This file contains a concatenation of the PCRE man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. There are
separate text files for the pcregrep and pcretest commands.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
DESCRIPTION
The PCRE library is a set of functions that implement regular expres-
sion pattern matching using the same syntax and semantics as Perl, with
just a few differences. The current implementation of PCRE (release
4.x) corresponds approximately with Perl 5.8, including support for
UTF-8 encoded strings. However, this support has to be explicitly
enabled; it is not the default.
PCRE is written in C and released as a C library. However, a number of
people have written wrappers and interfaces of various kinds. A C++
class is included in these contributions, which can be found in the
Contrib directory at the primary FTP site, which is:
ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
Details of exactly which Perl regular expression features are and are
not supported by PCRE are given in separate documents. See the pcrepat-
tern and pcrecompat pages.
Some features of PCRE can be included, excluded, or changed when the
library is built. The pcre_config() function makes it possible for a
client to discover which features are available. Documentation about
building PCRE for various operating systems can be found in the README
file in the source distribution.
USER DOCUMENTATION
The user documentation for PCRE has been split up into a number of dif-
ferent sections. In the "man" format, each of these is a separate "man
page". In the HTML format, each is a separate page, linked from the
index page. In the plain text format, all the sections are concate-
nated, for ease of searching. The sections are as follows:
pcre this document
pcreapi details of PCRE's native API
pcrebuild options for building PCRE
pcrecallout details of the callout feature
pcrecompat discussion of Perl compatibility
pcregrep description of the pcregrep command
pcrepattern syntax and semantics of supported
regular expressions
pcreperform discussion of performance issues
pcreposix the POSIX-compatible API
pcresample discussion of the sample program
pcretest the pcretest testing command
In addition, in the "man" and HTML formats, there is a short page for
each library function, listing its arguments and results.
LIMITATIONS
There are some size limitations in PCRE but it is hoped that they will
never in practice be relevant.
The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
is compiled with the default internal linkage size of 2. If you want to
process regular expressions that are truly enormous, you can compile
PCRE with an internal linkage size of 3 or 4 (see the README file in
the source distribution and the pcrebuild documentation for details).
If these cases the limit is substantially larger. However, the speed
of execution will be slower.
All values in repeating quantifiers must be less than 65536. The maxi-
mum number of capturing subpatterns is 65535.
There is no limit to the number of non-capturing subpatterns, but the
maximum depth of nesting of all kinds of parenthesized subpattern,
including capturing subpatterns, assertions, and other types of subpat-
tern, is 200.
The maximum length of a subject string is the largest positive number
that an integer variable can hold. However, PCRE uses recursion to han-
dle subpatterns and indefinite repetition. This means that the avail-
able stack space may limit the size of a subject string that can be
processed by certain patterns.
UTF-8 SUPPORT
Starting at release 3.3, PCRE has had some support for character
strings encoded in the UTF-8 format. For release 4.0 this has been
greatly extended to cover most common requirements.
In order process UTF-8 strings, you must build PCRE to include UTF-8
support in the code, and, in addition, you must call pcre_compile()
with the PCRE_UTF8 option flag. When you do this, both the pattern and
any subject strings that are matched against it are treated as UTF-8
strings instead of just strings of bytes.
If you compile PCRE with UTF-8 support, but do not use it at run time,
the library will be a bit bigger, but the additional run time overhead
is limited to testing the PCRE_UTF8 flag in several places, so should
not be very large.
The following comments apply when PCRE is running in UTF-8 mode:
1. When you set the PCRE_UTF8 flag, the strings passed as patterns and
subjects are checked for validity on entry to the relevant functions.
If an invalid UTF-8 string is passed, an error return is given. In some
situations, you may already know that your strings are valid, and
therefore want to skip these checks in order to improve performance. If
you set the PCRE_NO_UTF8_CHECK flag at compile time or at run time,
PCRE assumes that the pattern or subject it is given (respectively)
contains only valid UTF-8 codes. In this case, it does not diagnose an
invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE when
PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program may
crash.
2. In a pattern, the escape sequence \x{...}, where the contents of the
braces is a string of hexadecimal digits, is interpreted as a UTF-8
character whose code number is the given hexadecimal number, for exam-
ple: \x{1234}. If a non-hexadecimal digit appears between the braces,
the item is not recognized. This escape sequence can be used either as
a literal, or within a character class.
3. The original hexadecimal escape sequence, \xhh, matches a two-byte
UTF-8 character if the value is greater than 127.
4. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
vidual bytes, for example: \x{100}{3}.
5. The dot metacharacter matches one UTF-8 character instead of a
single byte.
6. The escape sequence \C can be used to match a single byte in UTF-8
mode, but its use can lead to some strange effects.
7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
test characters of any code value, but the characters that PCRE recog-
nizes as digits, spaces, or word characters remain the same set as
before, all with values less than 256.
8. Case-insensitive matching applies only to characters whose values
are less than 256. PCRE does not support the notion of "case" for
higher-valued characters.
9. PCRE does not support the use of Unicode tables and properties or
the Perl escapes \p, \P, and \X.
AUTHOR
Philip Hazel <ph10@cam.ac.uk>
University Computing Service,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714
Last updated: 20 August 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE BUILD-TIME OPTIONS
This document describes the optional features of PCRE that can be
selected when the library is compiled. They are all selected, or dese-
lected, by providing options to the configure script which is run
before the make command. The complete list of options for configure
(which includes the standard ones such as the selection of the instal-
lation directory) can be obtained by running
./configure --help
The following sections describe certain options whose names begin with
--enable or --disable. These settings specify changes to the defaults
for the configure command. Because of the way that configure works,
--enable and --disable always come in pairs, so the complementary
option always exists as well, but as it specifies the default, it is
not described.
UTF-8 SUPPORT
To build PCRE with support for UTF-8 character strings, add
--enable-utf8
to the configure command. Of itself, this does not make PCRE treat
strings as UTF-8. As well as compiling PCRE with this option, you also
have have to set the PCRE_UTF8 option when you call the pcre_compile()
function.
CODE VALUE OF NEWLINE
By default, PCRE treats character 10 (linefeed) as the newline charac-
ter. This is the normal newline character on Unix-like systems. You can
compile PCRE to use character 13 (carriage return) instead by adding
--enable-newline-is-cr
to the configure command. For completeness there is also a --enable-
newline-is-lf option, which explicitly specifies linefeed as the new-
line character.
BUILDING SHARED AND STATIC LIBRARIES
The PCRE building process uses libtool to build both shared and static
Unix libraries by default. You can suppress one of these by adding one
of
--disable-shared
--disable-static
to the configure command, as required.
POSIX MALLOC USAGE
When PCRE is called through the POSIX interface (see the pcreposix
documentation), additional working storage is required for holding the
pointers to capturing substrings because PCRE requires three integers
per substring, whereas the POSIX interface provides only two. If the
number of expected substrings is small, the wrapper function uses space
on the stack, because this is faster than using malloc() for each call.
The default threshold above which the stack is no longer used is 10; it
can be changed by adding a setting such as
--with-posix-malloc-threshold=20
to the configure command.
LIMITING PCRE RESOURCE USAGE
Internally, PCRE has a function called match() which it calls repeat-
edly (possibly recursively) when performing a matching operation. By
limiting the number of times this function may be called, a limit can
be placed on the resources used by a single call to pcre_exec(). The
limit can be changed at run time, as described in the pcreapi documen-
tation. The default is 10 million, but this can be changed by adding a
setting such as
--with-match-limit=500000
to the configure command.
HANDLING VERY LARGE PATTERNS
Within a compiled pattern, offset values are used to point from one
part to another (for example, from an opening parenthesis to an alter-
nation metacharacter). By default two-byte values are used for these
offsets, leading to a maximum size for a compiled pattern of around
64K. This is sufficient to handle all but the most gigantic patterns.
Nevertheless, some people do want to process enormous patterns, so it
is possible to compile PCRE to use three-byte or four-byte offsets by
adding a setting such as
--with-link-size=3
to the configure command. The value given must be 2, 3, or 4. Using
longer offsets slows down the operation of PCRE because it has to load
additional bytes when handling them.
If you build PCRE with an increased link size, test 2 (and test 5 if
you are using UTF-8) will fail. Part of the output of these tests is a
representation of the compiled pattern, and this changes with the link
size.
AVOIDING EXCESSIVE STACK USAGE
PCRE implements backtracking while matching by making recursive calls
to an internal function called match(). In environments where the size
of the stack is limited, this can severely limit PCRE's operation. (The
Unix environment does not usually suffer from this problem.) An alter-
native approach that uses memory from the heap to remember data,
instead of using recursive function calls, has been implemented to work
round this problem. If you want to build a version of PCRE that works
this way, add
--disable-stack-for-recursion
to the configure command. With this configuration, PCRE will use the
pcre_stack_malloc and pcre_stack_free variables to call memory
management functions. Separate functions are provided because the usage
is very predictable: the block sizes requested are always the same, and
the blocks are always freed in reverse order. A calling program might
be able to implement optimized functions that perform better than the
standard malloc() and free() functions. PCRE runs noticeably more
slowly when built in this way.
USING EBCDIC CODE
PCRE assumes by default that it will run in an environment where the
character code is ASCII (or UTF-8, which is a superset of ASCII). PCRE
can, however, be compiled to run in an EBCDIC environment by adding
--enable-ebcdic
to the configure command.
Last updated: 09 December 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
SYNOPSIS OF PCRE API
#include <pcre.h>
pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);
void pcre_free_substring(const char *stringptr);
void pcre_free_substring_list(const char **stringptr);
const unsigned char *pcre_maketables(void);
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
int pcre_config(int what, void *where);
char *pcre_version(void);
void *(*pcre_malloc)(size_t);
void (*pcre_free)(void *);
void *(*pcre_stack_malloc)(size_t);
void (*pcre_stack_free)(void *);
int (*pcre_callout)(pcre_callout_block *);
PCRE API
PCRE has its own native API, which is described in this document. There
is also a set of wrapper functions that correspond to the POSIX regular
expression API. These are described in the pcreposix documentation.
The native API function prototypes are defined in the header file
pcre.h, and on Unix systems the library itself is called libpcre.a, so
can be accessed by adding -lpcre to the command for linking an applica-
tion which calls it. The header file defines the macros PCRE_MAJOR and
PCRE_MINOR to contain the major and minor release numbers for the
library. Applications can use these to include support for different
releases.
The functions pcre_compile(), pcre_study(), and pcre_exec() are used
for compiling and matching regular expressions. A sample program that
demonstrates the simplest way of using them is given in the file pcre-
demo.c. The pcresample documentation describes how to run it.
There are convenience functions for extracting captured substrings from
a matched subject string. They are:
pcre_copy_substring()
pcre_copy_named_substring()
pcre_get_substring()
pcre_get_named_substring()
pcre_get_substring_list()
pcre_free_substring() and pcre_free_substring_list() are also provided,
to free the memory used for extracted strings.
The function pcre_maketables() is used (optionally) to build a set of
character tables in the current locale for passing to pcre_compile().
The function pcre_fullinfo() is used to find out information about a
compiled pattern; pcre_info() is an obsolete version which returns only
some of the available information, but is retained for backwards com-
patibility. The function pcre_version() returns a pointer to a string
containing the version of PCRE and its date of release.
The global variables pcre_malloc and pcre_free initially contain the
entry points of the standard malloc() and free() functions respec-
tively. PCRE calls the memory management functions via these variables,
so a calling program can replace them if it wishes to intercept the
calls. This should be done before calling any PCRE functions.
The global variables pcre_stack_malloc and pcre_stack_free are also
indirections to memory management functions. These special functions
are used only when PCRE is compiled to use the heap for remembering
data, instead of recursive function calls. This is a non-standard way
of building PCRE, for use in environments that have limited stacks.
Because of the greater use of memory management, it runs more slowly.
Separate functions are provided so that special-purpose external code
can be used for this case. When used, these functions are always called
in a stack-like manner (last obtained, first freed), and always for
memory blocks of the same size.
The global variable pcre_callout initially contains NULL. It can be set
by the caller to a "callout" function, which PCRE will then call at
specified points during a matching operation. Details are given in the
pcrecallout documentation.
MULTITHREADING
The PCRE functions can be used in multi-threading applications, with
the proviso that the memory management functions pointed to by
pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
callout function pointed to by pcre_callout, are shared by all threads.
The compiled form of a regular expression is not altered during match-
ing, so the same compiled pattern can safely be used by several threads
at once.
CHECKING BUILD-TIME OPTIONS
int pcre_config(int what, void *where);
The function pcre_config() makes it possible for a PCRE client to dis-
cover which optional features have been compiled into the PCRE library.
The pcrebuild documentation has more details about these optional fea-
tures.
The first argument for pcre_config() is an integer, specifying which
information is required; the second argument is a pointer to a variable
into which the information is placed. The following information is
available:
PCRE_CONFIG_UTF8
The output is an integer that is set to one if UTF-8 support is avail-
able; otherwise it is set to zero.
PCRE_CONFIG_NEWLINE
The output is an integer that is set to the value of the code that is
used for the newline character. It is either linefeed (10) or carriage
return (13), and should normally be the standard character for your
operating system.
PCRE_CONFIG_LINK_SIZE
The output is an integer that contains the number of bytes used for
internal linkage in compiled regular expressions. The value is 2, 3, or
4. Larger values allow larger regular expressions to be compiled, at
the expense of slower matching. The default value of 2 is sufficient
for all but the most massive patterns, since it allows the compiled
pattern to be up to 64K in size.
PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
The output is an integer that contains the threshold above which the
POSIX interface uses malloc() for output vectors. Further details are
given in the pcreposix documentation.
PCRE_CONFIG_MATCH_LIMIT
The output is an integer that gives the default limit for the number of
internal matching function calls in a pcre_exec() execution. Further
details are given with pcre_exec() below.
PCRE_CONFIG_STACKRECURSE
The output is an integer that is set to one if internal recursion is
implemented by recursive function calls that use the stack to remember
their state. This is the usual way that PCRE is compiled. The output is
zero if PCRE was compiled to use blocks of data on the heap instead of
recursive function calls. In this case, pcre_stack_malloc and
pcre_stack_free are called to manage memory blocks on the heap, thus
avoiding the use of the stack.
COMPILING A PATTERN
pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
The function pcre_compile() 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. A pointer to a single block of
memory that is obtained via pcre_malloc is returned. This contains the
compiled code and related data. The pcre type is defined for the
returned block; this is a typedef for a structure whose contents are
not externally defined. It is up to the caller to free the memory when
it is no longer required.
Although the compiled code of a PCRE regex is relocatable, that is, it
does not depend on memory location, the complete pcre data block is not
fully relocatable, because it contains a copy of the tableptr argument,
which is an address (see below).
The options argument contains independent bits that affect the compila-
tion. It should be zero if no options are required. Some of the
options, in particular, those that are compatible with Perl, can also
be set and unset from within the pattern (see the detailed description
of regular expressions in the pcrepattern documentation). For these
options, the contents of the options argument specifies their initial
settings at the start of compilation and execution. The PCRE_ANCHORED
option can be set at the time of matching as well as at compile time.
If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
if compilation of a pattern fails, pcre_compile() returns NULL, and
sets the variable pointed to by errptr to point to a textual error mes-
sage. The offset from the start of the pattern to the character where
the error was discovered is placed in the variable pointed to by
erroffset, which must not be NULL. If it is, an immediate error is
given.
If the final argument, tableptr, is NULL, PCRE uses a default set of
character tables which are built when it is compiled, using the default
C locale. Otherwise, tableptr must be the result of a call to
pcre_maketables(). See the section on locale support below.
This code fragment shows a typical straightforward call to pcre_com-
pile():
pcre *re;
const char *error;
int erroffset;
re = pcre_compile(
"^A.*Z", /* the pattern */
0, /* default options */
&error, /* for error message */
&erroffset, /* for error offset */
NULL); /* use default character tables */
The following option bits are defined:
PCRE_ANCHORED
If this bit is set, the pattern is forced to be "anchored", that is, it
is constrained to match only at the first matching point in the string
which is being searched (the "subject string"). This effect can also be
achieved by appropriate constructs in the pattern itself, which is the
only way to do it in Perl.
PCRE_CASELESS
If this bit is set, letters in the pattern match both upper and lower
case letters. It is equivalent to Perl's /i option, and it can be
changed within a pattern by a (?i) option setting.
PCRE_DOLLAR_ENDONLY
If this bit is set, a dollar metacharacter in the pattern matches only
at the end of the subject string. Without this option, a dollar also
matches immediately before the final character if it is a newline (but
not before any other newlines). The PCRE_DOLLAR_ENDONLY option is
ignored if PCRE_MULTILINE is set. There is no equivalent to this option
in Perl, and no way to set it within a pattern.
PCRE_DOTALL
If this bit is set, a dot metacharater in the pattern matches all char-
acters, including newlines. Without it, newlines are excluded. This
option is equivalent to Perl's /s option, and it can be changed within
a pattern by a (?s) option setting. A negative class such as [^a]
always matches a newline character, independent of the setting of this
option.
PCRE_EXTENDED
If this bit is set, whitespace data characters in the pattern are
totally ignored except when escaped or inside a character class.
Whitespace does not include the VT character (code 11). In addition,
characters between an unescaped # outside a character class and the
next newline character, inclusive, are also ignored. This is equivalent
to Perl's /x option, and it can be changed within a pattern by a (?x)
option setting.
This option makes it possible to include comments inside complicated
patterns. Note, however, that this applies only to data characters.
Whitespace characters may never appear within special character
sequences in a pattern, for example within the sequence (?( which
introduces a conditional subpattern.
PCRE_EXTRA
This option was invented in order to turn on additional functionality
of PCRE that is incompatible with Perl, but it is currently of very
little use. When set, any backslash in a pattern that is followed by a
letter that has no special meaning causes an error, thus reserving
these combinations for future expansion. By default, as in Perl, a
backslash followed by a letter with no special meaning is treated as a
literal. There are at present no other features controlled by this
option. It can also be set by a (?X) option setting within a pattern.
PCRE_MULTILINE
By default, PCRE treats the subject string as consisting of a single
"line" of characters (even if it actually contains several newlines).
The "start of line" metacharacter (^) matches only at the start of the
string, while the "end of line" metacharacter ($) matches only at the
end of the string, or before a terminating newline (unless PCRE_DOL-
LAR_ENDONLY is set). This is the same as Perl.
When PCRE_MULTILINE it is set, the "start of line" and "end of line"
constructs match immediately following or immediately before any new-
line in the subject string, respectively, as well as at the very start
and end. This is equivalent to Perl's /m option, and it can be changed
within a pattern by a (?m) option setting. If there are no "\n" charac-
ters in a subject string, or no occurrences of ^ or $ in a pattern,
setting PCRE_MULTILINE has no effect.
PCRE_NO_AUTO_CAPTURE
If this option is set, it disables the use of numbered capturing paren-
theses in the pattern. Any opening parenthesis that is not followed by
? behaves as if it were followed by ?: but named parentheses can still
be used for capturing (and they acquire numbers in the usual way).
There is no equivalent of this option in Perl.
PCRE_UNGREEDY
This option inverts the "greediness" of the quantifiers so that they
are not greedy by default, but become greedy if followed by "?". It is
not compatible with Perl. It can also be set by a (?U) option setting
within the pattern.
PCRE_UTF8
This option causes PCRE to regard both the pattern and the subject as
strings of UTF-8 characters instead of single-byte character strings.
However, it is available only if PCRE has been built to include UTF-8
support. If not, the use of this option provokes an error. Details of
how this option changes the behaviour of PCRE are given in the section
on UTF-8 support in the main pcre page.
PCRE_NO_UTF8_CHECK
When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
automatically checked. If an invalid UTF-8 sequence of bytes is found,
pcre_compile() returns an error. If you already know that your pattern
is valid, and you want to skip this check for performance reasons, you
can set the PCRE_NO_UTF8_CHECK option. When it is set, the effect of
passing an invalid UTF-8 string as a pattern is undefined. It may cause
your program to crash. Note that there is a similar option for sup-
pressing the checking of subject strings passed to pcre_exec().
STUDYING A PATTERN
pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);
When a pattern is going to be used several times, it is worth spending
more time analyzing it in order to speed up the time taken for match-
ing. The function pcre_study() takes a pointer to a compiled pattern as
its first argument. If studing the pattern produces additional informa-
tion that will help speed up matching, pcre_study() returns a pointer
to a pcre_extra block, in which the study_data field points to the
results of the study.
The returned value from a pcre_study() can be passed directly to
pcre_exec(). However, the pcre_extra block also contains other fields
that can be set by the caller before the block is passed; these are
described below. If studying the pattern does not produce any addi-
tional information, pcre_study() returns NULL. In that circumstance, if
the calling program wants to pass some of the other fields to
pcre_exec(), it must set up its own pcre_extra block.
The second argument contains option bits. At present, no options are
defined for pcre_study(), and this argument should always be zero.
The third argument for pcre_study() is a pointer for an error message.
If studying succeeds (even if no data is returned), the variable it
points to is set to NULL. Otherwise it points to a textual error mes-
sage. You should therefore test the error pointer for NULL after call-
ing pcre_study(), to be sure that it has run successfully.
This is a typical call to pcre_study():
pcre_extra *pe;
pe = pcre_study(
re, /* result of pcre_compile() */
0, /* no options exist */
&error); /* set to NULL or points to a message */
At present, studying a pattern is useful only for non-anchored patterns
that do not have a single fixed starting character. A bitmap of possi-
ble starting characters is created.
LOCALE SUPPORT
PCRE handles caseless matching, and determines whether characters are
letters, digits, or whatever, by reference to a set of tables. When
running in UTF-8 mode, this applies only to characters with codes less
than 256. The library contains a default set of tables that is created
in the default C locale when PCRE is compiled. This is used when the
final argument of pcre_compile() is NULL, and is sufficient for many
applications.
An alternative set of tables can, however, be supplied. Such tables are
built by calling the pcre_maketables() function, which has no argu-
ments, in the relevant locale. The result can then be passed to
pcre_compile() as often as necessary. For example, to build and use
tables that are appropriate for the French locale (where accented char-
acters with codes greater than 128 are treated as letters), the follow-
ing code could be used:
setlocale(LC_CTYPE, "fr");
tables = pcre_maketables();
re = pcre_compile(..., tables);
The tables are built in memory that is obtained via pcre_malloc. The
pointer that is passed to pcre_compile is saved with the compiled pat-
tern, and the same tables are used via this pointer by pcre_study() and
pcre_exec(). Thus, for any single pattern, compilation, studying and
matching all happen in the same locale, but different patterns can be
compiled in different locales. It is the caller's responsibility to
ensure that the memory containing the tables remains available for as
long as it is needed.
INFORMATION ABOUT A PATTERN
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
The pcre_fullinfo() function returns information about a compiled pat-
tern. It replaces the obsolete pcre_info() function, which is neverthe-
less retained for backwards compability (and is documented below).
The first argument for pcre_fullinfo() is a pointer to the compiled
pattern. The second argument is the result of pcre_study(), or NULL if
the pattern was not studied. The third argument specifies which piece
of information is required, and the fourth argument is a pointer to a
variable to receive the data. The yield of the function is zero for
success, or one of the following negative numbers:
PCRE_ERROR_NULL the argument code was NULL
the argument where was NULL
PCRE_ERROR_BADMAGIC the "magic number" was not found
PCRE_ERROR_BADOPTION the value of what was invalid
Here is a typical call of pcre_fullinfo(), to obtain the length of the
compiled pattern:
int rc;
unsigned long int length;
rc = pcre_fullinfo(
re, /* result of pcre_compile() */
pe, /* result of pcre_study(), or NULL */
PCRE_INFO_SIZE, /* what is required */
&length); /* where to put the data */
The possible values for the third argument are defined in pcre.h, and
are as follows:
PCRE_INFO_BACKREFMAX
Return the number of the highest back reference in the pattern. The
fourth argument should point to an int variable. Zero is returned if
there are no back references.
PCRE_INFO_CAPTURECOUNT
Return the number of capturing subpatterns in the pattern. The fourth
argument should point to an int variable.
PCRE_INFO_FIRSTBYTE
Return information about the first byte of any matched string, for a
non-anchored pattern. (This option used to be called
PCRE_INFO_FIRSTCHAR; the old name is still recognized for backwards
compatibility.)
If there is a fixed first byte, e.g. from a pattern such as
(cat|cow|coyote), it is returned in the integer pointed to by where.
Otherwise, if either
(a) the pattern was compiled with the PCRE_MULTILINE option, and every
branch starts with "^", or
(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
set (if it were set, the pattern would be anchored),
-1 is returned, indicating that the pattern matches only at the start
of a subject string or after any newline within the string. Otherwise
-2 is returned. For anchored patterns, -2 is returned.
PCRE_INFO_FIRSTTABLE
If the pattern was studied, and this resulted in the construction of a
256-bit table indicating a fixed set of bytes for the first byte in any
matching string, a pointer to the table is returned. Otherwise NULL is
returned. The fourth argument should point to an unsigned char * vari-
able.
PCRE_INFO_LASTLITERAL
Return the value of the rightmost literal byte that must exist in any
matched string, other than at its start, if such a byte has been
recorded. The fourth argument should point to an int variable. If there
is no such byte, -1 is returned. For anchored patterns, a last literal
byte is recorded only if it follows something of variable length. For
example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
/^a\dz\d/ the returned value is -1.
PCRE_INFO_NAMECOUNT
PCRE_INFO_NAMEENTRYSIZE
PCRE_INFO_NAMETABLE
PCRE supports the use of named as well as numbered capturing parenthe-
ses. The names are just an additional way of identifying the parenthe-
ses, which still acquire a number. A caller that wants to extract data
from a named subpattern must convert the name to a number in order to
access the correct pointers in the output vector (described with
pcre_exec() below). In order to do this, it must first use these three
values to obtain the name-to-number mapping table for the pattern.
The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
of each entry; both of these return an int value. The entry size
depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
a pointer to the first entry of the table (a pointer to char). The
first two bytes of each entry are the number of the capturing parenthe-
sis, most significant byte first. The rest of the entry is the corre-
sponding name, zero terminated. The names are in alphabetical order.
For example, consider the following pattern (assume PCRE_EXTENDED is
set, so white space - including newlines - is ignored):
(?P<date> (?P<year>(\d\d)?\d\d) -
(?P<month>\d\d) - (?P<day>\d\d) )
There are four named subpatterns, so the table has four entries, and
each entry in the table is eight bytes long. The table is as follows,
with non-printing bytes shows in hex, and undefined bytes shown as ??:
00 01 d a t e 00 ??
00 05 d a y 00 ?? ??
00 04 m o n t h 00
00 02 y e a r 00 ??
When writing code to extract data from named subpatterns, remember that
the length of each entry may be different for each compiled pattern.
PCRE_INFO_OPTIONS
Return a copy of the options with which the pattern was compiled. The
fourth argument should point to an unsigned long int variable. These
option bits are those specified in the call to pcre_compile(), modified
by any top-level option settings within the pattern itself.
A pattern is automatically anchored by PCRE if all of its top-level
alternatives begin with one of the following:
^ unless PCRE_MULTILINE is set
\A always
\G always
.* if PCRE_DOTALL is set and there are no back
references to the subpattern in which .* appears
For such patterns, the PCRE_ANCHORED bit is set in the options returned
by pcre_fullinfo().
PCRE_INFO_SIZE
Return the size of the compiled pattern, that is, the value that was
passed as the argument to pcre_malloc() when PCRE was getting memory in
which to place the compiled data. The fourth argument should point to a
size_t variable.
PCRE_INFO_STUDYSIZE
Returns the size of the data block pointed to by the study_data field
in a pcre_extra block. That is, it is the value that was passed to
pcre_malloc() when PCRE was getting memory into which to place the data
created by pcre_study(). The fourth argument should point to a size_t
variable.
OBSOLETE INFO FUNCTION
int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
The pcre_info() function is now obsolete because its interface is too
restrictive to return all the available data about a compiled pattern.
New programs should use pcre_fullinfo() instead. The yield of
pcre_info() is the number of capturing subpatterns, or one of the fol-
lowing negative numbers:
PCRE_ERROR_NULL the argument code was NULL
PCRE_ERROR_BADMAGIC the "magic number" was not found
If the optptr argument is not NULL, a copy of the options with which
the pattern was compiled is placed in the integer it points to (see
PCRE_INFO_OPTIONS above).
If the pattern is not anchored and the firstcharptr argument is not
NULL, it is used to pass back information about the first character of
any matched string (see PCRE_INFO_FIRSTBYTE above).
MATCHING A PATTERN
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
The function pcre_exec() is called to match a subject string against a
pre-compiled pattern, which is passed in the code argument. If the pat-
tern has been studied, the result of the study should be passed in the
extra argument.
Here is an example of a simple call to pcre_exec():
int rc;
int ovector[30];
rc = pcre_exec(
re, /* result of pcre_compile() */
NULL, /* we didn't study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector for substring information */
30); /* number of elements in the vector */
If the extra argument is not NULL, it must point to a pcre_extra data
block. The pcre_study() function returns such a block (when it doesn't
return NULL), but you can also create one for yourself, and pass addi-
tional information in it. The fields in the block are as follows:
unsigned long int flags;
void *study_data;
unsigned long int match_limit;
void *callout_data;
The flags field is a bitmap that specifies which of the other fields
are set. The flag bits are:
PCRE_EXTRA_STUDY_DATA
PCRE_EXTRA_MATCH_LIMIT
PCRE_EXTRA_CALLOUT_DATA
Other flag bits should be set to zero. The study_data field is set in
the pcre_extra block that is returned by pcre_study(), together with
the appropriate flag bit. You should not set this yourself, but you can
add to the block by setting the other fields.
The match_limit field provides a means of preventing PCRE from using up
a vast amount of resources when running patterns that are not going to
match, but which have a very large number of possibilities in their
search trees. The classic example is the use of nested unlimited
repeats. Internally, PCRE uses a function called match() which it calls
repeatedly (sometimes recursively). The limit is imposed on the number
of times this function is called during a match, which has the effect
of limiting the amount of recursion and backtracking that can take
place. For patterns that are not anchored, the count starts from zero
for each position in the subject string.
The default limit for the library can be set when PCRE is built; the
default default is 10 million, which handles all but the most extreme
cases. You can reduce the default by suppling pcre_exec() with a
pcre_extra block in which match_limit is set to a smaller value, and
PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
The pcre_callout field is used in conjunction with the "callout" fea-
ture, which is described in the pcrecallout documentation.
The PCRE_ANCHORED option can be passed in the options argument, whose
unused bits must be zero. This limits pcre_exec() to matching at the
first matching position. However, if a pattern was compiled with
PCRE_ANCHORED, or turned out to be anchored by virtue of its contents,
it cannot be made unachored at matching time.
When PCRE_UTF8 was set at compile time, the validity of the subject as
a UTF-8 string is automatically checked, and the value of startoffset
is also checked to ensure that it points to the start of a UTF-8 char-
acter. If an invalid UTF-8 sequence of bytes is found, pcre_exec()
returns the error PCRE_ERROR_BADUTF8. If startoffset contains an
invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
If you already know that your subject is valid, and you want to skip
these checks for performance reasons, you can set the
PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
do this for the second and subsequent calls to pcre_exec() if you are
making repeated calls to find all the matches in a single subject
string. However, you should be sure that the value of startoffset
points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
set, the effect of passing an invalid UTF-8 string as a subject, or a
value of startoffset that does not point to the start of a UTF-8 char-
acter, is undefined. Your program may crash.
There are also three further options that can be set only at matching
time:
PCRE_NOTBOL
The first character of the string is not the beginning of a line, so
the circumflex metacharacter should not match before it. Setting this
without PCRE_MULTILINE (at compile time) causes circumflex never to
match.
PCRE_NOTEOL
The end of the string is not the end of a line, so the dollar metachar-
acter should not match it nor (except in multiline mode) a newline
immediately before it. Setting this without PCRE_MULTILINE (at compile
time) causes dollar never to match.
PCRE_NOTEMPTY
An empty string is not considered to be a valid match if this option is
set. If there are alternatives in the pattern, they are tried. If all
the alternatives match the empty string, the entire match fails. For
example, if the pattern
a?b?
is applied to a string not beginning with "a" or "b", it matches the
empty string at the start of the subject. With PCRE_NOTEMPTY set, this
match is not valid, so PCRE searches further into the string for occur-
rences of "a" or "b".
Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
cial case of a pattern match of the empty string within its split()
function, and when using the /g modifier. It is possible to emulate
Perl's behaviour after matching a null string by first trying the match
again at the same offset with PCRE_NOTEMPTY set, and then if that fails
by advancing the starting offset (see below) and trying an ordinary
match again.
The subject string is passed to pcre_exec() as a pointer in subject, a
length in length, and a starting byte offset in startoffset. Unlike the
pattern string, the subject may contain binary zero bytes. When the
starting offset is zero, the search for a match starts at the beginning
of the subject, and this is by far the most common case.
If the pattern was compiled with the PCRE_UTF8 option, the subject must
be a sequence of bytes that is a valid UTF-8 string, and the starting
offset must point to the beginning of a UTF-8 character. If an invalid
UTF-8 string or offset is passed, an error (either PCRE_ERROR_BADUTF8
or PCRE_ERROR_BADUTF8_OFFSET) is returned, unless the option
PCRE_NO_UTF8_CHECK is set, in which case PCRE's behaviour is not
defined.
A non-zero starting offset is useful when searching for another match
in the same subject by calling pcre_exec() again after a previous suc-
cess. Setting startoffset differs from just passing over a shortened
string and setting PCRE_NOTBOL in the case of a pattern that begins
with any kind of lookbehind. For example, consider the pattern
\Biss\B
which finds occurrences of "iss" in the middle of words. (\B matches
only if the current position in the subject is not a word boundary.)
When applied to the string "Mississipi" the first call to pcre_exec()
finds the first occurrence. If pcre_exec() is called again with just
the remainder of the subject, namely "issipi", it does not match,
because \B is always false at the start of the subject, which is deemed
to be a word boundary. However, if pcre_exec() is passed the entire
string again, but with startoffset set to 4, it finds the second
occurrence of "iss" because it is able to look behind the starting
point to discover that it is preceded by a letter.
If a non-zero starting offset is passed when the pattern is anchored,
one attempt to match at the given offset is tried. This can only suc-
ceed if the pattern does not require the match to be at the start of
the subject.
In general, a pattern matches a certain portion of the subject, and in
addition, further substrings from the subject may be picked out by
parts of the pattern. Following the usage in Jeffrey Friedl's book,
this is called "capturing" in what follows, and the phrase "capturing
subpattern" is used for a fragment of a pattern that picks out a sub-
string. PCRE supports several other kinds of parenthesized subpattern
that do not cause substrings to be captured.
Captured substrings are returned to the caller via a vector of integer
offsets whose address is passed in ovector. The number of elements in
the vector is passed in ovecsize. The first two-thirds of the vector is
used to pass back captured substrings, each substring using a pair of
integers. The remaining third of the vector is used as workspace by
pcre_exec() while matching capturing subpatterns, and is not available
for passing back information. The length passed in ovecsize should
always be a multiple of three. If it is not, it is rounded down.
When a match has been successful, information about captured substrings
is returned in pairs of integers, starting at the beginning of ovector,
and continuing up to two-thirds of its length at the most. The first
element of a pair is set to the offset of the first character in a sub-
string, and the second is set to the offset of the first character
after the end of a substring. The first pair, ovector[0] and ovec-
tor[1], identify the portion of the subject string matched by the
entire pattern. The next pair is used for the first capturing subpat-
tern, and so on. The value returned by pcre_exec() is the number of
pairs that have been set. If there are no capturing subpatterns, the
return value from a successful match is 1, indicating that just the
first pair of offsets has been set.
Some convenience functions are provided for extracting the captured
substrings as separate strings. These are described in the following
section.
It is possible for an capturing subpattern number n+1 to match some
part of the subject when subpattern n has not been used at all. For
example, if the string "abc" is matched against the pattern (a|(z))(bc)
subpatterns 1 and 3 are matched, but 2 is not. When this happens, both
offset values corresponding to the unused subpattern are set to -1.
If a capturing subpattern is matched repeatedly, it is the last portion
of the string that it matched that gets returned.
If the vector is too small to hold all the captured substrings, it is
used as far as possible (up to two-thirds of its length), and the func-
tion returns a value of zero. In particular, if the substring offsets
are not of interest, pcre_exec() may be called with ovector passed as
NULL and ovecsize as zero. However, if the pattern contains back refer-
ences and the ovector isn't big enough to remember the related sub-
strings, PCRE has to get additional memory for use during matching.
Thus it is usually advisable to supply an ovector.
Note that pcre_info() can be used to find out how many capturing sub-
patterns there are in a compiled pattern. The smallest size for ovector
that will allow for n captured substrings, in addition to the offsets
of the substring matched by the whole pattern, is (n+1)*3.
If pcre_exec() fails, it returns a negative number. The following are
defined in the header file:
PCRE_ERROR_NOMATCH (-1)
The subject string did not match the pattern.
PCRE_ERROR_NULL (-2)
Either code or subject was passed as NULL, or ovector was NULL and
ovecsize was not zero.
PCRE_ERROR_BADOPTION (-3)
An unrecognized bit was set in the options argument.
PCRE_ERROR_BADMAGIC (-4)
PCRE stores a 4-byte "magic number" at the start of the compiled code,
to catch the case when it is passed a junk pointer. This is the error
it gives when the magic number isn't present.
PCRE_ERROR_UNKNOWN_NODE (-5)
While running the pattern match, an unknown item was encountered in the
compiled pattern. This error could be caused by a bug in PCRE or by
overwriting of the compiled pattern.
PCRE_ERROR_NOMEMORY (-6)
If a pattern contains back references, but the ovector that is passed
to pcre_exec() is not big enough to remember the referenced substrings,
PCRE gets a block of memory at the start of matching to use for this
purpose. If the call via pcre_malloc() fails, this error is given. The
memory is freed at the end of matching.
PCRE_ERROR_NOSUBSTRING (-7)
This error is used by the pcre_copy_substring(), pcre_get_substring(),
and pcre_get_substring_list() functions (see below). It is never
returned by pcre_exec().
PCRE_ERROR_MATCHLIMIT (-8)
The recursion and backtracking limit, as specified by the match_limit
field in a pcre_extra structure (or defaulted) was reached. See the
description above.
PCRE_ERROR_CALLOUT (-9)
This error is never generated by pcre_exec() itself. It is provided for
use by callout functions that want to yield a distinctive error code.
See the pcrecallout documentation for details.
PCRE_ERROR_BADUTF8 (-10)
A string that contains an invalid UTF-8 byte sequence was passed as a
subject.
PCRE_ERROR_BADUTF8_OFFSET (-11)
The UTF-8 byte sequence that was passed as a subject was valid, but the
value of startoffset did not point to the beginning of a UTF-8 charac-
ter.
EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);
Captured substrings can be accessed directly by using the offsets
returned by pcre_exec() in ovector. For convenience, the functions
pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
string_list() are provided for extracting captured substrings as new,
separate, zero-terminated strings. These functions identify substrings
by number. The next section describes functions for extracting named
substrings. A substring that contains a binary zero is correctly
extracted and has a further zero added on the end, but the result is
not, of course, a C string.
The first three arguments are the same for all three of these func-
tions: subject is the subject string which has just been successfully
matched, ovector is a pointer to the vector of integer offsets that was
passed to pcre_exec(), and stringcount is the number of substrings that
were captured by the match, including the substring that matched the
entire regular expression. This is the value returned by pcre_exec if
it is greater than zero. If pcre_exec() returned zero, indicating that
it ran out of space in ovector, the value passed as stringcount should
be the size of the vector divided by three.
The functions pcre_copy_substring() and pcre_get_substring() extract a
single substring, whose number is given as stringnumber. A value of
zero extracts the substring that matched the entire pattern, while
higher values extract the captured substrings. For pcre_copy_sub-
string(), the string is placed in buffer, whose length is given by
buffersize, while for pcre_get_substring() a new block of memory is
obtained via pcre_malloc, and its address is returned via stringptr.
The yield of the function is the length of the string, not including
the terminating zero, or one of
PCRE_ERROR_NOMEMORY (-6)
The buffer was too small for pcre_copy_substring(), or the attempt to
get memory failed for pcre_get_substring().
PCRE_ERROR_NOSUBSTRING (-7)
There is no substring whose number is stringnumber.
The pcre_get_substring_list() function extracts all available sub-
strings and builds a list of pointers to them. All this is done in a
single block of memory which is obtained via pcre_malloc. The address
of the memory block is returned via listptr, which is also the start of
the list of string pointers. The end of the list is marked by a NULL
pointer. The yield of the function is zero if all went well, or
PCRE_ERROR_NOMEMORY (-6)
if the attempt to get the memory block failed.
When any of these functions encounter a substring that is unset, which
can happen when capturing subpattern number n+1 matches some part of
the subject, but subpattern n has not been used at all, they return an
empty string. This can be distinguished from a genuine zero-length sub-
string by inspecting the appropriate offset in ovector, which is nega-
tive for unset substrings.
The two convenience functions pcre_free_substring() and
pcre_free_substring_list() can be used to free the memory returned by a
previous call of pcre_get_substring() or pcre_get_substring_list(),
respectively. They do nothing more than call the function pointed to by
pcre_free, which of course could be called directly from a C program.
However, PCRE is used in some situations where it is linked via a spe-
cial interface to another programming language which cannot use
pcre_free directly; it is for these cases that the functions are pro-
vided.
EXTRACTING CAPTURED SUBSTRINGS BY NAME
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
To extract a substring by name, you first have to find associated num-
ber. This can be done by calling pcre_get_stringnumber(). The first
argument is the compiled pattern, and the second is the name. For exam-
ple, for this pattern
ab(?<xxx>\d+)...
the number of the subpattern called "xxx" is 1. Given the number, you
can then extract the substring directly, or use one of the functions
described in the previous section. For convenience, there are also two
functions that do the whole job.
Most of the arguments of pcre_copy_named_substring() and
pcre_get_named_substring() are the same as those for the functions that
extract by number, and so are not re-described here. There are just two
differences.
First, instead of a substring number, a substring name is given. Sec-
ond, there is an extra argument, given at the start, which is a pointer
to the compiled pattern. This is needed in order to gain access to the
name-to-number translation table.
These functions call pcre_get_stringnumber(), and if it succeeds, they
then call pcre_copy_substring() or pcre_get_substring(), as appropri-
ate.
Last updated: 09 December 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE CALLOUTS
int (*pcre_callout)(pcre_callout_block *);
PCRE provides a feature called "callout", which is a means of temporar-
ily passing control to the caller of PCRE in the middle of pattern
matching. The caller of PCRE provides an external function by putting
its entry point in the global variable pcre_callout. By default, this
variable contains NULL, which disables all calling out.
Within a regular expression, (?C) indicates the points at which the
external function is to be called. Different callout points can be
identified by putting a number less than 256 after the letter C. The
default value is zero. For example, this pattern has two callout
points:
(?C1)abc(?C2)def
During matching, when PCRE reaches a callout point (and pcre_callout is
set), the external function is called. Its only argument is a pointer
to a pcre_callout block. This contains the following variables:
int version;
int callout_number;
int *offset_vector;
const char *subject;
int subject_length;
int start_match;
int current_position;
int capture_top;
int capture_last;
void *callout_data;
The version field is an integer containing the version number of the
block format. The current version is zero. The version number may
change in future if additional fields are added, but the intention is
never to remove any of the existing fields.
The callout_number field contains the number of the callout, as com-
piled into the pattern (that is, the number after ?C).
The offset_vector field is a pointer to the vector of offsets that was
passed by the caller to pcre_exec(). The contents can be inspected in
order to extract substrings that have been matched so far, in the same
way as for extracting substrings after a match has completed.
The subject and subject_length fields contain copies the values that
were passed to pcre_exec().
The start_match field contains the offset within the subject at which
the current match attempt started. If the pattern is not anchored, the
callout function may be called several times for different starting
points.
The current_position field contains the offset within the subject of
the current match pointer.
The capture_top field contains one more than the number of the highest
numbered captured substring so far. If no substrings have been
captured, the value of capture_top is one.
The capture_last field contains the number of the most recently cap-
tured substring.
The callout_data field contains a value that is passed to pcre_exec()
by the caller specifically so that it can be passed back in callouts.
It is passed in the pcre_callout field of the pcre_extra data struc-
ture. If no such data was passed, the value of callout_data in a
pcre_callout block is NULL. There is a description of the pcre_extra
structure in the pcreapi documentation.
RETURN VALUES
The callout function returns an integer. If the value is zero, matching
proceeds as normal. If the value is greater than zero, matching fails
at the current point, but backtracking to test other possibilities goes
ahead, just as if a lookahead assertion had failed. If the value is
less than zero, the match is abandoned, and pcre_exec() returns the
value.
Negative values should normally be chosen from the set of
PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
reserved for use by callout functions; it will never be used by PCRE
itself.
Last updated: 21 January 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
DIFFERENCES FROM PERL
This document describes the differences in the ways that PCRE and Perl
handle regular expressions. The differences described here are with
respect to Perl 5.8.
1. PCRE does not have full UTF-8 support. Details of what it does have
are given in the section on UTF-8 support in the main pcre page.
2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
permits them, but they do not mean what you might think. For example,
(?!a){3} does not assert that the next three characters are not "a". It
just asserts that the next character is not "a" three times.
3. Capturing subpatterns that occur inside negative lookahead asser-
tions are counted, but their entries in the offsets vector are never
set. Perl sets its numerical variables from any such patterns that are
matched before the assertion fails to match something (thereby succeed-
ing), but only if the negative lookahead assertion contains just one
branch.
4. Though binary zero characters are supported in the subject string,
they are not allowed in a pattern string because it is passed as a nor-
mal C string, terminated by zero. The escape sequence "\0" can be used
in the pattern to represent a binary zero.
5. The following Perl escape sequences are not supported: \l, \u, \L,
\U, \P, \p, \N, and \X. In fact these are implemented by Perl's general
string-handling and are not part of its pattern matching engine. If any
of these are encountered by PCRE, an error is generated.
6. PCRE does support the \Q...\E escape for quoting substrings. Charac-
ters in between are treated as literals. This is slightly different
from Perl in that $ and @ are also handled as literals inside the
quotes. In Perl, they cause variable interpolation (but of course PCRE
does not have variables). Note the following examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the
contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
constructions. However, there is some experimental support for recur-
sive patterns using the non-Perl items (?R), (?number) and (?P>name).
Also, the PCRE "callout" feature allows an external function to be
called during pattern matching.
8. There are some differences that are concerned with the settings of
captured strings when part of a pattern is repeated. For example,
matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
unset, but in PCRE it is set to "b".
9. PCRE provides some extensions to the Perl regular expression
facilities:
(a) Although lookbehind assertions must match fixed length strings,
each alternative branch of a lookbehind assertion can match a different
length of string. Perl requires them all to have the same length.
(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
meta-character matches only at the very end of the string.
(c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
cial meaning is faulted.
(d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
fiers is inverted, that is, by default they are not greedy, but if fol-
lowed by a question mark they are.
(e) PCRE_ANCHORED can be used to force a pattern to be tried only at
the first matching position in the subject string.
(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
TURE options for pcre_exec() have no Perl equivalents.
(g) The (?R), (?number), and (?P>name) constructs allows for recursive
pattern matching (Perl can do this using the (?p{code}) construct,
which PCRE cannot support.)
(h) PCRE supports named capturing substrings, using the Python syntax.
(i) PCRE supports the possessive quantifier "++" syntax, taken from
Sun's Java package.
(j) The (R) condition, for testing recursion, is a PCRE extension.
(k) The callout facility is PCRE-specific.
Last updated: 09 December 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions supported by PCRE
are described below. Regular expressions are also described in the Perl
documentation and in a number of other books, some of which have copi-
ous examples. Jeffrey Friedl's "Mastering Regular Expressions", pub-
lished by O'Reilly, covers them in great detail. The description here
is intended as reference documentation.
The basic operation of PCRE is on strings of bytes. However, there is
also support for UTF-8 character strings. To use this support you must
build PCRE to include UTF-8 support, and then call pcre_compile() with
the PCRE_UTF8 option. How this affects the pattern matching is men-
tioned in several places below. There is also a summary of UTF-8 fea-
tures in the section on UTF-8 support in the main pcre page.
A regular expression is a pattern that is matched against a subject
string from left to right. Most characters stand for themselves in a
pattern, and match the corresponding characters in the subject. As a
trivial example, the pattern
The quick brown fox
matches a portion of a subject string that is identical to itself. The
power of regular expressions comes from the ability to include alterna-
tives and repetitions in the pattern. These are encoded in the pattern
by the use of meta-characters, which do not stand for themselves but
instead are interpreted in some special way.
There are two different sets of meta-characters: those that are recog-
nized anywhere in the pattern except within square brackets, and those
that are recognized in square brackets. Outside square brackets, the
meta-characters are as follows:
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
Part of a pattern that is in square brackets is called a "character
class". In a character class the only meta-characters are:
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (only if followed by POSIX
syntax)
] terminates the character class
The following sections describe the use of each of the meta-characters.
BACKSLASH
The backslash character has several uses. Firstly, if it is followed by
a non-alphameric character, it takes away any special meaning that
character may have. This use of backslash as an escape character
applies both inside and outside character classes.
For example, if you want to match a * character, you write \* in the
pattern. This escaping action applies whether or not the following
character would otherwise be interpreted as a meta-character, so it is
always safe to precede a non-alphameric with backslash to specify that
it stands for itself. In particular, if you want to match a backslash,
you write \\.
If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
the pattern (other than in a character class) and characters between a
# outside a character class and the next newline character are ignored.
An escaping backslash can be used to include a whitespace or # charac-
ter as part of the pattern.
If you want to remove the special meaning from a sequence of charac-
ters, you can do so by putting them between \Q and \E. This is differ-
ent from Perl in that $ and @ are handled as literals in \Q...\E
sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
tion. Note the following examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the
contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
A second use of backslash provides a way of encoding non-printing char-
acters in patterns in a visible manner. There is no restriction on the
appearance of non-printing characters, apart from the binary zero that
terminates a pattern, but when a pattern is being prepared by text
editing, it is usually easier to use one of the following escape
sequences than the binary character it represents:
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any character
\e escape (hex 1B)
\f formfeed (hex 0C)
\n newline (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\ddd character with octal code ddd, or backreference
\xhh character with hex code hh
\x{hhh..} character with hex code hhh... (UTF-8 mode only)
The precise effect of \cx is as follows: if x is a lower case letter,
it is converted to upper case. Then bit 6 of the character (hex 40) is
inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
becomes hex 7B.
After \x, from zero to two hexadecimal digits are read (letters can be
in upper or lower case). In UTF-8 mode, any number of hexadecimal dig-
its may appear between \x{ and }, but the value of the character code
must be less than 2**31 (that is, the maximum hexadecimal value is
7FFFFFFF). If characters other than hexadecimal digits appear between
\x{ and }, or if there is no terminating }, this form of escape is not
recognized. Instead, the initial \x will be interpreted as a basic hex-
adecimal escape, with no following digits, giving a byte whose value is
zero.
Characters whose value is less than 256 can be defined by either of the
two syntaxes for \x when PCRE is in UTF-8 mode. There is no difference
in the way they are handled. For example, \xdc is exactly the same as
\x{dc}.
After \0 up to two further octal digits are read. In both cases, if
there are fewer than two digits, just those that are present are used.
Thus the sequence \0\x\07 specifies two binary zeros followed by a BEL
character (code value 7). Make sure you supply two digits after the
initial zero if the character that follows is itself an octal digit.
The handling of a backslash followed by a digit other than 0 is compli-
cated. Outside a character class, PCRE reads it and any following dig-
its as a decimal number. If the number is less than 10, or if there
have been at least that many previous capturing left parentheses in the
expression, the entire sequence is taken as a back reference. A
description of how this works is given later, following the discussion
of parenthesized subpatterns.
Inside a character class, or if the decimal number is greater than 9
and there have not been that many capturing subpatterns, PCRE re-reads
up to three octal digits following the backslash, and generates a sin-
gle byte from the least significant 8 bits of the value. Any subsequent
digits stand for themselves. For example:
\040 is another way of writing a space
\40 is the same, provided there are fewer than 40
previous capturing subpatterns
\7 is always a back reference
\11 might be a back reference, or another way of
writing a tab
\011 is always a tab
\0113 is a tab followed by the character "3"
\113 might be a back reference, otherwise the
character with octal code 113
\377 might be a back reference, otherwise
the byte consisting entirely of 1 bits
\81 is either a back reference, or a binary zero
followed by the two characters "8" and "1"
Note that octal values of 100 or greater must not be introduced by a
leading zero, because no more than three octal digits are ever read.
All the sequences that define a single byte value or a single UTF-8
character (in UTF-8 mode) can be used both inside and outside character
classes. In addition, inside a character class, the sequence \b is
interpreted as the backspace character (hex 08). Outside a character
class it has a different meaning (see below).
The third use of backslash is for specifying generic character types:
\d any decimal digit
\D any character that is not a decimal digit
\s any whitespace character
\S any character that is not a whitespace character
\w any "word" character
\W any "non-word" character
Each pair of escape sequences partitions the complete set of characters
into two disjoint sets. Any given character matches one, and only one,
of each pair.
In UTF-8 mode, characters with values greater than 255 never match \d,
\s, or \w, and always match \D, \S, and \W.
For compatibility with Perl, \s does not match the VT character (code
11). This makes it different from the the POSIX "space" class. The \s
characters are HT (9), LF (10), FF (12), CR (13), and space (32).
A "word" character is any letter or digit or the underscore character,
that is, any character which can be part of a Perl "word". The defini-
tion of letters and digits is controlled by PCRE's character tables,
and may vary if locale- specific matching is taking place (see "Locale
support" in the pcreapi page). For example, in the "fr" (French)
locale, some character codes greater than 128 are used for accented
letters, and these are matched by \w.
These character type sequences can appear both inside and outside char-
acter classes. They each match one character of the appropriate type.
If the current matching point is at the end of the subject string, all
of them fail, since there is no character to match.
The fourth use of backslash is for certain simple assertions. An asser-
tion specifies a condition that has to be met at a particular point in
a match, without consuming any characters from the subject string. The
use of subpatterns for more complicated assertions is described below.
The backslashed assertions are
\b matches at a word boundary
\B matches when not at a word boundary
\A matches at start of subject
\Z matches at end of subject or before newline at end
\z matches at end of subject
\G matches at first matching position in subject
These assertions may not appear in character classes (but note that \b
has a different meaning, namely the backspace character, inside a char-
acter class).
A word boundary is a position in the subject string where the current
character and the previous character do not both match \w or \W (i.e.
one matches \w and the other matches \W), or the start or end of the
string if the first or last character matches \w, respectively.
The \A, \Z, and \z assertions differ from the traditional circumflex
and dollar (described below) in that they only ever match at the very
start and end of the subject string, whatever options are set. Thus,
they are independent of multiline mode.
They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the
startoffset argument of pcre_exec() is non-zero, indicating that match-
ing is to start at a point other than the beginning of the subject, \A
can never match. The difference between \Z and \z is that \Z matches
before a newline that is the last character of the string as well as at
the end of the string, whereas \z matches only at the end.
The \G assertion is true only when the current matching position is at
the start point of the match, as specified by the startoffset argument
of pcre_exec(). It differs from \A when the value of startoffset is
non-zero. By calling pcre_exec() multiple times with appropriate argu-
ments, you can mimic Perl's /g option, and it is in this kind of imple-
mentation where \G can be useful.
Note, however, that PCRE's interpretation of \G, as the start of the
current match, is subtly different from Perl's, which defines it as the
end of the previous match. In Perl, these can be different when the
previously matched string was empty. Because PCRE does just one match
at a time, it cannot reproduce this behaviour.
If all the alternatives of a pattern begin with \G, the expression is
anchored to the starting match position, and the "anchored" flag is set
in the compiled regular expression.
CIRCUMFLEX AND DOLLAR
Outside a character class, in the default matching mode, the circumflex
character is an assertion which is true only if the current matching
point is at the start of the subject string. If the startoffset argu-
ment of pcre_exec() is non-zero, circumflex can never match if the
PCRE_MULTILINE option is unset. Inside a character class, circumflex
has an entirely different meaning (see below).
Circumflex need not be the first character of the pattern if a number
of alternatives are involved, but it should be the first thing in each
alternative in which it appears if the pattern is ever to match that
branch. If all possible alternatives start with a circumflex, that is,
if the pattern is constrained to match only at the start of the sub-
ject, it is said to be an "anchored" pattern. (There are also other
constructs that can cause a pattern to be anchored.)
A dollar character is an assertion which is true only if the current
matching point is at the end of the subject string, or immediately
before a newline character that is the last character in the string (by
default). Dollar need not be the last character of the pattern if a
number of alternatives are involved, but it should be the last item in
any branch in which it appears. Dollar has no special meaning in a
character class.
The meaning of dollar can be changed so that it matches only at the
very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
compile time. This does not affect the \Z assertion.
The meanings of the circumflex and dollar characters are changed if the
PCRE_MULTILINE option is set. When this is the case, they match immedi-
ately after and immediately before an internal newline character,
respectively, in addition to matching at the start and end of the sub-
ject string. For example, the pattern /^abc$/ matches the subject
string "def\nabc" in multiline mode, but not otherwise. Consequently,
patterns that are anchored in single line mode because all branches
start with ^ are not anchored in multiline mode, and a match for cir-
cumflex is possible when the startoffset argument of pcre_exec() is
non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE
is set.
Note that the sequences \A, \Z, and \z can be used to match the start
and end of the subject in both modes, and if all branches of a pattern
start with \A it is always anchored, whether PCRE_MULTILINE is set or
not.
FULL STOP (PERIOD, DOT)
Outside a character class, a dot in the pattern matches any one charac-
ter in the subject, including a non-printing character, but not (by
default) newline. In UTF-8 mode, a dot matches any UTF-8 character,
which might be more than one byte long, except (by default) for new-
line. If the PCRE_DOTALL option is set, dots match newlines as well.
The handling of dot is entirely independent of the handling of circum-
flex and dollar, the only relationship being that they both involve
newline characters. Dot has no special meaning in a character class.
MATCHING A SINGLE BYTE
Outside a character class, the escape sequence \C matches any one byte,
both in and out of UTF-8 mode. Unlike a dot, it always matches a new-
line. The feature is provided in Perl in order to match individual
bytes in UTF-8 mode. Because it breaks up UTF-8 characters into indi-
vidual bytes, what remains in the string may be a malformed UTF-8
string. For this reason it is best avoided.
PCRE does not allow \C to appear in lookbehind assertions (see below),
because in UTF-8 mode it makes it impossible to calculate the length of
the lookbehind.
SQUARE BRACKETS
An opening square bracket introduces a character class, terminated by a
closing square bracket. A closing square bracket on its own is not spe-
cial. If a closing square bracket is required as a member of the class,
it should be the first data character in the class (after an initial
circumflex, if present) or escaped with a backslash.
A character class matches a single character in the subject. In UTF-8
mode, the character may occupy more than one byte. A matched character
must be in the set of characters defined by the class, unless the first
character in the class definition is a circumflex, in which case the
subject character must not be in the set defined by the class. If a
circumflex is actually required as a member of the class, ensure it is
not the first character, or escape it with a backslash.
For example, the character class [aeiou] matches any lower case vowel,
while [^aeiou] matches any character that is not a lower case vowel.
Note that a circumflex is just a convenient notation for specifying the
characters which are in the class by enumerating those that are not. It
is not an assertion: it still consumes a character from the subject
string, and fails if the current pointer is at the end of the string.
In UTF-8 mode, characters with values greater than 255 can be included
in a class as a literal string of bytes, or by using the \x{ escaping
mechanism.
When caseless matching is set, any letters in a class represent both
their upper case and lower case versions, so for example, a caseless
[aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
match "A", whereas a caseful version would. PCRE does not support the
concept of case for characters with values greater than 255.
The newline character is never treated in any special way in character
classes, whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE
options is. A class such as [^a] will always match a newline.
The minus (hyphen) character can be used to specify a range of charac-
ters in a character class. For example, [d-m] matches any letter
between d and m, inclusive. If a minus character is required in a
class, it must be escaped with a backslash or appear in a position
where it cannot be interpreted as indicating a range, typically as the
first or last character in the class.
It is not possible to have the literal character "]" as the end charac-
ter of a range. A pattern such as [W-]46] is interpreted as a class of
two characters ("W" and "-") followed by a literal string "46]", so it
would match "W46]" or "-46]". However, if the "]" is escaped with a
backslash it is interpreted as the end of range, so [W-\]46] is inter-
preted as a single class containing a range followed by two separate
characters. The octal or hexadecimal representation of "]" can also be
used to end a range.
Ranges operate in the collating sequence of character values. They can
also be used for characters specified numerically, for example
[\000-\037]. In UTF-8 mode, ranges can include characters whose values
are greater than 255, for example [\x{100}-\x{2ff}].
If a range that includes letters is used when caseless matching is set,
it matches the letters in either case. For example, [W-c] is equivalent
to [][\^_`wxyzabc], matched caselessly, and if character tables for the
"fr" locale are in use, [\xc8-\xcb] matches accented E characters in
both cases.
The character types \d, \D, \s, \S, \w, and \W may also appear in a
character class, and add the characters that they match to the class.
For example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
conveniently be used with the upper case character types to specify a
more restricted set of characters than the matching lower case type.
For example, the class [^\W_] matches any letter or digit, but not
underscore.
All non-alphameric characters other than \, -, ^ (at the start) and the
terminating ] are non-special in character classes, but it does no harm
if they are escaped.
POSIX CHARACTER CLASSES
Perl supports the POSIX notation for character classes, which uses
names enclosed by [: and :] within the enclosing square brackets. PCRE
also supports this notation. For example,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The supported class
names are
alnum letters and digits
alpha letters
ascii character codes 0 - 127
blank space or tab only
cntrl control characters
digit decimal digits (same as \d)
graph printing characters, excluding space
lower lower case letters
print printing characters, including space
punct printing characters, excluding letters and digits
space white space (not quite the same as \s)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
and space (32). Notice that this list includes the VT character (code
11). This makes "space" different to \s, which does not include VT (for
Perl compatibility).
The name "word" is a Perl extension, and "blank" is a GNU extension
from Perl 5.8. Another Perl extension is negation, which is indicated
by a ^ character after the colon. For example,
[12[:^digit:]]
matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
these are not supported, and an error is given if they are encountered.
In UTF-8 mode, characters with values greater than 255 do not match any
of the POSIX character classes.
VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. For
example, the pattern
gilbert|sullivan
matches either "gilbert" or "sullivan". Any number of alternatives may
appear, and an empty alternative is permitted (matching the empty
string). The matching process tries each alternative in turn, from
left to right, and the first one that succeeds is used. If the alterna-
tives are within a subpattern (defined below), "succeeds" means match-
ing the rest of the main pattern as well as the alternative in the sub-
pattern.
INTERNAL OPTION SETTING
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
PCRE_EXTENDED options can be changed from within the pattern by a
sequence of Perl option letters enclosed between "(?" and ")". The
option letters are
i for PCRE_CASELESS
m for PCRE_MULTILINE
s for PCRE_DOTALL
x for PCRE_EXTENDED
For example, (?im) sets caseless, multiline matching. It is also possi-
ble to unset these options by preceding the letter with a hyphen, and a
combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
is also permitted. If a letter appears both before and after the
hyphen, the option is unset.
When an option change occurs at top level (that is, not inside subpat-
tern parentheses), the change applies to the remainder of the pattern
that follows. If the change is placed right at the start of a pattern,
PCRE extracts it into the global options (and it will therefore show up
in data extracted by the pcre_fullinfo() function).
An option change within a subpattern affects only that part of the cur-
rent pattern that follows it, so
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
used). By this means, options can be made to have different settings
in different parts of the pattern. Any changes made in one alternative
do carry on into subsequent branches within the same subpattern. For
example,
(a(?i)b|c)
matches "ab", "aB", "c", and "C", even though when matching "C" the
first branch is abandoned before the option setting. This is because
the effects of option settings happen at compile time. There would be
some very weird behaviour otherwise.
The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed
in the same way as the Perl-compatible options by using the characters
U and X respectively. The (?X) flag setting is special in that it must
always occur earlier in the pattern than any of the additional features
it turns on, even when it is at top level. It is best put at the start.
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be
nested. Marking part of a pattern as a subpattern does two things:
1. It localizes a set of alternatives. For example, the pattern
cat(aract|erpillar|)
matches one of the words "cat", "cataract", or "caterpillar". Without
the parentheses, it would match "cataract", "erpillar" or the empty
string.
2. It sets up the subpattern as a capturing subpattern (as defined
above). When the whole pattern matches, that portion of the subject
string that matched the subpattern is passed back to the caller via the
ovector argument of pcre_exec(). Opening parentheses are counted from
left to right (starting from 1) to obtain the numbers of the capturing
subpatterns.
For example, if the string "the red king" is matched against the pat-
tern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king", and are num-
bered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not always
helpful. There are often times when a grouping subpattern is required
without a capturing requirement. If an opening parenthesis is followed
by a question mark and a colon, the subpattern does not do any captur-
ing, and is not counted when computing the number of any subsequent
capturing subpatterns. For example, if the string "the white queen" is
matched against the pattern
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and are numbered
1 and 2. The maximum number of capturing subpatterns is 65535, and the
maximum depth of nesting of all subpatterns, both capturing and non-
capturing, is 200.
As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern, the option letters may appear
between the "?" and the ":". Thus the two patterns
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative branches are
tried from left to right, and options are not reset until the end of
the subpattern is reached, an option setting in one branch does affect
subsequent branches, so the above patterns match "SUNDAY" as well as
"Saturday".
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated regular expres-
sions. Furthermore, if an expression is modified, the numbers may
change. To help with the difficulty, PCRE supports the naming of sub-
patterns, something that Perl does not provide. The Python syntax
(?P<name>...) is used. Names consist of alphanumeric characters and
underscores, and must be unique within a pattern.
Named capturing parentheses are still allocated numbers as well as
names. The PCRE API provides function calls for extracting the name-to-
number translation table from a compiled pattern. For further details
see the pcreapi documentation.
REPETITION
Repetition is specified by quantifiers, which can follow any of the
following items:
a literal data character
the . metacharacter
the \C escape sequence
escapes such as \d that match single characters
a character class
a back reference (see next section)
a parenthesized subpattern (unless it is an assertion)
The general repetition quantifier specifies a minimum and maximum num-
ber of permitted matches, by giving the two numbers in curly brackets
(braces), separated by a comma. The numbers must be less than 65536,
and the first must be less than or equal to the second. For example:
z{2,4}
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
special character. If the second number is omitted, but the comma is
present, there is no upper limit; if the second number and the comma
are both omitted, the quantifier specifies an exact number of required
matches. Thus
[aeiou]{3,}
matches at least 3 successive vowels, but may match many more, while
\d{8}
matches exactly 8 digits. An opening curly bracket that appears in a
position where a quantifier is not allowed, or one that does not match
the syntax of a quantifier, is taken as a literal character. For exam-
ple, {,6} is not a quantifier, but a literal string of four characters.
In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
acters, each of which is represented by a two-byte sequence.
The quantifier {0} is permitted, causing the expression to behave as if
the previous item and the quantifier were not present.
For convenience (and historical compatibility) the three most common
quantifiers have single-character abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
It is possible to construct infinite loops by following a subpattern
that can match no characters with a quantifier that has no upper limit,
for example:
(a?)*
Earlier versions of Perl and PCRE used to give an error at compile time
for such patterns. However, because there are cases where this can be
useful, such patterns are now accepted, but if any repetition of the
subpattern does in fact match no characters, the loop is forcibly bro-
ken.
By default, the quantifiers are "greedy", that is, they match as much
as possible (up to the maximum number of permitted times), without
causing the rest of the pattern to fail. The classic example of where
this gives problems is in trying to match comments in C programs. These
appear between the sequences /* and */ and within the sequence, indi-
vidual * and / characters may appear. An attempt to match C comments by
applying the pattern
/\*.*\*/
to the string
/* first command */ not comment /* second comment */
fails, because it matches the entire string owing to the greediness of
the .* item.
However, if a quantifier is followed by a question mark, it ceases to
be greedy, and instead matches the minimum number of times possible, so
the pattern
/\*.*?\*/
does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of
matches. Do not confuse this use of question mark with its use as a
quantifier in its own right. Because it has two uses, it can sometimes
appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that is the
only way the rest of the pattern matches.
If the PCRE_UNGREEDY option is set (an option which is not available in
Perl), the quantifiers are not greedy by default, but individual ones
can be made greedy by following them with a question mark. In other
words, it inverts the default behaviour.
When a parenthesized subpattern is quantified with a minimum repeat
count that is greater than 1 or with a limited maximum, more store is
required for the compiled pattern, in proportion to the size of the
minimum or maximum.
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
alent to Perl's /s) is set, thus allowing the . to match newlines, the
pattern is implicitly anchored, because whatever follows will be tried
against every character position in the subject string, so there is no
point in retrying the overall match at any position after the first.
PCRE normally treats such a pattern as though it were preceded by \A.
In cases where it is known that the subject string contains no new-
lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
mization, or alternatively using ^ to indicate anchoring explicitly.
However, there is one situation where the optimization cannot be used.
When .* is inside capturing parentheses that are the subject of a
backreference elsewhere in the pattern, a match at the start may fail,
and a later one succeed. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the fourth charac-
ter. For this reason, such a pattern is not implicitly anchored.
When a capturing subpattern is repeated, the value captured is the sub-
string that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the captured substring
is "tweedledee". However, if there are nested capturing subpatterns,
the corresponding captured values may have been set in previous itera-
tions. For example, after
/(a|(b))+/
matches "aba" the value of the second captured substring is "b".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing and minimizing repetition, failure of what follows
normally causes the repeated item to be re-evaluated to see if a dif-
ferent number of repeats allows the rest of the pattern to match. Some-
times it is useful to prevent this, either to change the nature of the
match, or to cause it fail earlier than it otherwise might, when the
author of the pattern knows there is no point in carrying on.
Consider, for example, the pattern \d+foo when applied to the subject
line
123456bar
After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the
\d+ item, and then with 4, and so on, before ultimately failing.
"Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
the means for specifying that once a subpattern has matched, it is not
to be re-evaluated in this way.
If we use atomic grouping for the previous example, the matcher would
give up immediately on failing to match "foo" the first time. The nota-
tion is a kind of special parenthesis, starting with (?> as in this
example:
(?>\d+)foo
This kind of parenthesis "locks up" the part of the pattern it con-
tains once it has matched, and a failure further into the pattern is
prevented from backtracking into it. Backtracking past it to previous
items, however, works as normal.
An alternative description is that a subpattern of this type matches
the string of characters that an identical standalone pattern would
match, if anchored at the current point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases
such as the above example can be thought of as a maximizing repeat that
must swallow everything it can. So, while both \d+ and \d+? are pre-
pared to adjust the number of digits they match in order to make the
rest of the pattern match, (?>\d+) can only match an entire sequence of
digits.
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an
atomic group is just a single repeated item, as in the example above, a
simpler notation, called a "possessive quantifier" can be used. This
consists of an additional + character following a quantifier. Using
this notation, the previous example can be rewritten as
\d++bar
Possessive quantifiers are always greedy; the setting of the
PCRE_UNGREEDY option is ignored. They are a convenient notation for the
simpler forms of atomic group. However, there is no difference in the
meaning or processing of a possessive quantifier and the equivalent
atomic group.
The possessive quantifier syntax is an extension to the Perl syntax. It
originates in Sun's Java package.
When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of an
atomic group is the only way to avoid some failing matches taking a
very long time indeed. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of non-
digits, or digits enclosed in <>, followed by either ! or ?. When it
matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because the
string can be divided between the two repeats in a large number of
ways, and all have to be tried. (The example used [!?] rather than a
single character at the end, because both PCRE and Perl have an opti-
mization that allows for fast failure when a single character is used.
They remember the last single character that is required for a match,
and fail early if it is not present in the string.) If the pattern is
changed to
((?>\D+)|<\d+>)*[!?]
sequences of non-digits cannot be broken, and failure happens quickly.
BACK REFERENCES
Outside a character class, a backslash followed by a digit greater than
0 (and possibly further digits) is a back reference to a capturing sub-
pattern earlier (that is, to its left) in the pattern, provided there
have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is less than 10,
it is always taken as a back reference, and causes an error only if
there are not that many capturing left parentheses in the entire pat-
tern. In other words, the parentheses that are referenced need not be
to the left of the reference for numbers less than 10. See the section
entitled "Backslash" above for further details of the handling of dig-
its following a backslash.
A back reference matches whatever actually matched the capturing sub-
pattern in the current subject string, rather than anything matching
the subpattern itself (see "Subpatterns as subroutines" below for a way
of doing that). So the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If caseful matching is in force at the
time of the back reference, the case of letters is relevant. For exam-
ple,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capturing subpattern is matched caselessly.
Back references to named subpatterns use the Python syntax (?P=name).
We could rewrite the above example as follows:
(?<p1>(?i)rah)\s+(?P=p1)
There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back
references to it always fail. For example, the pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc". Because there
may be many capturing parentheses in a pattern, all digits following
the backslash are taken as part of a potential back reference number.
If the pattern continues with a digit character, some delimiter must be
used to terminate the back reference. If the PCRE_EXTENDED option is
set, this can be whitespace. Otherwise an empty comment can be used.
A back reference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, (a\1) never
matches. However, such references can be useful inside repeated sub-
patterns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
ation of the subpattern, the back reference matches the character
string corresponding to the previous iteration. In order for this to
work, the pattern must be such that the first iteration does not need
to match the back reference. This can be done using alternation, as in
the example above, or by a quantifier with a minimum of zero.
ASSERTIONS
An assertion is a test on the characters following or preceding the
current matching point that does not actually consume any characters.
The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
described above. More complicated assertions are coded as subpatterns.
There are two kinds: those that look ahead of the current position in
the subject string, and those that look behind it.
An assertion subpattern is matched in the normal way, except that it
does not cause the current matching position to be changed. Lookahead
assertions start with (?= for positive assertions and (?! for negative
assertions. For example,
\w+(?=;)
matches a word followed by a semicolon, but does not include the semi-
colon in the match, and
foo(?!bar)
matches any occurrence of "foo" that is not followed by "bar". Note
that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something
other than "foo"; it finds any occurrence of "bar" whatsoever, because
the assertion (?!foo) is always true when the next three characters are
"bar". A lookbehind assertion is needed to achieve this effect.
If you want to force a matching failure at some point in a pattern, the
most convenient way to do it is with (?!) because an empty string
always matches, so an assertion that requires there not to be an empty
string must always fail.
Lookbehind assertions start with (?<= for positive assertions and (?<!
for negative assertions. For example,
(?<!foo)bar
does find an occurrence of "bar" that is not preceded by "foo". The
contents of a lookbehind assertion are restricted such that all the
strings it matches must have a fixed length. However, if there are sev-
eral alternatives, they do not all have to have the same fixed length.
Thus
(?<=bullock|donkey)
is permitted, but
(?<!dogs?|cats?)
causes an error at compile time. Branches that match different length
strings are permitted only at the top level of a lookbehind assertion.
This is an extension compared with Perl (at least for 5.8), which
requires all branches to match the same length of string. An assertion
such as
(?<=ab(c|de))
is not permitted, because its single top-level branch can match two
different lengths, but it is acceptable if rewritten to use two top-
level branches:
(?<=abc|abde)
The implementation of lookbehind assertions is, for each alternative,
to temporarily move the current position back by the fixed width and
then try to match. If there are insufficient characters before the cur-
rent position, the match is deemed to fail.
PCRE does not allow the \C escape (which matches a single byte in UTF-8
mode) to appear in lookbehind assertions, because it makes it impossi-
ble to calculate the length of the lookbehind.
Atomic groups can be used in conjunction with lookbehind assertions to
specify efficient matching at the end of the subject string. Consider a
simple pattern such as
abcd$
when applied to a long string that does not match. Because matching
proceeds from left to right, PCRE will look for each "a" in the subject
and then see if what follows matches the rest of the pattern. If the
pattern is specified as
^.*abcd$
the initial .* matches the entire string at first, but when this fails
(because there is no following "a"), it backtracks to match all but the
last character, then all but the last two characters, and so on. Once
again the search for "a" covers the entire string, from right to left,
so we are no better off. However, if the pattern is written as
^(?>.*)(?<=abcd)
or, equivalently,
^.*+(?<=abcd)
there can be no backtracking for the .* item; it can match only the
entire string. The subsequent lookbehind assertion does a single test
on the last four characters. If it fails, the match fails immediately.
For long strings, this approach makes a significant difference to the
processing time.
Several assertions (of any sort) may occur in succession. For example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999". Notice that
each of the assertions is applied independently at the same point in
the subject string. First there is a check that the previous three
characters are all digits, and then there is a check that the same
three characters are not "999". This pattern does not match "foo" pre-
ceded by six characters, the first of which are digits and the last
three of which are not "999". For example, it doesn't match "123abc-
foo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters,
checking that the first three are digits, and then the second assertion
checks that the preceding three characters are not "999".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar" which in turn
is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern which matches "foo" preceded by three digits and any
three characters that are not "999".
Assertion subpatterns are not capturing subpatterns, and may not be
repeated, because it makes no sense to assert the same thing several
times. If any kind of assertion contains capturing subpatterns within
it, these are counted for the purposes of numbering the capturing sub-
patterns in the whole pattern. However, substring capturing is carried
out only for positive assertions, because it does not make sense for
negative assertions.
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern con-
ditionally or to choose between two alternative subpatterns, depending
on the result of an assertion, or whether a previous capturing
subpattern matched or not. The two possible forms of conditional sub-
pattern are
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. If there are more than two alterna-
tives in the subpattern, a compile-time error occurs.
There are three kinds of condition. If the text between the parentheses
consists of a sequence of digits, the condition is satisfied if the
capturing subpattern of that number has previously matched. The number
must be greater than zero. Consider the following pattern, which con-
tains non-significant white space to make it more readable (assume the
PCRE_EXTENDED option) and to divide it into three parts for ease of
discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The sec-
ond part matches one or more characters that are not parentheses. The
third part is a conditional subpattern that tests whether the first set
of parentheses matched or not. If they did, that is, if subject started
with an opening parenthesis, the condition is true, and so the yes-pat-
tern is executed and a closing parenthesis is required. Otherwise,
since no-pattern is not present, the subpattern matches nothing. In
other words, this pattern matches a sequence of non-parentheses,
optionally enclosed in parentheses.
If the condition is the string (R), it is satisfied if a recursive call
to the pattern or subpattern has been made. At "top level", the condi-
tion is false. This is a PCRE extension. Recursive patterns are
described in the next section.
If the condition is not a sequence of digits or (R), it must be an
assertion. This may be a positive or negative lookahead or lookbehind
assertion. Consider this pattern, again containing non-significant
white space, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an
optional sequence of non-letters followed by a letter. In other words,
it tests for the presence of at least one letter in the subject. If a
letter is found, the subject is matched against the first alternative;
otherwise it is matched against the second. This pattern matches
strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
letters and dd are digits.
COMMENTS
The sequence (?# marks the start of a comment which continues up to the
next closing parenthesis. Nested parentheses are not permitted. The
characters that make up a comment play no part in the pattern matching
at all.
If the PCRE_EXTENDED option is set, an unescaped # character outside a
character class introduces a comment that continues up to the next new-
line character in the pattern.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best
that can be done is to use a pattern that matches up to some fixed
depth of nesting. It is not possible to handle an arbitrary nesting
depth. Perl has provided an experimental facility that allows regular
expressions to recurse (amongst other things). It does this by interpo-
lating Perl code in the expression at run time, and the code can refer
to the expression itself. A Perl pattern to solve the parentheses prob-
lem can be created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and in this case
refers recursively to the pattern in which it appears. Obviously, PCRE
cannot support the interpolation of Perl code. Instead, it supports
some special syntax for recursion of the entire pattern, and also for
individual subpattern recursion.
The special item that consists of (? followed by a number greater than
zero and a closing parenthesis is a recursive call of the subpattern of
the given number, provided that it occurs inside that subpattern. (If
not, it is a "subroutine" call, which is described in the next sec-
tion.) The special item (?R) is a recursive call of the entire regular
expression.
For example, this PCRE pattern solves the nested parentheses problem
(assume the PCRE_EXTENDED option is set so that white space is
ignored):
\( ( (?>[^()]+) | (?R) )* \)
First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a
recursive match of the pattern itself (that is a correctly parenthe-
sized substring). Finally there is a closing parenthesis.
If this were part of a larger pattern, you would not want to recurse
the entire pattern, so instead you could use this:
( \( ( (?>[^()]+) | (?1) )* \) )
We have put the pattern into parentheses, and caused the recursion to
refer to them instead of the whole pattern. In a larger pattern, keep-
ing track of parenthesis numbers can be tricky. It may be more conve-
nient to use named parentheses instead. For this, PCRE uses (?P>name),
which is an extension to the Python syntax that PCRE uses for named
parentheses (Perl does not provide named parentheses). We could rewrite
the above example as follows:
(?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )
This particular example pattern contains nested unlimited repeats, and
so the use of atomic grouping for matching strings of non-parentheses
is important when applying the pattern to strings that do not match.
For example, when this pattern is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if atomic grouping is not used,
the match runs for a very long time indeed because there are so many
different ways the + and * repeats can carve up the subject, and all
have to be tested before failure can be reported.
At the end of a match, the values set for any capturing subpatterns are
those from the outermost level of the recursion at which the subpattern
value is set. If you want to obtain intermediate values, a callout
function can be used (see below and the pcrecallout documentation). If
the pattern above is matched against
(ab(cd)ef)
the value for the capturing parentheses is "ef", which is the last
value taken on at the top level. If additional parentheses are added,
giving
\( ( ( (?>[^()]+) | (?R) )* ) \)
^ ^
^ ^
the string they capture is "ab(cd)ef", the contents of the top level
parentheses. If there are more than 15 capturing parentheses in a pat-
tern, PCRE has to obtain extra memory to store data during a recursion,
which it does by using pcre_malloc, freeing it via pcre_free after-
wards. If no memory can be obtained, the match fails with the
PCRE_ERROR_NOMEMORY error.
Do not confuse the (?R) item with the condition (R), which tests for
recursion. Consider this pattern, which matches text in angle brack-
ets, allowing for arbitrary nesting. Only digits are allowed in nested
brackets (that is, when recursing), whereas any characters are permit-
ted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpattern, with
two different alternatives for the recursive and non-recursive cases.
The (?R) item is the actual recursive call.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern reference (either by number or
by name) is used outside the parentheses to which it refers, it oper-
ates like a subroutine in a programming language. An earlier example
pointed out that the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If instead the pattern
(sens|respons)e and (?1)ibility
is used, it does match "sense and responsibility" as well as the other
two strings. Such references must, however, follow the subpattern to
which they refer.
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary
Perl code to be obeyed in the middle of matching a regular expression.
This makes it possible, amongst other things, to extract different sub-
strings that match the same pair of parentheses when there is a repeti-
tion.
PCRE provides a similar feature, but of course it cannot obey arbitrary
Perl code. The feature is called "callout". The caller of PCRE provides
an external function by putting its entry point in the global variable
pcre_callout. By default, this variable contains NULL, which disables
all calling out.
Within a regular expression, (?C) indicates the points at which the
external function is to be called. If you want to identify different
callout points, you can put a number less than 256 after the letter C.
The default value is zero. For example, this pattern has two callout
points:
(?C1)abc(?C2)def
During matching, when PCRE reaches a callout point (and pcre_callout is
set), the external function is called. It is provided with the number
of the callout, and, optionally, one item of data originally supplied
by the caller of pcre_exec(). The callout function may cause matching
to backtrack, or to fail altogether. A complete description of the
interface to the callout function is given in the pcrecallout documen-
tation.
Last updated: 03 February 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE PERFORMANCE
Certain items that may appear in regular expression patterns are more
efficient than others. It is more efficient to use a character class
like [aeiou] than a set of alternatives such as (a|e|i|o|u). In gen-
eral, the simplest construction that provides the required behaviour is
usually the most efficient. Jeffrey Friedl's book contains a lot of
discussion about optimizing regular expressions for efficient perfor-
mance.
When a pattern begins with .* not in parentheses, or in parentheses
that are not the subject of a backreference, and the PCRE_DOTALL option
is set, the pattern is implicitly anchored by PCRE, since it can match
only at the start of a subject string. However, if PCRE_DOTALL is not
set, PCRE cannot make this optimization, because the . metacharacter
does not then match a newline, and if the subject string contains new-
lines, the pattern may match from the character immediately following
one of them instead of from the very start. For example, the pattern
.*second
matches the subject "first\nand second" (where \n stands for a newline
character), with the match starting at the seventh character. In order
to do this, PCRE has to retry the match starting after every newline in
the subject.
If you are using such a pattern with subject strings that do not con-
tain newlines, the best performance is obtained by setting PCRE_DOTALL,
or starting the pattern with ^.* to indicate explicit anchoring. That
saves PCRE from having to scan along the subject looking for a newline
to restart at.
Beware of patterns that contain nested indefinite repeats. These can
take a long time to run when applied to a string that does not match.
Consider the pattern fragment
(a+)*
This can match "aaaa" in 33 different ways, and this number increases
very rapidly as the string gets longer. (The * repeat can match 0, 1,
2, 3, or 4 times, and for each of those cases other than 0, the +
repeats can match different numbers of times.) When the remainder of
the pattern is such that the entire match is going to fail, PCRE has in
principle to try every possible variation, and this can take an
extremely long time.
An optimization catches some of the more simple cases such as
(a+)*b
where a literal character follows. Before embarking on the standard
matching procedure, PCRE checks that there is a "b" later in the sub-
ject string, and if there is not, it fails the match immediately. How-
ever, when there is no following literal this optimization cannot be
used. You can see the difference by comparing the behaviour of
(a+)*\d
with the pattern above. The former gives a failure almost instantly
when applied to a whole line of "a" characters, whereas the latter
takes an appreciable time with strings longer than about 20 characters.
Last updated: 03 February 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions.
SYNOPSIS OF POSIX API
#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 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:
REG_ICASE
The PCRE_CASELESS option is set when the expression is passed for com-
pilation to the native function.
REG_NEWLINE
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
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 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:
REG_NOTBOL
The PCRE_NOTBOL option is set when calling the underlying PCRE matching
function.
REG_NOTEOL
The PCRE_NOTEOL option is set when calling the underlying PCRE matching
function.
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"
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 func-
tion is the size of buffer needed to hold the whole message.
STORAGE
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-
sion.
AUTHOR
Philip Hazel <ph10@cam.ac.uk>
University Computing Service,
Cambridge CB2 3QG, England.
Last updated: 03 February 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE SAMPLE PROGRAM
A simple, complete demonstration program, to get you started with using
PCRE, is supplied in the file pcredemo.c in the PCRE distribution.
The program compiles the regular expression that is its first argument,
and matches it against the subject string in its second argument. No
PCRE options are set, and default character tables are used. If match-
ing succeeds, the program outputs the portion of the subject that
matched, together with the contents of any captured substrings.
If the -g option is given on the command line, the program then goes on
to check for further matches of the same regular expression in the same
subject string. The logic is a little bit tricky because of the possi-
bility of matching an empty string. Comments in the code explain what
is going on.
On a Unix system that has PCRE installed in /usr/local, you can compile
the demonstration program using a command like this:
gcc -o pcredemo pcredemo.c -I/usr/local/include \
-L/usr/local/lib -lpcre
Then you can run simple tests like this:
./pcredemo 'cat|dog' 'the cat sat on the mat'
./pcredemo -g 'cat|dog' 'the dog sat on the cat'
Note that there is a much more comprehensive test program, called
pcretest, which supports many more facilities for testing regular
expressions and the PCRE library. The pcredemo program is provided as a
simple coding example.
On some operating systems (e.g. Solaris) you may get an error like this
when you try to run pcredemo:
ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or
directory
This is caused by the way shared library support works on those sys-
tems. You need to add
-R/usr/local/lib
to the compile command to get round this problem.
Last updated: 28 January 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
