--- name: "pcre2posix" section: "3" platform: "linux" package: "pcre2" version: "10.49-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/pcre2posix" --- # pcre2posix(3) Perl-compatible regular expressions (revised API) ## NAME PCRE2 - Perl-compatible regular expressions (revised API) ## SYNOPSIS **#include \** ``` int pcre2_regcomp(regex_t *preg, const char *pattern, int cflags); int pcre2_regexec(const regex_t *preg, const char *string, size_t nmatch, regmatch_t pmatch[], int eflags); size_t pcre2_regerror(int errcode, const regex_t *preg, char *errbuf, size_t errbuf_size); void pcre2_regfree(regex_t *preg); ``` ## DESCRIPTION This set of functions provides a POSIX-style API for the PCRE2 regular expression 8-bit library. There are no POSIX-style wrappers for PCRE2's 16-bit and 32-bit libraries. See the **pcre2api** documentation for a description of PCRE2's native API, which contains much additional functionality. **IMPORTANT NOTE**: The functions described here are NOT thread-safe, and should not be used in multi-threaded applications. They are also limited to processing subjects that are not bigger than 2GB. Use the native API instead. These functions are wrapper functions that ultimately call the PCRE2 native API. Their prototypes are defined in the **pcre2posix.h** header file, and they all have unique names starting with **pcre2\_**. However, the **pcre2posix.h** header also contains macro definitions that convert the standard POSIX names such **regcomp()** into **pcre2\_regcomp()** etc. This means that a program can use the usual POSIX names without running the risk of accidentally linking with POSIX functions from a different library. On Unix-like systems the PCRE2 POSIX library is called **libpcre2-posix**, so can be accessed by adding **-lpcre2-posix** to the command for linking an application. Because the POSIX functions call the native ones, it is also necessary to add **-lpcre2-8**. On Windows systems, if you are linking to a DLL version of the library, it is recommended that **PCRE2POSIX\_SHARED** is defined before including the **pcre2posix.h** header, as it will allow for a more efficient way to invoke the functions by adding the **\_\_declspec(dllimport)** decorator. Although they were not defined as prototypes in **pcre2posix.h**, releases 10.33 to 10.36 of the library contained functions with the POSIX names **regcomp()** etc. These simply passed their arguments to the PCRE2 functions. These functions were provided for backwards compatibility with earlier versions of PCRE2, which had only POSIX names. However, this has proved troublesome in situations where a program links with several libraries, some of which use PCRE2's POSIX interface while others use the real POSIX functions. For this reason, the POSIX names have been removed since release 10.37. Calling the header file **pcre2posix.h** avoids any conflict with other POSIX libraries. It can, of course, be renamed or aliased as **regex.h**, which is the "correct" name, if there is no clash. It provides two structure types, *regex\_t* for compiled internal forms, and *regmatch\_t* for returning captured substrings. It also defines some constants whose names start with "REG\_"; these are used for setting options and identifying error codes. ## USING THE POSIX FUNCTIONS Note that these functions are just POSIX-style wrappers for PCRE2's native API. They do not give POSIX regular expression behaviour, and they are not thread-safe or even POSIX compatible. 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 or to add BSD or GNU functionality. 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 expressions 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 descriptions below use the actual names of the functions, but, as described above, the standard POSIX names (without the **pcre2\_** prefix) may also be used. ## COMPILING A PATTERN The function **pcre2\_regcomp()** is called to compile a pattern into an internal form. By default, the pattern is a C string terminated by a binary zero (but see REG\_PEND below). 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. It is also used for input when REG\_PEND is set. The **regex\_t** structure used by **pcre2\_regcomp()** is defined in **pcre2posix.h** and is not the same as the structure used by other libraries that provide POSIX-style matching. 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 section). REG\_NOSPEC The PCRE2\_LITERAL option is set when the regular expression is passed for compilation to the native function. This disables all meta characters in the pattern, causing it to be treated as a literal string. The only other options that are allowed with REG\_NOSPEC are REG\_ICASE, REG\_NOSUB, REG\_PEND, and REG\_UTF. Note that REG\_NOSPEC is not part of the POSIX standard. REG\_NOSUB When a pattern that is compiled with this flag is passed to **pcre2\_regexec()** for matching, the *nmatch* and *pmatch* arguments are ignored, and no captured strings are returned. Versions of the PCRE2 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 backreferences. REG\_PEND If this option is set, the **reg\_endp** field in the *preg* structure (which has the type const char \*) must be set to point to the character beyond the end of the pattern before calling **pcre2\_regcomp()**. The pattern itself may now contain binary zeros, which are treated as data characters. Without REG\_PEND, a binary zero terminates the pattern and the **re\_endp** field is ignored. This is a GNU extension to the POSIX standard and should be used with caution in software intended to be portable to other systems. 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 matching \\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 that 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 metacharacter (they are not) or by a negative class such as \[^a\] (they are). The yield of **pcre2\_regcomp()** is zero on success, and non-zero otherwise. The *preg* structure is filled in on success, and one other member of the structure (as well as *re\_endp*) 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 **pcre2\_regcomp()** is non-zero, you must not attempt to use the contents of the *preg* structure. If, for example, you pass it to **pcre2\_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 **pcre2\_regcomp()** function causes PCRE2\_MULTILINE to be passed to **pcre2\_compile()**, and REG\_DOTALL passes PCRE2\_DOTALL. There is no way to pass PCRE2\_DOLLAR\_ENDONLY. ## MATCHING A PATTERN The function **pcre2\_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 matching 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 behaviour in some situations. REG\_NOTEOL The PCRE2\_NOTEOL option is set when calling the underlying PCRE2 matching function. REG\_STARTEND When this option is set, the subject string starts at *string* + *pmatch\[0\].rm\_so* and ends at *string* + *pmatch\[0\].rm\_eo*, which should point to the first character beyond the string. There may be binary zeros within the subject string, and indeed, using REG\_STARTEND is the only way to pass a subject string that contains a binary zero. Whatever the value of *pmatch\[0\].rm\_so*, the offsets of the matched string and any captured substrings are still given relative to the start of *string* itself. (Before PCRE2 release 10.30 these were given relative to *string* + *pmatch\[0\].rm\_so*, but this differs from other implementations.) 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 and length 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 **pcre2\_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. *regmatch\_t* as well as the *regoff\_t* typedef it uses are defined in **pcre2posix.h** and are not warranted to have the same size or layout as other similarly named types from other libraries that provide POSIX-style matching. 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 **pcre2\_regerror()** function maps a non-zero errorcode from either **pcre2\_regcomp()** or **pcre2\_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 associated with the *preg* structure. The function **pcre2\_regfree()** frees all such memory, after which *preg* may no longer be used as a compiled expression. ## AUTHOR ``` Philip Hazel Retired from University Computing Service Cambridge, England. ``` ## REVISION ``` Last updated: 27 November 2024 Copyright (c) 1997-2024 University of Cambridge. ```