PCRE was originally written for the Exim MTA, but is now used by many high-profile open source projects, including Python, Apache, PHP, KDE, Postfix, Analog, and nmap. Other interesting projects using PCRE include Ferite, Onyx, Hypermail, and Askemos.
-----------------------------------------------------------------------------
NAME
PCRE - Perl-compatible regular expressions
DESCRIPTION
The PCRE library is a set of functions that implement regu-
lar expression pattern matching using the same syntax and
semantics as Perl, with just a few differences. The current
implementation of PCRE (release 4.x) corresponds approxi-
mately 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 contribu-
tions, 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 pcrepattern 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 different 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 concatenated, 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 maximum number of capturing subpatterns is 65535.
There is no limit to the number of non-capturing subpat-
terns, but the maximum depth of nesting of all kinds of
parenthesized subpattern, including capturing subpatterns,
assertions, and other types of subpattern, is 200.
The maximum length of a subject string is the largest posi-
tive number that an integer variable can hold. However, PCRE
uses recursion to handle subpatterns and indefinite repeti-
tion. This means that the available stack space may limit
the size of a subject string that can be processed by cer-
tain patterns.
UTF-8 SUPPORT
Starting at release 3.3, PCRE has had some support for char-
acter strings encoded in the UTF-8 format. For release 4.0
this has been greatly extended to cover most common require-
ments.
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 addi-
tional 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 con-
tents 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 example: \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 individual 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 charac-
ters that PCRE recognizes as digits, spaces, or word charac-
ters 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 pro-
perties 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.
-----------------------------------------------------------------------------
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 deselected, by providing options to the config-
ure script which is run before the make command. The com-
plete list of options for configure (which includes the
standard ones such as the selection of the installation
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 new-
line character. 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 newline 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 mal-
loc() 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 repeatedly (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
documentation. 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 alternation 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 pat-
terns, 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.
Last updated: 21 January 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
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, *firstcharptr);
int pcre_config(int what, void *where);
char *pcre_version(void);
void *(*pcre_malloc)(size_t);
void (*pcre_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 application 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 pcredemo.c. The pcresample docu-
mentation describes how to run it.
There are convenience functions for extracting captured sub-
strings 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 compatibility. 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 respectively. 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 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 opera-
tion. Details are given in the pcrecallout documentation.
MULTITHREADING
The PCRE functions can be used in multi-threading applica-
tions, with the proviso that the memory management functions
pointed to by pcre_malloc and pcre_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 matching, 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 discover which optional features have been com-
piled into the PCRE library. The pcrebuild documentation has
more details about these optional features.
The first argument for pcre_config() is an integer, specify-
ing 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 available; 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 match-
ing. 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.
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 con-
tains a copy of the tableptr argument, which is an address
(see below).
The options argument contains independent bits that affect
the compilation. 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 expres-
sions 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 message. 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_compile():
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 match-
ing point in the string which is being searched (the "sub-
ject string"). This effect can also be achieved by appropri-
ate 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 new-
lines). 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 characters, including newlines. Without it, new-
lines 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 pat-
tern are totally ignored except when escaped or inside a
character class. Whitespace does not include the VT charac-
ter (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 sub-
pattern.
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_DOLLAR_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 immedi-
ately before any newline in the subject string, respec-
tively, 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"
characters 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 cap-
turing parentheses 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 sec-
tion 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 suppressing 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 matching. The function pcre_study() takes
a pointer to a compiled pattern as its first argument. If
studing the pattern produces additional information 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 additional information,
pcre_study() returns NULL. In that circumstance, if the cal-
ling 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. Other-
wise it points to a textual error message. You should there-
fore test the error pointer for NULL after calling
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 possible starting characters is
created.
LOCALE SUPPORT
PCRE handles caseless matching, and determines whether char-
acters 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 arguments, 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 characters with codes
greater than 128 are treated as letters), the following 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 pattern, 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 responsi-
bility 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 pattern. It replaces the obsolete pcre_info() func-
tion, which is nevertheless retained for backwards compabil-
ity (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 pat-
tern. 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 con-
struction 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 * variable.
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
parentheses. The names are just an additional way of identi-
fying the parentheses, 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 cap-
turing parenthesis, most significant byte first. The rest of
the entry is the corresponding 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 com-
piled. 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, *firstcharptr);
The pcre_info() function is now obsolete because its inter-
face 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 following
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 pattern 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 additional 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 pat-
terns 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 dur-
ing 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 "cal-
lout" feature, which is described in the pcrecallout docu-
mentation.
The PCRE_ANCHORED option can be passed in the options argu-
ment, whose unused bits must be zero. This limits
pcre_exec() to matching at the first matching position. How-
ever, if a pattern was compiled with PCRE_ANCHORED, or
turned out to be anchored by virtue of its contents, it can-
not 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. If an
invalid UTF-8 sequence of bytes is found, pcre_exec()
returns the error PCRE_ERROR_BADUTF8. If you already know
that your subject is valid, and you want to skip this check
for performance reasons, you can set the PCRE_NO_UTF8_CHECK
option when calling pcre_exec(). When this option is set,
the effect of passing an invalid UTF-8 string as a subject
is undefined. It may cause your program to 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 dol-
lar metacharacter should not match it nor (except in multi-
line 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 pat-
tern, 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 occurrences of "a" or "b".
Perl has no direct equivalent of PCRE_NOTEMPTY, but it does
make a special 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 offset in star-
toffset. 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. If an invalid UTF-8 string is passed, 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 success. 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 succeed 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 sub-
ject, 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 substring.
PCRE supports several other kinds of parenthesized subpat-
tern 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 substring, 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 subpattern, 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 sub-
strings, it is used as far as possible (up to two-thirds of
its length), and the function 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
references and the ovector isn't big enough to remember the
related substrings, 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 cap-
turing subpatterns 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 fol-
lowing 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 com-
piled 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 encoun-
tered 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() func-
tions (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.
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_substring_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 sub-
strings. 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 functions: 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 cap-
tured 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 ovec-
tor, 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 string-
number. A value of zero extracts the substring that matched
the entire pattern, while higher values extract the captured
substrings. For pcre_copy_substring(), 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 avail-
able substrings 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 substring 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. How-
ever, PCRE is used in some situations where it is linked via
a special interface to another programming language which
cannot use pcre_free directly; it is for these cases that
the functions are provided.
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 asso-
ciated number. This can be done by calling
pcre_get_stringnumber(). The first argument is the compiled
pattern, and the second is the name. For example, 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. Second, 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 trans-
lation table.
These functions call pcre_get_stringnumber(), and if it
succeeds, they then call pcre_copy_substring() or
pcre_get_substring(), as appropriate.
Last updated: 20 August 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
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 temporarily 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 cal-
lout 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)9abc(?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 compiled 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 sub-
strings have been captured, the value of capture_top is one.
The capture_last field contains the number of the most
recently captured 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 structure. 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 back-
tracking 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 standard "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.
-----------------------------------------------------------------------------
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 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.
2. Capturing subpatterns that occur inside negative looka-
head assertions are counted, but their entries in the
offsets vector are never set. Perl sets its numerical vari-
ables from any such patterns that are matched before the
assertion fails to match something (thereby succeeding), but
only if the negative lookahead assertion contains just one
branch.
3. Though binary zero characters are supported in the sub-
ject string, they are not allowed in a pattern string
because it is passed as a normal C string, terminated by
zero. The escape sequence "\0" can be used in the pattern to
represent a binary zero.
4. The following Perl escape sequences are not supported:
\l, \u, \L, \U, \P, \p, and \X. In fact these are imple-
mented 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.
5. PCRE does support the \Q...\E escape for quoting sub-
strings. Characters 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$xyz
In PCRE, the \Q...\E mechanism is not recognized inside a
character class.
8. Fairly obviously, PCRE does not support the (?{code}) and
(?p{code}) constructions. However, there is some experimen-
tal support for recursive patterns using the non-Perl items
(?R), (?number) and (?P>name). Also, the PCRE "callout"
feature allows an external function to be called during pat-
tern matching.
9. There are some differences that are concerned with the
settings of captured strings when part of a pattern is
repeated. For example, matching "aba" against the pattern
/^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set
to "b".
10. 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 special meaning is faulted.
(d) If PCRE_UNGREEDY is set, the greediness of the repeti-
tion quantifiers is inverted, that is, by default they are
not greedy, but if followed 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_CAPTURE 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: 03 February 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions sup-
ported 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 copious examples. Jeffrey
Friedl's "Mastering Regular Expressions", published 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 mentioned in several
places below. There is also a summary of UTF-8 features 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 charac-
ters 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 alternatives and repetitions in the pat-
tern. 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 recognized 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, whi-
tespace 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 # character as part
of the pattern.
If you want to remove the special meaning from a sequence of
characters, you can do so by putting them between \Q and \E.
This is different from Perl in that $ and @ are handled as
literals in \Q...\E sequences in PCRE, whereas in Perl, $
and @ cause variable interpolation. Note the following exam-
ples:
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 characters in patterns in a visible manner. There
is no restriction on the appearance of non-printing charac-
ters, 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 digits 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 inter-
preted as a basic hexadecimal 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 complicated. Outside a character class, PCRE reads it
and any following digits 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 follow-
ing the backslash, and generates a single 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 intro-
duced 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 sin-
gle UTF-8 character (in UTF-8 mode) can be used both inside
and outside character classes. In addition, inside a charac-
ter class, the sequence \b is interpreted as the backspace
character (hex 08). Outside a character class it has a dif-
ferent meaning (see below).
The third use of backslash is for specifying generic charac-
ter 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 char-
acter (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 definition of letters and digits is con-
trolled 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 character 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 asser-
tions. An assertion 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 character 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 mul-
tiline 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 matching 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 arguments, 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 expres-
sion.
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 argument 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 alter-
natives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is
said to be an "anchored" pattern. (There are also other con-
structs 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 immediately after and immediately
before an internal newline character, respectively, in addi-
tion to matching at the start and end of the subject string.
For example, the pattern /^abc$/ matches the subject string
"def\nabc" in multiline mode, but not otherwise. Conse-
quently, patterns that are anchored in single line mode
because all branches start with ^ are not anchored in multi-
line mode, and a match for circumflex 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 character in the subject, including a non-printing char-
acter, 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 newline. If the
PCRE_DOTALL option is set, dots match newlines as well. The
handling of dot is entirely independent of the handling of
circumflex 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 newline. The feature is provided in Perl
in order to match individual bytes in UTF-8 mode. Because
it breaks up UTF-8 characters into individual 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, ter-
minated by a closing square bracket. A closing square
bracket on its own is not special. 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 cir-
cumflex, 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 defin-
ition 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 con-
venient 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 case-
ful 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 characters 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 inter-
preted 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 character of a range. A pattern such as [W-]46] is
interpreted as a class of two characters ("W" and "-") fol-
lowed 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 res-
tricted 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 exam-
ple,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The sup-
ported 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" dif-
ferent to \s, which does not include VT (for Perl compati-
bility).
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 alter-
natives 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 alternatives are within a
subpattern (defined below), "succeeds" means matching the
rest of the main pattern as well as the alternative in the
subpattern.
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 possible to unset these options by preceding the letter
with a hyphen, and a combined setting and unsetting such as
(?im-sx), which sets PCRE_CASELESS 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 subpattern 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 current 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 oth-
erwise.
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 subpat-
tern does two things:
1. It localizes a set of alternatives. For example, the pat-
tern
cat(aract|erpillar|)
matches one of the words "cat", "cataract", or "caterpil-
lar". 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 por-
tion 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 captur-
ing subpatterns.
For example, if the string "the red king" is matched against
the pattern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king",
and are numbered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not
always helpful. There are often times when a grouping sub-
pattern 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 capturing, and is not
counted when computing the number of any subsequent captur-
ing 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 sub-
patterns 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 compli-
cated regular expressions. Furthermore, if an expression is
modified, the numbers may change. To help with the diffi-
culty, PCRE supports the naming of subpatterns, something
that Perl does not provide. The Python syntax (?P<name>...)
is used. Names consist of alphanumeric characters and under-
scores, 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 com-
piled pattern. For further details see the pcreapi documen-
tation.
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 number 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 example, {,6} is not a quantif-
ier, 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 characters, 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 broken.
By default, the quantifiers are "greedy", that is, they
match as much as possible (up to the maximum number of per-
mitted 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, individual
* and / characters may appear. An attempt to match C com-
ments 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 pre-
ferred number of matches. Do not confuse this use of ques-
tion 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 max-
imum, 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 (equivalent 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 charac-
ter 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 pre-
ceded by \A.
In cases where it is known that the subject string contains
no newlines, it is worth setting PCRE_DOTALL in order to
obtain this optimization, or alternatively using ^ to indi-
cate anchoring explicitly.
However, there is one situation where the optimization can-
not 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. Con-
sider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the
fourth character. For this reason, such a pattern is not
implicitly anchored.
When a capturing subpattern is repeated, the value captured
is the substring that matched the final iteration. For exam-
ple, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the cap-
tured substring is "tweedledee". However, if there are
nested capturing subpatterns, the corresponding captured
values may have been set in previous iterations. For exam-
ple, 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 different number of repeats allows the
rest of the pattern to match. Sometimes 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 specify-
ing 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 notation is a kind of special
parenthesis, starting with (?> as in this example:
(?>\d+)bar
This kind of parenthesis "locks up" the part of the pattern
it contains once it has matched, and a failure further into
the pattern is prevented from backtracking into it. Back-
tracking past it to previous items, however, works as nor-
mal.
An alternative description is that a subpattern of this type
matches the string of characters that an identical stan-
dalone 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 prepared 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 nota-
tion for the simpler forms of atomic group. However, there
is no difference in the meaning or processing of a posses-
sive 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 subpat-
tern 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 con-
sist 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 exam-
ple used [!?] rather than a single character at the end,
because both PCRE and Perl have an optimization 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 hap-
pens 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 subpattern 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 pattern. 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 han-
dling of digits following a backslash.
A back reference matches whatever actually matched the cap-
turing subpattern 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 responsi-
bility", 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 example,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even
though the original capturing subpattern is matched case-
lessly.
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 sub-
pattern. 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 pat-
tern, all digits following the backslash are taken as part
of a potential back reference number. If the pattern contin-
ues 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 subpatterns. For example, the pat-
tern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At
each iteration of the subpattern, the back reference matches
the character string corresponding to the previous itera-
tion. 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 com-
plicated 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 semicolon 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 asser-
tions 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 several 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 dif-
ferent 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 rewrit-
ten 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 current 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 impossible 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 back-
tracks 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" preceded by six
characters, the first of which are digits and the last three
of which are not "999". For example, it doesn't match
"123abcfoo". 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 subpatterns 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 sub-
pattern conditionally 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 subpattern are
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used; oth-
erwise the no-pattern (if present) is used. If there are
more than two alternatives 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 contains 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 second 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-pattern 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 condition 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 looka-
head 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 contin-
ues 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 contin-
ues up to the next newline 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 pro-
vided an experimental facility that allows regular expres-
sions to recurse (amongst other things). It does this by
interpolating Perl code in the expression at run time, and
the code can refer to the expression itself. A Perl pattern
to solve the parentheses problem 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 section.) The special
item (?R) is a recursive call of the entire regular expres-
sion.
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 parenthesized 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, keeping track of parenthesis numbers can
be tricky. It may be more convenient 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:
(?<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 sub-
patterns 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 pattern, PCRE has to obtain extra memory to
store data during a recursion, which it does by using
pcre_malloc, freeing it via pcre_free afterwards. 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 brackets, allowing for arbitrary nesting. Only
digits are allowed in nested brackets (that is, when recurs-
ing), whereas any characters are permitted at the outer
level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpat-
tern, 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 operates like a subroutine in a program-
ming language. An earlier example pointed out that the pat-
tern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsi-
bility", 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 substrings 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)9abc(?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 pcre-
callout documentation.
Last updated: 03 February 2003
Copyright (c) 1997-2003 University of Cambridge.
-----------------------------------------------------------------------------
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 general, the simplest construction
that provides the required behaviour is usually the most
efficient. Jeffrey Friedl's book contains a lot of discus-
sion about optimizing regular expressions for efficient per-
formance.
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 newlines, 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 contain 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 princi-
ple 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 subject string, and if there is not, it fails
the match immediately. However, 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.
-----------------------------------------------------------------------------
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 addi-
tional 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 applica-
tion 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 rea-
sonably 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 expressions themselves are still those of Perl, sub-
ject 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 regmatch_t for returning
captured substrings. It also defines some constants whose
names start with "REG_"; these are used for setting options
and identifying error codes.
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 expres-
sion.
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 compilation to the native function.
REG_NEWLINE
The PCRE_MULTILINE option is set when the expression is
passed for compilation to the native function. Note that
this does not mimic the defined POSIX behaviour for
REG_NEWLINE (see the following section).
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 oth-
erwise. 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 dif-
ferent 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 pos-
sibilities 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 equivalent 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 set-
ting 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 substrings, are returned via the pmatch argument,
which points to an array of nmatch structures of type
regmatch_t, containing the members rm_so and rm_eo. These
contain the offset to the first character of each substring
and the offset to the first character after the end of each
substring, respectively. The 0th element of the vector
relates to the entire portion of string that was matched;
subsequent elements relate to the capturing subpatterns of
the regular expression. Unused entries in the array have
both structure members set to -1.
A successful match yields a zero return; various error codes
are defined in the header file, of which REG_NOMATCH is the
"expected" failure code.
ERROR MESSAGES
The regerror() function maps a non-zero errorcode from
either regcomp() or regexec() to a printable message. If
preg is not NULL, the error should have arisen from the use
of that structure. A message terminated by a binary zero is
placed in errbuf. The length of the message, including the
zero, is limited to errbuf_size. The yield of the function
is the size of buffer needed to hold the whole message.
STORAGE
Compiling a regular expression causes memory to be allocated
and associated with the preg structure. The function reg-
free() frees all such memory, after which preg may no longer
be used as a compiled expression.
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.
-----------------------------------------------------------------------------
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 matching 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 regu-
lar expression in the same subject string. The logic is a
little bit tricky because of the possibility 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 systems. 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.
-----------------------------------------------------------------------------
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construction
I,ve been studying these codes for 3 days and it seems to me that some of the routines are incorrect
incorrect portions
Can you point out the portions that are incorrect?