Provided by: libpcre2-dev_10.42-4ubuntu3_amd64 
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
SYNOPSIS
#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);
DESCRIPTION
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.
USING THE POSIX FUNCTIONS
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.
COMPILING A PATTERN
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.
MATCHING NEWLINE CHARACTERS
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.
MATCHING A PATTERN
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.
ERROR MESSAGES
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.
MEMORY USAGE
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.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 26 April 2021
Copyright (c) 1997-2021 University of Cambridge.