NAME

SYNOPSIS

int regcomp(regex_t *preg, const char *pattern,
     int cflags);

int regexec(regex_t *preg, const char *string,
     size_t nmatch, regmatch_t pmatch[], int eflags);
     size_t regerror(int errcode, const regex_t *preg,
     char *errbuf, size_t errbuf_size);

void regfree(regex_t *preg);

DESCRIPTION

The functions described here are just wrapper functions that ultimately call the PCRE native API. Their prototypes are defined in the pcreposix.h header file, and on Unix systems the library itself is called pcreposix.a, so can be accessed by adding -lpcreposix to the command for linking an application that uses them. Because the POSIX functions call the native ones, it is also necessary to add -lpcre.

I have implemented only those POSIX option bits that can be reasonably mapped to PCRE native options. 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 PCRE as a replacement library. Other POSIX options are not even defined.

There are also some other options that are not defined by POSIX. These have been added at the request of users who want to make use of certain PCRE-specific features via the POSIX calling interface.

When PCRE 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 PCRE 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-byte encoding domains it is probably even less compatible.

The header for these functions is supplied as pcreposix.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 argument cflags is either zero, or contains one or more of the bits defined by the following macros:

REG_DOTALL

The PCRE_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 PCRE_CASELESS option is set when the regular expression is passed for compilation to the native function.

REG_NEWLINE

The PCRE_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

The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is passed for compilation to the native function. In addition, 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.

REG_UCP

The PCRE_UCP option is set when the regular expression is passed for compilation to the native function. This causes PCRE to use Unicode properties when matchine \d, \w, etc., instead of just recognizing ASCII values. Note that REG_UTF8 is not part of the POSIX standard.

REG_UNGREEDY

The PCRE_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_UTF8

The PCRE_UTF8 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_UTF8 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 PCRE 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 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. It does not affect the way newlines are matched by . (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

Default Change with

. matches newline no PCRE_DOTALL newline matches [^a] yes not changeable $ matches \n at end yes PCRE_DOLLARENDONLY $ matches \n in middle no PCRE_MULTILINE ^ matches \n in middle no PCRE_MULTILINE

This is the equivalent table for POSIX:

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

PCRE's behaviour is the same as Perl's, except that there is no equivalent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is no way to stop newline from matching [^a].

The default POSIX newline handling can be obtained by setting PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE behave exactly as for the REG_NEWLINE action.

MATCHING A PATTERN

REG_NOTBOL

The PCRE_NOTBOL option is set when calling the underlying PCRE matching function.

REG_NOTEMPTY

The PCRE_NOTEMPTY option is set when calling the underlying PCRE 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 PCRE_NOTEOL option is set when calling the underlying PCRE 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.

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.

If the value of nmatch is zero, or if the value pmatch is NULL, 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 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

MEMORY USAGE

AUTHOR

Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.

REVISION

Last updated: 09 January 2012
Copyright (c) 1997-2012 University of Cambridge.