PCRE(3)PCRE(3)NAMEpcre - Perl-compatible regular expressions.
SYNOPSIS
#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_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);
void pcre_free_substring(const char *stringptr);
void pcre_free_substring_list(const char **stringptr);
const unsigned char *pcre_maketables(void);
int pcre_fullinfo(const pcre *code, const pcre_extra
*extra,
int what, void *where);
int pcre_info(const pcre *code, int *optptr, int
*firstcharptr);
char *pcre_version(void);
void *(*pcre_malloc)(size_t);
void (*pcre_free)(void *);
DESCRIPTION
The PCRE library is a set of functions that implement reg
ular expression pattern matching using the same syntax and
semantics as Perl 5, with just a few differences (see
below). The current implementation corresponds to Perl
5.005, with some additional features from later versions.
This includes some experimental, incomplete support for
UTF-8 encoded strings. Details of exactly what is and what
is not supported are given below.
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 sim
plest way of using them is given in the file pcredemo.c.
The last section of this man page describes how to run it.
The functions pcre_copy_substring(), pcre_get_substring(),
and pcre_get_substring_list() are convenience functions
for extracting captured substrings from a matched subject
string; pcre_free_substring() and pcre_free_sub
string_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 informa
tion about a compiled pattern; pcre_info() is an obsolete
version which returns only some of the available informa
tion, but is retained for backwards compatibility. The
function pcre_version() returns a pointer to a string con
taining 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 man
agement functions via these variables, so a calling pro
gram can replace them if it wishes to intercept the calls.
This should be done before calling any PCRE functions.
MULTI-THREADING
The PCRE functions can be used in multi-threading applica
tions, with the proviso that the memory management func
tions pointed to by pcre_malloc and pcre_free 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.
COMPILING A PATTERN
The function pcre_compile() is called to compile a pattern
into an internal form. The pattern is a C string termi
nated by a binary zero, and is passed in the argument pat_
tern. 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 com
plete pcre data block is not fully relocatable, because it
contains a copy of the tableptr argument, which is an
address (see below).
The size of a compiled pattern is roughly proportional to
the length of the pattern string, except that each charac
ter class (other than those containing just a single char
acter, negated or not) requires 33 bytes, and repeat quan
tifiers with a minimum greater than one or a bounded maxi
mum cause the relevant portions of the compiled pattern to
be replicated.
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 regu
lar expressions below). 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 immedi
ately. 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 off
set 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 in the header file:
PCRE_ANCHORED
If this bit is set, the pattern is forced to be
"anchored", that is, it is constrained to match only at
the start of the string which is being searched (the "sub
ject string"). This effect can also be achieved by appro
priate 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.
PCRE_DOLLAR_ENDONLY
If this bit is set, a dollar metacharacter in the pattern
matches only at the end of the subject string. Without
this option, a dollar also matches immediately before the
final character if it is a newline (but not before any
other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
if PCRE_MULTILINE is set. There is no equivalent to this
option in Perl.
PCRE_DOTALL
If this bit is set, a dot metacharater in the pattern
matches all characters, including newlines. Without it,
newlines are excluded. This option is equivalent to Perl's
/s option. 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, and 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 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 pat
tern, for example within the sequence (?( which introduces
a conditional subpattern.
PCRE_EXTRA
This option was invented in order to turn on additional
functionality of PCRE that is incompatible with Perl, but
it is currently of very little use. When set, any back
slash 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 con
tains 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
immediately before any newline in the subject string,
respectively, as well as at the very start and end. This
is equivalent to Perl's /m option. 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_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 just
byte 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. Support for UTF-8 is new,
experimental, and incomplete. Details of exactly what it
entails are given below.
STUDYING A PATTERN
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 argu
ment, and returns a pointer to a pcre_extra block (another
typedef for a structure with hidden contents) containing
additional information about the pattern; this can be
passed to pcre_exec(). If no additional information is
available, NULL is returned.
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 to an
error message. If studying succeeds (even if no data is
returned), the variable it points to is set to NULL. Oth
erwise it points to a textual error message.
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
characters are letters, digits, or whatever, by reference
to a set of tables. The library contains a default set of
tables which 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 appli
cations.
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 let
ters), 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 pat
terns can be compiled in different locales. It is the
caller's responsibility to ensure that the memory contain
ing the tables remains available for as long as it is
needed.
INFORMATION ABOUT A PATTERN
The pcre_fullinfo() function returns information about a
compiled pattern. It replaces the obsolete pcre_info()
function, which is nevertheless retained for backwards
compability (and is documented below).
The first argument for pcre_fullinfo() is a pointer to the
compiled pattern. The second argument is the result of
pcre_study(), or NULL if the pattern was not studied. The
third argument specifies which piece of information is
required, while 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 num
bers:
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_OPTIONS
Return a copy of the options with which the pattern was
compiled. The fourth argument should point to an unsigned
long int variable. These option bits are those specified
in the call to pcre_compile(), modified by any top-level
option settings within the pattern itself, and with the
PCRE_ANCHORED bit forcibly set if the form of the pattern
implies that it can match only at the start of a subject
string.
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 com
piled data. The fourth argument should point to a size_t
variable.
PCRE_INFO_CAPTURECOUNT
Return the number of capturing subpatterns in the pattern.
The fourth argument should point to an int variable.
PCRE_INFO_BACKREFMAX
Return the number of the highest back reference in the
pattern. The fourth argument should point to an int vari
able. Zero is returned if there are no back references.
PCRE_INFO_FIRSTCHAR
Return information about the first character of any
matched string, for a non-anchored pattern. If there is a
fixed first character, 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 "\n" within
the string. Otherwise -2 is returned. For anchored pat
terns, -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
characters for the first character 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
For a non-anchored pattern, return the value of the right
most literal character which must exist in any matched
string, other than at its start. The fourth argument
should point to an int variable. If there is no such char
acter, or if the pattern is anchored, -1 is returned. For
example, for the pattern /a\d+z\d+/ the returned value is
'z'.
The pcre_info() function is now obsolete because its
interface is too restrictive to return all the available
data about a compiled pattern. New programs should use
pcre_fullinfo() instead. The yield of pcre_info() is the
number of capturing subpatterns, or one of the 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 inte
ger it points to (see PCRE_INFO_OPTIONS above).
If the pattern is not anchored and the firstcharptr argu
ment is not NULL, it is used to pass back information
about the first character of any matched string (see
PCRE_INFO_FIRSTCHAR above).
MATCHING A PATTERN
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 argu
ment. Otherwise this must be NULL.
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 */
The PCRE_ANCHORED option can be passed in the options
argument, whose unused bits must be zero. However, if a
pattern was compiled with PCRE_ANCHORED, or turned out to
be anchored by virtue of its contents, it cannot be made
unachored at matching time.
There are also three further options that can be set only
at matching time:
PCRE_NOTBOL
The first character of the string is not the beginning of
a line, so the circumflex metacharacter should not match
before it. Setting this without PCRE_MULTILINE (at compile
time) causes circumflex never to match.
PCRE_NOTEOL
The end of the string is not the end of a line, so the
dollar metacharacter should not match it nor (except in
multiline mode) a newline immediately before it. Setting
this without PCRE_MULTILINE (at compile time) causes dol
lar 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 as a pointer in subject, a
length in length, and a starting offset in startoffset.
Unlike the pattern string, the subject may contain binary
zero characters. 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.
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 dif
fers 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 "Missis
sipi" the first call to pcre_exec() finds the first occur
rence. 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 sub
ject, 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
subject, and in addition, further substrings from the sub
ject may be picked out by parts of the pattern. Following
the usage in Jeffrey Friedl's book, this is called "cap
turing" in what follows, and the phrase "capturing subpat
tern" is used for a fragment of a pattern that picks out a
substring. PCRE supports several other kinds of parenthe
sized subpattern that do not cause substrings to be cap
tured.
Captured substrings are returned to the caller via a vec
tor of integer offsets whose address is passed in ovector.
The number of elements in the vector is passed in ovec_
size. 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 subpat
terns, 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 cap
tured substrings is returned in pairs of integers, start
ing at the beginning of ovector, and continuing up to two-
thirds of its length at the most. The first element of a
pair is set to the offset of the first character in a sub
string, and the second is set to the offset of the first
character after the end of a substring. The first pair,
ovector[0] and ovector[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 sub
patterns, 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 off
set 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 inter
est, 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 remem
ber the related substrings, PCRE has to get additional
memory for use during matching. Thus it is usually advis
able to supply an ovector.
Note that pcre_info() can be used to find out how many
capturing 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
following are defined in the header file:
PCRE_ERROR_NOMATCH (-1)
The subject string did not match the pattern.
PCRE_ERROR_NULL (-2)
Either code or subject was passed as NULL, or ovector was
NULL and ovecsize was not zero.
PCRE_ERROR_BADOPTION (-3)
An unrecognized bit was set in the options argument.
PCRE_ERROR_BADMAGIC (-4)
PCRE stores a 4-byte "magic number" at the start of the
compiled code, to catch the case when it is passed a junk
pointer. This is the error it gives when the magic number
isn't present.
PCRE_ERROR_UNKNOWN_NODE (-5)
While running the pattern match, an unknown item was
encountered in the compiled pattern. This error could be
caused by a bug in PCRE or by overwriting of the compiled
pattern.
PCRE_ERROR_NOMEMORY (-6)
If a pattern contains back references, but the ovector
that is passed to pcre_exec() is not big enough to remem
ber 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.
EXTRACTING CAPTURED SUBSTRINGS
Captured substrings can be accessed directly by using the
offsets returned by pcre_exec() in ovector. For conve
nience, the functions pcre_copy_substring(), pcre_get_sub
string(), and pcre_get_substring_list() are provided for
extracting captured substrings as new, separate, zero-ter
minated strings. A substring that contains a binary zero
is correctly extracted and has a further zero added on the
end, but the result does not, of course, function as a C
string.
The first three arguments are the same for all three func
tions: subject is the subject string which has just been
successfully matched, ovector is a pointer to the vector
of integer offsets that was passed to pcre_exec(), and
stringcount is the number of substrings that were captured
by the match, including the substring that matched the
entire regular expression. This is the value returned by
pcre_exec if it is greater than zero. If pcre_exec()
returned zero, indicating that it ran out of space in
ovector, the value passed as stringcount should be the
size of the vector divided by three.
The functions pcre_copy_substring() and pcre_get_sub
string() extract a single substring, whose number is given
as stringnumber. A value of zero extracts the substring
that matched the entire pattern, while higher values
extract the captured substrings. For pcre_copy_sub
string(), the string is placed in buffer, whose length is
given by buffersize, while for pcre_get_substring() a new
block of memory is obtained via pcre_malloc, and its
address is returned via stringptr. The yield of the func
tion is the length of the string, not including the termi
nating 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
negative 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.
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. All
values in repeating quantifiers must be less than 65536.
There 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 pos
itive number that an integer variable can hold. However,
PCRE uses recursion to handle subpatterns and indefinite
repetition. This means that the available stack space may
limit the size of a subject string that can be processed
by certain patterns.
DIFFERENCES FROM PERL
The differences described here are with respect to Perl
5.005.
1. By default, a whitespace character is any character
that the C library function isspace() recognizes, though
it is possible to compile PCRE with alternative character
type tables. Normally isspace() matches space, formfeed,
newline, carriage return, horizontal tab, and vertical
tab. Perl 5 no longer includes vertical tab in its set of
whitespace characters. The \v escape that was in the Perl
documentation for a long time was never in fact recog
nized. However, the character itself was treated as
whitespace at least up to 5.002. In 5.004 and 5.005 it
does not match \s.
2. PCRE does not allow repeat quantifiers on lookahead
assertions. Perl permits them, but they do not mean what
you might think. For example, (?!a){3} does not assert
that the next three characters are not "a". It just
asserts that the next character is not "a" three times.
3. Capturing subpatterns that occur inside negative looka
head assertions are counted, but their entries in the off
sets 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.
4. 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.
5. The following Perl escape sequences are not supported:
\l, \u, \L, \U, \E, \Q. In fact these are implemented by
Perl's general string-handling and are not part of its
pattern matching engine.
6. The Perl \G assertion is not supported as it is not
relevant to single pattern matches.
7. Fairly obviously, PCRE does not support the (?{code})
and (?p{code}) constructions. However, there is some
experimental support for recursive patterns using the non-
Perl item (?R).
8. There are at the time of writing some oddities in Perl
5.005_02 concerned with the settings of captured strings
when part of a pattern is repeated. For example, matching
"aba" against the pattern /^(a(b)?)+$/ sets $2 to the
value "b", but matching "aabbaa" against /^(aa(bb)?)+$/
leaves $2 unset. However, if the pattern is changed to
/^(aa(b(b))?)+$/ then $2 (and $3) are set.
In Perl 5.004 $2 is set in both cases, and that is also
true of PCRE. If in the future Perl changes to a consis
tent state that is different, PCRE may change to follow.
9. Another as yet unresolved discrepancy is that in Perl
5.005_02 the pattern /^(a)?(?(1)a|b)+$/ matches the string
"a", whereas in PCRE it does not. However, in both Perl
and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
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 5.005
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 start of the subject.
(f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY
options for pcre_exec() have no Perl equivalents.
(g) The (?R) construct allows for recursive pattern match
ing (Perl 5.6 can do this using the (?p{code}) construct,
which PCRE cannot of course support.)
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 num
ber of other books, some of which have copious examples.
Jeffrey Friedl's "Mastering Regular Expressions", pub
lished by O'Reilly (ISBN 1-56592-257), covers them in
great detail.
The description here is intended as reference documenta
tion. The basic operation of PCRE is on strings of bytes.
However, there is the beginnings of some support for UTF-8
character strings. To use this support you must configure
PCRE to include it, and then call pcre_compile() with the
PCRE_UTF8 option. How this affects the pattern matching is
described in the final section of this document.
A regular expression is a pattern that is matched against
a subject string from left to right. Most characters stand
for themselves in a pattern, and match the corresponding
characters in the subject. As a trivial example, the pat
tern
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
pattern. These are encoded in the pattern by the use of
meta-characters, which do not stand for themselves but
instead are interpreted in some special way.
There are two different sets of meta-characters: those
that are 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 subject (or line, in multiline
mode)
$ assert end of subject (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
{ 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
] 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 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 "\" to specify that it stands for itself.
In particular, if you want to match a backslash, you write
"\\".
If a pattern is compiled with the PCRE_EXTENDED option,
whitespace in the pattern (other than in a character
class) and characters between a "#" outside a character
class and the next newline character are ignored. An
escaping backslash can be used to include a whitespace or
"#" character as part of the pattern.
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 char
acters, apart from the binary zero that terminates a pat
tern, 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)
\xhh character with hex code hh
\ddd character with octal code ddd, or backreference
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", up to two hexadecimal digits are read (letters
can be in upper or lower case).
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.
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 refer_
ence. A description of how this works is given later, fol
lowing 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 fol
lowing the backslash, and generates a single byte from the
least significant 8 bits of the value. Any subsequent dig
its 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 is the character with octal code 113 (since there
can be no more than 99 back references)
\377 is a byte consisting entirely of 1 bits
\81 is either a back reference, or a binary zero
followed by the two characters "8" and "1"
Note that octal values of 100 or greater must not be
introduced by a leading zero, because no more than three
octal digits are ever read.
All the sequences that define a single byte value can be
used both inside and outside character classes. In addi
tion, inside a character class, the sequence "\b" is
interpreted as the backspace character (hex 08). Outside a
character class it has a different meaning (see below).
The third use of backslash is for specifying generic char
acter 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.
A "word" character is any letter or digit or the under
score character, that is, any character which can be part
of a Perl "word". The definition of letters and digits is
controlled by PCRE's character tables, and may vary if
locale- specific matching is taking place (see "Locale
support" above). 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 subpat
terns for more complicated assertions is described below.
The backslashed assertions are
\b word boundary
\B not a word boundary
\A start of subject (independent of multiline mode)
\Z end of subject or newline at end (independent of
multiline mode)
\z end of subject (independent of multiline mode)
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. They are not affected by
the PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset
argument of pcre_exec() is non-zero, \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.
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. Inside a charac
ter class, circumflex has an entirely different meaning
(see below).
Circumflex need not be the first character of the pattern
if a number of alternatives are involved, but it should be
the first thing in each alternative in which it appears if
the pattern is ever to match that branch. If all possible
alternatives start with a circumflex, that is, if the pat
tern is constrained to match only at the start of the sub
ject, it is said to be an "anchored" pattern. (There are
also other constructs that can cause a pattern to be
anchored.)
A dollar character is an assertion which is true only if
the current matching point is at the end of the subject
string, or immediately before a newline character that is
the last character in the string (by default). Dollar need
not be the last character of the pattern if a number of
alternatives are involved, but it should be the last item
in any branch in which it appears. Dollar has no special
meaning in a character class.
The meaning of dollar can be changed so that it matches
only at the very end of the string, by setting the
PCRE_DOLLAR_ENDONLY option at compile or matching 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 "\n" character, respectively, in addi
tion to matching at the start and end of the subject
string. For example, the pattern /^abc$/ matches the sub
ject string "def\nabc" in multiline mode, but not other
wise. Consequently, patterns that are anchored in single
line mode because all branches start with "^" are not
anchored in multiline 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
character, but not (by default) newline. If the
PCRE_DOTALL option is set, dots match newlines as well.
The handling of dot is entirely independent of the han
dling of circumflex and dollar, the only relationship
being that they both involve newline characters. Dot has
no special meaning in a character class.
SQUARE BRACKETS
An opening square bracket introduces a character class,
terminated by a closing square bracket. A closing square
bracket on its own is not 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
circumflex, if present) or escaped with a backslash.
A character class matches a single character in the sub
ject; the character must be in the set of characters
defined by the class, unless the first character in the
class is a circumflex, in which case the subject character
must not be in the set defined by the class. If a circum
flex is actually required as a member of the class, ensure
it is not the first character, or escape it with a back
slash.
For example, the character class [aeiou] matches any lower
case vowel, while [^aeiou] matches any character that is
not a lower case vowel. Note that a circumflex is just a
convenient notation for specifying the characters which
are in the class by enumerating those that are not. It is
not an assertion: it still consumes a character from the
subject string, and fails if the current pointer is at the
end of the string.
When caseless matching is set, any letters in a class rep
resent both their upper case and lower case versions, so
for example, a caseless [aeiou] matches "A" as well as
"a", and a caseless [^aeiou] does not match "A", whereas a
caseful version would.
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 interpreted as indicating a range, typically as the
first or last character in the class.
It is not possible to have the literal character "]" as
the end character of a range. A pattern such as [W-]46] is
interpreted as a class of two characters ("W" and "-")
followed by a literal string "46]", so it would match
"W46]" or "-46]". However, if the "]" is escaped with a
backslash it is interpreted as the end of range, so
[W-\]46] is interpreted 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 ASCII collating sequence. They can also
be used for characters specified numerically, for example
[\000-\037]. If a range that includes letters is used when
caseless matching is set, it matches the letters in either
case. For example, [W-c] is equivalent to [][\^_`wxyzabc],
matched caselessly, and if character tables for the "fr"
locale are in use, [\xc8-\xcb] matches accented E
characters in both cases.
The character types \d, \D, \s, \S, \w, and \W may also
appear in a character class, and add the characters that
they match to the class. For example, [\dABCDEF] matches
any hexadecimal digit. A circumflex can conveniently be
used with the upper case character types to specify a more
restricted set of characters than the matching lower case
type. For example, the class [^\W_] matches any letter or
digit, but not underscore.
All non-alphameric characters other than \, -, ^ (at the
start) and the terminating ] are non-special in character
classes, but it does no harm if they are escaped.
POSIX CHARACTER CLASSES
Perl 5.6 (not yet released at the time of writing) is
going to support the POSIX notation for character classes,
which uses names enclosed by [: and :] within the enclos
ing square brackets. PCRE supports this notation. For
example,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The
supported class names are
alnum letters and digits
alpha letters
ascii character codes 0 - 127
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 dig
its
space white space (same as \s)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
The names "ascii" and "word" are Perl extensions. 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.
VERTICAL BAR
Vertical bar characters are used to separate alternative
patterns. For example, the pattern
gilbert|sullivan
matches either "gilbert" or "sullivan". Any number of
alternatives may appear, and an empty alternative is per
mitted (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 PCRE_CASELESS, PCRE_MULTILINE,
PCRE_DOTALL, and PCRE_EXTENDED 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_MULTI
LINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is
also permitted. If a letter appears both before and after
the hyphen, the option is unset.
The scope of these option changes depends on where in the
pattern the setting occurs. For settings that are outside
any subpattern (defined below), the effect is the same as
if the options were set or unset at the start of matching.
The following patterns all behave in exactly the same way:
(?i)abc
a(?i)bc
ab(?i)c
abc(?i)
which in turn is the same as compiling the pattern abc
with PCRE_CASELESS set. In other words, such "top level"
settings apply to the whole pattern (unless there are
other changes inside subpatterns). If there is more than
one setting of the same option at top level, the rightmost
setting is used.
If an option change occurs inside a subpattern, the effect
is different. This is a change of behaviour in Perl 5.005.
An option change inside a subpattern affects only that
part of the subpattern 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 match
ing "C" the first branch is abandoned before the option
setting. This is because the effects of option settings
happen at compile time. There would be some very weird
behaviour otherwise.
The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can
be changed in the same way as the Perl-compatible options
by using the characters U and X respectively. The (?X)
flag setting is special in that it must always occur ear
lier 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 sub
pattern does two things:
1. It localizes a set of alternatives. For example, the
pattern
cat(aract|erpillar|)
matches one of the words "cat", "cataract", or "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 cap
turing 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
subpattern is required without a capturing requirement. If
an opening parenthesis is followed by "?:", the subpattern
does not do any capturing, and is not counted when comput
ing the number of any subsequent capturing subpatterns.
For example, if the string "the white queen" is matched
against the pattern
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and
are numbered 1 and 2. The maximum number of captured sub
strings is 99, and the maximum number 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".
REPETITION
Repetition is specified by quantifiers, which can follow
any of the following items:
a single character, possibly escaped
the . metacharacter
a character class
a back reference (see next section)
a parenthesized subpattern (unless it is an assertion -
see below)
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 omit
ted, 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
quantifier, but a literal string of four characters.
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 abbrevia
tions:
* 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
permitted times), without causing the rest of the pattern
to fail. The classic example of where this gives problems
is in trying to match comments in C programs. These appear
between the sequences /* and */ and within the sequence,
individual * and / characters may appear. An attempt to
match C comments by applying the pattern
/\*.*\*/
to the string
/* first command */ not comment /* second comment */
fails, because it matches the entire string owing to the
greediness of the .* item.
However, if a quantifier is followed by a question mark,
it ceases to be greedy, and instead matches the minimum
number of times possible, so the pattern
/\*.*?\*/
does the right thing with the C comments. The meaning of
the various quantifiers is not otherwise changed, just the
preferred number of matches. Do not confuse this use of
question mark with its use as a quantifier in its own
right. Because it has two uses, it can sometimes appear
doubled, as in
\d??\d
which matches one digit by preference, but can match two
if that is the only way the rest of the pattern matches.
If the PCRE_UNGREEDY option is set (an option which is not
available in Perl), the quantifiers are not greedy by
default, but individual ones can be made greedy by follow
ing them with a question mark. In other words, it inverts
the default behaviour.
When a parenthesized subpattern is quantified with a mini
mum repeat count that is greater than 1 or with a limited
maximum, more store is required for the compiled pattern,
in proportion to the size of the minimum or maximum.
If a pattern starts with .* or .{0,} and the PCRE_DOTALL
option (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 char
acter position in the subject string, so there is no point
in retrying the overall match at any position after the
first. PCRE 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 when the pattern begins with .* in order to
obtain this optimization, or alternatively using ^ to
indicate anchoring explicitly.
When a capturing subpattern is repeated, the value
captured is the substring that matched the final itera
tion. For example, 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".
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 (i.e. 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 handling of digits following a backslash.
A back reference matches whatever actually matched the
capturing subpattern in the current subject string, rather
than anything matching the subpattern itself. So the pat
tern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and respon
sibility", 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.
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 up to 99 back references, all digits
following the backslash are taken as part of a potential
back reference number. If the pattern continues with a
digit character, some delimiter must be used to terminate
the back reference. If the PCRE_EXTENDED option is set,
this can be whitespace. Otherwise an empty comment can be
used.
A back reference that occurs inside the parentheses to
which it refers fails when the subpattern is first used,
so, for example, (a\1) never matches. However, such ref
erences can be useful inside repeated subpatterns. For
example, the pattern
(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
iteration. In order for this to work, the pattern must be
such that the first iteration does not need to match the
back reference. This can be done using alternation, as in
the example above, or by a quantifier with a minimum of
zero.
ASSERTIONS
An assertion is a test on the characters following or pre
ceding the current matching point that does not actually
consume any characters. The simple assertions coded as \b,
\B, \A, \Z, \z, ^ and $ are described above. More compli
cated 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 posi
tion 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 lookbe
hind assertion is needed to achieve this effect.
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 alterna
tives, 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 5.005, which requires all branches to match the
same length of string. An assertion such as
(?<=ab(c|de))
is not permitted, because its single top-level branch can
match two different lengths, but it is acceptable if
rewritten to use two top-level branches:
(?<=abc|abde)
The implementation of lookbehind assertions is, for each
alternative, to temporarily move the current position back
by the fixed width and then try to match. If there are
insufficient characters before the current position, the
match is deemed to fail. Lookbehinds in conjunction with
once-only subpatterns can be particularly useful for
matching at the ends of strings; an example is given at
the end of the section on once-only subpatterns.
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 indepen
dently 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 sub
patterns in the whole pattern. However, substring captur
ing is carried out only for positive assertions, because
it does not make sense for negative assertions.
Assertions count towards the maximum of 200 parenthesized
subpatterns.
ONCE-ONLY SUBPATTERNS
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 car
rying 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. Once-only subpat
terns provide the means for specifying that once a portion
of the pattern has matched, it is not to be re-evaluated
in this way, so the matcher would give up immediately on
failing to match "foo" the first time. The notation is
another kind of special parenthesis, starting with (?> as
in this example:
(?>\d+)bar
This kind of parenthesis "locks up" the part of the pat
tern it contains once it has matched, and a failure fur
ther into the pattern is prevented from backtracking into
it. Backtracking past it to previous items, however, works
as normal.
An alternative description is that a subpattern of this
type matches the string of characters that an identical
standalone pattern would match, if anchored at the current
point in the subject string.
Once-only subpatterns are not capturing subpatterns. Sim
ple 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 pat
tern match, (?>\d+) can only match an entire sequence of
digits.
This construction can of course contain arbitrarily com
plicated subpatterns, and it can be nested.
Once-only subpatterns 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 which does not match.
Because matching proceeds from left to right, PCRE will
look for each "a" in the subject and then see if what fol
lows matches the rest of the pattern. If the pattern is
specified as
^.*abcd$
the initial .* matches the entire string at first, but
when this fails (because there is no following "a"), it
backtracks to match all but the last character, then all
but the last two characters, and so on. Once again the
search for "a" covers the entire string, from right to
left, so we are no better off. However, if the pattern is
written as
^(?>.*)(?<=abcd)
there can be no backtracking for the .* item; it can match
only the entire string. The subsequent lookbehind asser
tion 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.
When a pattern contains an unlimited repeat inside a sub
pattern that can itself be repeated an unlimited number of
times, the use of a once-only subpattern 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
example 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.
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a
subpattern conditionally or to choose between two alterna
tive 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;
otherwise the no-pattern (if present) is used. If there
are more than two alternatives in the subpattern, a com
pile-time error occurs.
There are two kinds of condition. If the text between the
parentheses consists of a sequence of digits, the condi
tion is satisfied if the capturing subpattern of that num
ber 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 sub
ject 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 not a sequence of digits, it must be
an assertion. This may be a positive or negative lookahead
or lookbehind assertion. Consider this pattern, again con
taining 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; other
wise 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 con
tinues up to the next closing parenthesis. Nested paren
theses 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 # charac
ter outside a character class introduces a comment that
continues 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 pat
tern that matches up to some fixed depth of nesting. It is
not possible to handle an arbitrary nesting depth. Perl
5.6 has provided an experimental facility that allows reg
ular expressions 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, the special item (?R) is provided
for the specific case of recursion. This PCRE pattern
solves the 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 (i.e. a correctly parenthesized substring). Finally
there is a closing parenthesis.
This particular example pattern contains nested unlimited
repeats, and so the use of a once-only subpattern for
matching strings of non-parentheses is important when
applying the pattern to strings that do not match. For
example, when it is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if a once-only sub
pattern 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.
The values set for any capturing subpatterns are those
from the outermost level of the recursion at which the
subpattern value is set. 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, it
saves data for the first 15 capturing parentheses only, as
there is no way to give an out-of-memory error from within
a recursion.
PERFORMANCE
Certain items that may appear in patterns are more effi
cient 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 effi
cient. Jeffrey Friedl's book contains a lot of discussion
about optimizing regular expressions for efficient perfor
mance.
When a pattern begins with .* 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. How
ever, if PCRE_DOTALL is not set, PCRE cannot make this
optimization, because the . metacharacter does not then
match a newline, and if the subject string contains new
lines, the pattern may match from the character immedi
ately 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 first captured substring
being "and". 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 hav
ing 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 num
ber 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 differ
ent numbers of times.) When the remainder of the pattern
is such that the entire match is going to fail, PCRE has
in principle to try every possible variation, and this can
take an extremely long time.
An optimization catches some of the more simple cases such
as
(a+)*b
where a literal character follows. Before embarking on the
standard matching procedure, PCRE checks that there is a
"b" later in the 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.
UTF-8 SUPPORT
Starting at release 3.3, PCRE has some support for charac
ter strings encoded in the UTF-8 format. This is incom
plete, and is regarded as experimental. In order to use
it, you must configure 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, but only in the cases that are mentioned below.
If you compile PCRE with UTF-8 support, but do not use it
at run time, the library will be a bit bigger, but the
additional run time overhead is limited to testing the
PCRE_UTF8 flag in several places, so should not be very
large.
PCRE assumes that the strings it is given contain valid
UTF-8 codes. It does not diagnose invalid UTF-8 strings.
If you pass invalid UTF-8 strings to PCRE, the results are
undefined.
Running with PCRE_UTF8 set causes these changes in the way
PCRE works:
1. In a pattern, the escape sequence \x{...}, where the
contents of the braces is a string of hexadecimal digits,
is interpreted as a UTF-8 character whose code number is
the given hexadecimal number, for example: \x{1234}. This
inserts from one to six literal bytes into the pattern,
using the UTF-8 encoding. If a non-hexadecimal digit
appears between the braces, the item is not recognized.
2. The original hexadecimal escape sequence, \xhh, gener
ates a two-byte UTF-8 character if its value is greater
than 127.
3. Repeat quantifiers are NOT correctly handled if they
follow a multibyte character. For example, \x{100}* and
\xc3+ do not work. If you want to repeat such characters,
you must enclose them in non-capturing parentheses, for
example (?:\x{100}), at present.
4. The dot metacharacter matches one UTF-8 character
instead of a single byte.
5. Unlike literal UTF-8 characters, the dot metacharacter
followed by a repeat quantifier does operate correctly on
UTF-8 characters instead of single bytes.
4. Although the \x{...} escape is permitted in a character
class, characters whose values are greater than 255 cannot
be included in a class.
5. A class is matched against a UTF-8 character instead of
just a single byte, but it can match only characters whose
values are less than 256. Characters with greater values
always fail to match a class.
6. Repeated classes work correctly on multiple characters.
7. Classes containing just a single character whose value
is greater than 127 (but less than 256), for example,
[\x80] or [^\x{93}], do not work because these are opti
mized into single byte matches. In the first case, of
course, the class brackets are just redundant.
8. Lookbehind assertions move backwards in the subject by
a fixed number of characters instead of a fixed number of
bytes. Simple cases have been tested to work correctly,
but there may be hidden gotchas herein.
9. The character types such as \d and \w do not work cor
rectly with UTF-8 characters. They continue to test a sin
gle byte.
10. Anything not explicitly mentioned here continues to
work in bytes rather than in characters.
The following UTF-8 features of Perl 5.6 are not imple
mented:
1. The escape sequence \C to match a single byte.
2. The use of Unicode tables and properties and escapes
\p, \P, and \X.
SAMPLE PROGRAM
The code below is a simple, complete demonstration pro
gram, to get you started with using PCRE. This code is
also supplied in the file pcredemo.c in the PCRE distribu
tion.
The program compiles the regular expression that is its
first argument, and matches it against the subject string
in its second argument. No options are set, and default
character tables are used. If matching succeeds, the pro
gram outputs the portion of the subject that matched,
together with the contents of any captured substrings.
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'
Note that there is a much more comprehensive test program,
called pcretest, which supports many more facilities for
testing regular expressions. The pcredemo program is pro
vided 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. Here's
the code:
#include <stdio.h>
#include <string.h>
#include <pcre.h>
#define OVECCOUNT 30 /* should be a multiple of 3 */
int main(int argc, char **argv)
{
pcre *re;
const char *error;
int erroffset;
int ovector[OVECCOUNT];
int rc, i;
if (argc != 3)
{
printf("Two arguments required: a regex and a "
"subject string\n");
return 1;
}
/* Compile the regular expression in the first argument
*/
re = pcre_compile(
argv[1], /* the pattern */
0, /* default options */
&error, /* for error message */
&erroffset, /* for error offset */
NULL); /* use default character tables */
/* Compilation failed: print the error message and exit
*/
if (re == NULL)
{
printf("PCRE compilation failed at offset %d: %s\n",
erroffset, error);
return 1;
}
/* Compilation succeeded: match the subject in the sec
ond
argument */
rc = pcre_exec(
re, /* the compiled pattern */
NULL, /* we didn't study the pattern */
argv[2], /* the subject string */
(int)strlen(argv[2]), /* the length of the subject */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector for substring information */
OVECCOUNT); /* number of elements in the vector */
/* Matching failed: handle error cases */
if (rc < 0)
{
switch(rc)
{
case PCRE_ERROR_NOMATCH: printf("No match\n");
break;
/*
Handle other special cases if you like
*/
default: printf("Matching error %d\n", rc); break;
}
return 1;
}
/* Match succeded */
printf("Match succeeded\n");
/* The output vector wasn't big enough */
if (rc == 0)
{
rc = OVECCOUNT/3;
printf("ovector only has room for %d captured "
substrings\n", rc - 1);
}
/* Show substrings stored in the output vector */
for (i = 0; i < rc; i++)
{
char *substring_start = argv[2] + ovector[2*i];
int substring_length = ovector[2*i+1] - ovector[2*i];
printf("%2d: %.*s\n", i, substring_length,
substring_start);
}
return 0;
}
AUTHOR
Philip Hazel <ph10@cam.ac.uk>
University Computing Service,
New Museums Site,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714
Last updated: 15 August 2001
Copyright (c) 1997-2001 University of Cambridge.
PCRE(3)
