PCRE2 — Perl-compatible regular expressions (revised API)
#include <pcre2posix.h> int regcomp(regex_t *preg
, const char *pattern
, intcflags
); int regexec(const regex_t *preg
, const char *string
, size_tnmatch
, regmatch_tpmatch
[], inteflags
); size_t regerror(interrcode
, const regex_t *preg
, char *errbuf
, size_terrbuf_size
);
void
regfree( |
regex_t
*preg) ; |
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 is no POSIX-style wrapper 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 other 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.
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
The PCRE2_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 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.
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Note |
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If the yield of |
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 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 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
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].
The default POSIX newline handling can be obtained by setting PCRE2_DOTALL and PCRE2_DOLLAR_ENDONLY, but there is no way to make PCRE2 behave exactly as for the REG_NEWLINE action.
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.
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
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.
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.
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.
Last updated: 20 October 2014 Copyright (c) 1997-2014 University of Cambridge.
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This manual page is taken from the PCRE library, which is distributed under the BSD license. |