PCREPOSIX(3) Introduction to Library Functions PCREPOSIX(3)

NAME

PCRE - Perl-compatible regular expressions.

SYNOPSIS

#include <pcreposix.h>

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

This set of functions provides a POSIX-style API for the PCRE regular
expression 8-bit library. See the pcreapi documentation for a
description of PCRE's native API, which contains much additional
functionality. There is no POSIX-style wrapper for PCRE's 16-bit and
32-bit library.

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 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 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

This area is not simple, because POSIX and Perl take different views
of things. It is not possible to get PCRE to obey POSIX semantics,
but then PCRE was never intended to be a POSIX engine. The following
table lists the different possibilities for matching newline
characters in PCRE:

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

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 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

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. The length
of the message, including the zero, is limited to errbuf_size. The
yield of the function is the size of buffer needed to hold the whole
message.

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 CB2 3QH, England.

REVISION

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

PCRE 8.30 09 January 2012 PCREPOSIX(3)