2_ERROR_NOMATCH or PCRE2_ERROR_PARTIAL, pcre2_get_subject() may return NULL. If the lengthptr argument is not NULL, it receives the length in code units of the string which is returned, excluding the terminating zero (the subject string may contain embedded NUL characters).

After a successful match, a partial match (PCRE2_ERROR_PARTIAL), or a failure to match (PCRE2_ERROR_NOMATCH), a mark name may be available. The function pcre2_get_mark() can be called to access this name, which can be specified in the pattern by any of the backtracking control verbs, not just (*MARK). The same function applies to all the verbs. It returns a pointer to the zero-terminated name, which is within the compiled pattern. If no name is available, NULL is returned. The length of the name (excluding the terminating zero) is stored in the code unit that precedes the name. You should use this length instead of relying on the terminating zero if the name might contain a binary zero.

After a successful match, the name that is returned is the last mark name encountered on the matching path through the pattern. Instances of backtracking verbs without names do not count. Thus, for example, if the matching path contains (*MARK:A)(*PRUNE), the name A is returned. After a "no match" or a partial match, the last encountered name is returned. For example, consider this pattern:

When it matches bc, the returned name 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 name 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.

If pcre2_match() fails, it returns a negative number. This can be converted to a text string by calling the pcre2_get_error_message() function (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():

A text message for an error code from any PCRE2 function (compile, match, or auxiliary) can be obtained by calling pcre2_get_error_message(). The code is passed as the first argument, with the remaining two arguments specifying a code unit buffer and its length in code units, into which the text message is placed. 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 function 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 message is truncated (but still with a trailing zero), and the negative error code PCRE2_ERROR_NOMEMORY is returned. None of the messages is very long; a buffer size of 120 code units is ample.

A common task for applications is to implement "global" matching behaviour, for example, replacing all matches in the subject; splitting the subject on all matches; or simply counting the number of matches. The pcre2_next_match() function helps with this task by providing the appropriate parameters for the next match attempt (available since PCRE2 10.47).

First, a match attempt should be made using one of the matching functions (pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match()). Then, pcre2_next_match() can be called, providing the same match_data parameter.

It returns 0 ("false") if there is no need to make a further match attempt, or 1 ("true") if another match should be attempted. Returning 1 does not imply that there is another match, only that another match should be attempted (which may return PCRE2_ERROR_NOMATCH).

The *pstart_offset and *poptions are set if the function returns 1. The *pstart_offset should be passed to the next match attempt directly, and the *poptions should be passed to the next match attempt by combining with the application’s match options using OR.

There is some code that demonstrates how to do this in the pcre2demo sample program. The general pattern is:

The guarantees provided by pcre2_next_match() are that the start_offset will advance, so the loop will definitely terminate. The conditions which ensure this are that either:

A loop implemented as shown above will always terminate, unless there is a bug in PCRE2. As a measure of "defensive programming", applications are encouraged to add an assertion or check to break their loop if it does not make progress (and report the issue as a bug).

If an application does not use the flag PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK, then each match is "well-behaved" and satisfies:

In this case, the matches found by pcre2_match() with pcre2_next_match() will be sorted, non-overlapping (possibly touching), and with no duplicates.

Otherwise, if PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK is used, then the guarantees are considerably weaker. We do not guarantee that the matches will always advance: only that the start_offset will. The matches found by pcre2_match() with pcre2_next_match() will be a finite sequence (as pcre2_next_match() ensures that start_offset advances, so the search will terminate). The matches can however be overlapping, can contain duplicates, and (in truly pathological examples) may not even be sorted by ovector[0]. Additionally, each match itself can end before it starts (ovector[1] < ovector[0]). We recommend that applications do not set PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK.

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 referring 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 section Extracting a list of all captured substrings describes similar functions for extracting captured substrings by name.

If a pattern uses the \K escape sequence within a positive lookahead 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 substring 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 arguments of these functions are a pointer to the match data block and a capture 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:

The pcre2_substring_list_get() function extracts all available substrings 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 argument 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 memory 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 capture group number n+1 matches some part of the subject, but group 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_substring_length_bynumber().

To extract a substring by name, you first have to find associated number. For example, for this pattern:

the number of the capture group 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 argument is the compiled pattern, and the second is the name. The yield of the function is the group number, PCRE2_ERROR_NOSUBSTRING if there is no group with that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if there is more than one group with that name. Given the number, you can extract the substring directly from the ovector, or use one of the "bynumber" 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 captured substring from the first named group 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.

This function optionally calls pcre2_match() and then makes a copy of the subject string in buffer, replacing parts that were matched with the replacement string, whose length is supplied in rlength, which can be given as PCRE2_ZERO_TERMINATED for a zero-terminated string. As a special case, if replacement is NULL and rlength is zero, the replacement is assumed to be an empty string. If rlength is non-zero, an error occurs if replacement is NULL.

There is an option (see PCRE2_SUBSTITUTE_REPLACEMENT_ONLY below) to return just the replacement string(s). The default action is to perform just one replacement if the pattern matches, but there is an option that requests multiple replacements (see PCRE2_SUBSTITUTE_GLOBAL below).

If successful, pcre2_substitute() returns the number of substitutions that were carried out. This may be zero if no match was found, and is never greater than one unless PCRE2_SUBSTITUTE_GLOBAL is set. A negative value is returned if an error is detected.

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. For global replacements, matches in which \K in a lookbehind causes the match to start earlier than the point that was reached in the previous iteration are also not supported. (These cases are only possible if the pattern was compiled with the backwards-compatibility option PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK.)

The first seven arguments of pcre2_substitute() are the same as for pcre2_match(), except that the partial matching options are not permitted, and match_data may be passed as NULL, in which case a match data block is obtained and freed within this function, using memory management functions from the match context, if provided, or else those that were used to allocate memory for the compiled code.

If match_data is not NULL and PCRE2_SUBSTITUTE_MATCHED is not set, the provided block is used for all calls to pcre2_match(), and its contents afterwards are the result of the final call made internally by pcre2_substitute() to the matching function. For global changes, this will always be a no-match error. The contents of the ovector within the match data block may or may not have been changed.

As well as the usual options for pcre2_match(), a number of additional options can be set in the options argument of pcre2_substitute(). One such option is PCRE2_SUBSTITUTE_MATCHED. When this is set, an external match_data block must be provided, and it must have already been used for an external call to pcre2_match() (or pcre2_jit_match()) with the same pattern, subject pointer, effective subject length, start offset, and match option arguments (substitute-specific options can be added to the options argument). If any of these parameters is changed, pcre2_substitute() returns an error. The data in the match_data block (return code, offset vector) is used for the first substitution instead of calling pcre2_match() from within pcre2_substitute(). This allows an application to check for a match before choosing to substitute, without having to repeat the match.

If the contents of the subject buffer are mutated in between pcre2_match() and a call to pcre2_substitute() with PCRE2_SUBSTITUTE_MATCHED, the behaviour is unsafe; in particular, in this case, PCRE2 is unable to ensure that the offsets in the ovector point to the start of characters (with UTF-encoded input).

The contents of the externally supplied match data block are not changed when PCRE2_SUBSTITUTE_MATCHED is set, and so the match block is permitted for use in another call using PCRE2_SUBSTITUTE_MATCHED. If PCRE2_SUBSTITUTE_GLOBAL is also set, pcre2_match() is called after the first substitution to check for furthe matches, but this is done using an internally obtained match data block, thus always leaving the external block unchanged.

The code argument is not used for matching before the first substitution when PCRE2_SUBSTITUTE_MATCHED is set, but it must be provided, even when PCRE2_SUBSTITUTE_GLOBAL is not set, because it contains information such as the UTF setting and the number of capturing parentheses in the pattern.

When using PCRE2_SUBSTITUTE_MATCHED, you should not modify the subject string in between the prior call to pcre2_match() and pcre2_substitute(), as the substitution assumes that the passed-in ovector is compatible with the subject string. Although PCRE2 does verify that the subject is a pointer to the same buffer, it cannot in general verify whether the contents of the buffer have changed. For example, if the subject buffer is mutated from one valid UTF-8 string to another valid string, of the same length in code units, the ovector offsets are no longer guaranteed to point to the start of a character. Beware that with PCRE2_SUBSTITUTE_MATCHED in UTF mode, the subject string is not re-scanned for UTF validity when pcre2_substitute() first uses it.

The default action of pcre2_substitute() is to return a copy of the subject string with matched substrings replaced. However, if PCRE2_SUBSTITUTE_REPLACEMENT_ONLY is set, only the replacement substrings are returned. In the global case, multiple replacements are concatenated in the output buffer. Substitution callouts (see below) can be used to separate them if necessary.

Partial matching is supported, with limitations: if matching succeeds but with a partial match, then pcre2_substitute() returns PCRE2_ERROR_PARTIAL. When partial-matching (either of PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT is passed), then PCRE2_SUBSTITUTE_REPLACEMENT_ONLY must also be set, or else PCRE2_ERROR_BADOPTION is returned. Similarly, certain replacement items ($' and $_) cause PCRE2_ERROR_PARTIALSUBS to be returned when partial-matching, even if a complete match is found.

The blength argument of pcre2_substitute() must point to a variable that contains the length, in code units, of the output buffer. If the function is successful, the value is updated to contain the length in code units of the new string, excluding the trailing zero that is automatically added.

If the function is not successful, the value set via blength 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.

PCRE2_SUBSTITUTE_OVERFLOW_LENGTH changes what happens when the output buffer is too small. The default action is to return PCRE2_ERROR_NOMEMORY immediately. If this option is set, however, pcre2_substitute() continues to go through the motions of matching and substituting (without, of course, writing anything) in order to compute the size of buffer that is needed, which will include the extra space for the terminating NUL. This value is passed back via the blength variable, with the result of the function still being PCRE2_ERROR_NOMEMORY.

Passing a buffer size of zero (with buffer optionally NULL) 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 application, it may be more efficient to allocate a large buffer and free the excess afterwards, instead of using PCRE2_SUBSTITUTE_OVERFLOW_LENGTH.

The replacement string, which is interpreted as a UTF string in UTF mode, is checked for UTF validity unless PCRE2_NO_UTF_CHECK is set. An invalid UTF replacement string causes an immediate return with the relevant UTF error code.

If PCRE2_SUBSTITUTE_LITERAL is set, the replacement string is not interpreted in any way. By default, however, a dollar character is an escape character that can specify the insertion of characters from capture groups and names from (*MARK) or other control verbs in the pattern. Dollar is the only escape character (backslash is treated as literal). The following forms are recognized:

Either a group number or a group name can be given for n, for example $2 or $NAME. Curly brackets are required only if the following character would be interpreted 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 JavaScript form $<name>, where the angle brackets are part of the syntax, is also recognized for group names, but not for group numbers or *MARK.

$*MARK inserts the name from the last encountered backtracking control verb on the matching path that has a name. (*MARK) must always include a name, but the other verbs need not. For example, in the case of (*MARK:A)(*PRUNE) the name inserted is A, but for (*MARK:A)(*PRUNE:B) the relevant name is B. This facility can be used to perform simple simultaneous substitutions, as this pcre2test example shows:

PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject string, replacing every matching substring. If this option is not set, only the first matching substring is replaced. The search for matches takes place in the original subject string (that is, previous replacements do not affect it). Iteration is implemented by advancing the start_offset value for each search, which is always passed the entire subject string. If an offset limit is set in the match context, searching stops when that limit is reached.

Because global substitutions apply the pattern repeatedly to the subject string, and always iterate over non-overlapping matches, the substitutions done by pcre2_substitute() do not match and substitute text inside the replacement strings themselves (no recursive/iterative substitution). However, applications can easily implement other alternative replacement strategies, such as iteratively replacing, then matching and replacing on the result. The replacement loop inside pcre2_substitute() is simple and can be emulated in client code by allocating a buffer, searching for matches in a loop, and calling pcre2_substitute() with PCRE2_SUBSTITUTE_REPLACEMENT_ONLY an PCRE2_SUBSTITUTE_MATCHED, and without PCRE2_SUBSTITUTE_GLOBAL.

You can restrict the effect of a global substitution to a portion of the subject string by setting either or both of start_offset and an offset limit. Here is a pcre2test example:

When continuing with global substitutions after matching a substring with zero length, an attempt to find a non-empty match at the same offset is performed. If this is not successful, the offset 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 offset is advanced by two characters.

PCRE2_SUBSTITUTE_UNKNOWN_UNSET causes references to capture 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 capture 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, several things change:

Firstly, backslash in a replacement string is interpreted as an escape character. The usual forms such as \x{ddd} can be used to specify particular character codes, and backslash followed by any non-alphanumeric character quotes that character. Extended quoting can be coded using \Q...\E, exactly as in pattern strings. The escapes \b and \v are interpreted as the characters backspace and vertical tab, respectively.

The interpretation of backslash followed by one or more digits is the same as in a pattern, which in Perl has some ambiguities. Details are given in the pcre2pattern page.

The Python form \g<n>, where the angle brackets are part of the syntax and n is either a group name or number, is recognized as an alternative way of inserting the contents of a group, for example \g<3>.

There are also four escape sequences for forcing the case of inserted letters. Case forcing applies to all inserted characters, including those from capture groups and letters within \Q...\E quoted sequences. 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, respectively, 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.

However, if \u is immediately followed by \L or \l is immediately followed by \U, the next character’s case is forced by the first escape sequence, and subsequent characters by the second. This provides a "title casing" facility that can be applied to group captures. For example, if group 1 has captured heLLo, the replacement string \u\L$1 becomes Hello.

If either PCRE2_UTF or PCRE2_UCP was set when the pattern was compiled, Unicode properties are used for case forcing characters whose code points are greater than 127. However, only simple case folding, as determined by the Unicode file CaseFolding.txt is supported. PCRE2 does not support language-specific special casing rules such as using different lower case Greek sigmas in the middle and ends of words (as defined in the Unicode file SpecialCasing.txt).

Note that case forcing sequences such as \U...\E do not nest. For example, the result of processing \Uaa\LBB\Ecc\E is AAbbcc; the final \E has no effect. Note also that the PCRE2_ALT_BSUX and PCRE2_EXTRA_ALT_BSUX options do not apply to replacement strings.

The final effect of setting PCRE2_SUBSTITUTE_EXTENDED is to add more flexibility to capture group substitution. The syntax is similar to that used by Bash:

As in the simple case, n may be a group number or a name. The first form specifies a default value. If group n is set, its value is inserted; if not, the 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:

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 PCRE2_SUBSTITUTE_LITERAL is set, PCRE2_SUBSTITUTE_UNKNOWN_UNSET, PCRE2_SUBSTITUTE_UNSET_EMPTY, and PCRE2_SUBSTITUTE_EXTENDED are irrelevant and are ignored.

When a pattern is compiled with the PCRE2_DUPNAMES option, names for capture groups are not required to be unique. Duplicate names are always allowed for groups with the same number, created by using the (?| feature. Indeed, if such groups are named, they are required to use the same names.

Normally, patterns that use duplicate names are such that in any one match, only one of each set of identically-named groups 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 Information about a compiled pattern. Given all the relevant entries for the name, you can extract each of their numbers, and hence the captured data.

The traditional matching function uses a similar algorithm to Perl, which stops when it finds the first match at a given point in the subject. 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 function, 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 pattern. When your callout function is called, extract and save the current 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.

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 (not counting lookaround assertions), and does not backtrack (except when processing lookaround assertions). This has different characteristics to the normal algorithm, and is not compatible with Perl. Some of the features of PCRE2 patterns are not supported. Nevertheless, 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 documentation.

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 common 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():

pcre2build, pcre2callout, pcre2demo, pcre2matching, pcre2partial, pcre2posix, pcre2sample, pcre2unicode.

Philip Hazel
Retired from University Computing Service
Cambridge, England.

Last updated: 08 December 2025
Copyright © 1997-2024 University of Cambridge.