PCRE2 - Perl-compatible regular expressions (revised API)
#include <pcre2posix.h>
int pcre2_regcomp(regex_t *preg, const char *pattern,
int cflags);
int pcre2_regexec(const regex_t *preg, const char *string,
size_t nmatch, regmatch_t pmatch[], int eflags);
size_t pcre2_regerror(int errcode, const regex_t *preg,
char *errbuf, size_t errbuf_size);
void pcre2_regfree(regex_t *preg);
This set of functions provides a POSIX-style API for the PCRE2 regular
expression 8-bit library. There are no POSIX-style wrappers for PCRE2's 16-bit
and 32-bit libraries. See the
pcre2api documentation for a description
of PCRE2's native API, which contains much additional functionality.
The functions described here are wrapper functions that ultimately call the
PCRE2 native API. Their prototypes are defined in the
pcre2posix.h
header file, and they all have unique names starting with
pcre2_.
However, the
pcre2posix.h header also contains macro definitions that
convert the standard POSIX names such
regcomp() into
pcre2_regcomp() etc. This means that a program can use the usual POSIX
names without running the risk of accidentally linking with POSIX functions
from a different library.
On Unix-like systems the PCRE2 POSIX library is called
libpcre2-posix, so
can be accessed by adding
-lpcre2-posix to the command for linking an
application. Because the POSIX functions call the native ones, it is also
necessary to add
-lpcre2-8.
Although they were not defined as protypes in
pcre2posix.h, releases
10.33 to 10.36 of the library contained functions with the POSIX names
regcomp() etc. These simply passed their arguments to the PCRE2
functions. These functions were provided for backwards compatibility with
earlier versions of PCRE2, which had only POSIX names. However, this has
proved troublesome in situations where a program links with several libraries,
some of which use PCRE2's POSIX interface while others use the real POSIX
functions. For this reason, the POSIX names have been removed since release
10.37.
Calling the header file
pcre2posix.h avoids any conflict with other POSIX
libraries. It can, of course, be renamed or aliased as
regex.h, which
is the "correct" name, if there is no clash. 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.
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 or to add BSD or GNU functionality.
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 descriptions below use the actual names of the functions, but, as described
above, the standard POSIX names (without the
pcre2_ prefix) may also be
used.
The function
pcre2_regcomp() is called to compile a pattern into an
internal form. By default, the pattern is a C string terminated by a binary
zero (but see REG_PEND below). 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. (It is also used for input when REG_PEND is
set.)
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_NOSPEC
The PCRE2_LITERAL option is set when the regular expression is passed for
compilation to the native function. This disables all meta characters in the
pattern, causing it to be treated as a literal string. The only other options
that are allowed with REG_NOSPEC are REG_ICASE, REG_NOSUB, REG_PEND, and
REG_UTF. Note that REG_NOSPEC is not part of the POSIX standard.
REG_NOSUB
When a pattern that is compiled with this flag is passed to
pcre2_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
backreferences.
REG_PEND
If this option is set, the
reg_endp field in the
preg structure
(which has the type const char *) must be set to point to the character beyond
the end of the pattern before calling
pcre2_regcomp(). The pattern
itself may now contain binary zeros, which are treated as data characters.
Without REG_PEND, a binary zero terminates the pattern and the
re_endp
field is ignored. This is a GNU extension to the POSIX standard and should be
used with caution in software intended to be portable to other systems.
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
pcre2_regcomp() is zero on success, and non-zero otherwise.
The
preg structure is filled in on success, and one other member of the
structure (as well as
re_endp) 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
pcre2_regcomp() is non-zero, you must not attempt
to use the contents of the
preg structure. If, for example, you pass it
to
pcre2_regexec(), the result is undefined and your program is likely
to crash.
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
pcre2_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.
The function
pcre2_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
When this option is set, the subject string starts at
string +
pmatch[0].rm_so and ends at
string +
pmatch[0].rm_eo,
which should point to the first character beyond the string. There may be
binary zeros within the subject string, and indeed, using REG_STARTEND is the
only way to pass a subject string that contains a binary zero.
Whatever the value of
pmatch[0].rm_so, the offsets of the matched string
and any captured substrings are still given relative to the start of
string itself. (Before PCRE2 release 10.30 these were given relative to
string +
pmatch[0].rm_so, but this differs from other
implementations.)
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 and length 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
pcre2_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.
The
pcre2_regerror() function maps a non-zero errorcode from either
pcre2_regcomp() or
pcre2_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.
Compiling a regular expression causes memory to be allocated and associated with
the
preg structure. The function
pcre2_regfree() frees all such
memory, after which
preg may no longer be used as a compiled
expression.
Philip Hazel
University Computing Service
Cambridge, England.
Last updated: 26 April 2021
Copyright (c) 1997-2021 University of Cambridge.