PCRE - Perl-compatible regular expressions.
SYNOPSIS OF POSIX API
int regcomp(regex_t *preg, const char *pattern,
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);
This set of functions provides a POSIX-style API to the PCRE regular
expression package. See the pcreapi documentation for a description of
the native API, which contains additional functionality.
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 which uses them. Because the POSIX
functions call the native ones, it is also necessary to add -lpcre.
I have implemented only those option bits that can be reasonably mapped
to PCRE native options. In addition, the options REG_EXTENDED and
REG_NOSUB are defined with the value zero. They have no effect, but
since programs that are written to the POSIX interface often use them,
this makes it easier to slot in PCRE as a replacement library. Other
POSIX options are not even defined.
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 expres‐
sions 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 reg‐
match_t for returning captured substrings. It also defines some con‐
stants 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 which is used as a base for storing information about
the compiled expression.
The argument cflags is either zero, or contains one or more of the bits
defined by the following macros:
The PCRE_CASELESS option is set when the expression is passed for com‐
pilation to the native function.
The PCRE_MULTILINE option is set when the expression is passed for com‐
pilation to the native function. Note that this does not mimic the
defined POSIX behaviour for REG_NEWLINE (see the following section).
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 aren't) 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.
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
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 equiva‐
lent for PCRE_DOLLARENDONLY 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_DOLLARENDONLY, 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 pre-compiled pattern preg
against a given string, which is terminated by a zero byte, subject to
the options in eflags. These can be:
The PCRE_NOTBOL option is set when calling the underlying PCRE matching
The PCRE_NOTEOL option is set when calling the underlying PCRE matching
The portion of the string that was matched, and also any captured sub‐
strings, 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 sub‐
string and the offset to the first character after the end of each sub‐
string, 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"
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 func‐
tion is the size of buffer needed to hold the whole message.
Compiling a regular expression causes memory to be allocated and asso‐
ciated with the preg structure. The function regfree() frees all such
memory, after which preg may no longer be used as a compiled expres‐
Philip Hazel <firstname.lastname@example.org>
University Computing Service,
Cambridge CB2 3QG, England.
Last updated: 03 February 2003Copyright (c) 1997-2003 University of Cambridge.