DESCRIPTION

This set of functions provides a POSIX-style API for the PCRE2 regular expression 8-bit library. See the pcre2api(3) documentation for a description of PCRE2's native API, which contains much additional functionality. 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 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 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 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.

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 section).

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 captured 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 metacharacter (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 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 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

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 associated with the preg structure. The function regfree() frees all such memory, after which preg may no longer be used as a compiled expression.

AUTHOR

Philip Hazel
University Computing Service
Cambridge, England.

REVISION

Last updated: 31 January 2016
Copyright (c) 1997-2016 University of Cambridge.