NAME
pcre - 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);
const unsigned char *pcre_maketables(void);
int pcre_info(const pcre *code, int *optptr, *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 regu-
lar 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.
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 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 functions pcre_compile(), pcre_study(), and pcre_exec()
are used for compiling and matching regular expressions,
while pcre_copy_substring(), pcre_get_substring(), and
pcre_get_substring_list() are convenience functions for
extracting captured substrings from a matched subject
string. 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_info() is used to find out information
about a compiled pattern, while 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.
MULTI-THREADING
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 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 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 this for conveni-
ence, but in fact pcre is just a typedef for void, since the
contents of the block are not externally defined. It is up
to the caller to free the memory when it is no longer
required.
The size of a compiled pattern is roughly proportional to
the length of the pattern string, except that each character
class (other than those containing just a single character,
negated or not) requires 33 bytes, and repeat quantifiers
with a minimum greater than one or a bounded maximum cause
the relevant portions of the compiled pattern to be repli-
cated.
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 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 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.
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 "subject string"). This
effect can also be achieved by appropriate constructs in the
pattern itself, which is the only way to do it in Perl.
PCRE_CASELESS
If this bit is set, letters in the pattern match both upper
and lower case letters. It is equivalent to Perl's /i
option.
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.
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. A negative class such as [^a] always matches a new-
line 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 # out-
side 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 pattern, for example
within the sequence (?( which introduces a conditional sub-
pattern.
PCRE_EXTRA
This option turns on additional functionality of PCRE that
is incompatible with Perl. 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.
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
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" charac-
ters 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.
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 argument, and
returns a pointer to a pcre_extra block (another void
typedef) containing additional information about the pat-
tern; 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. Otherwise it
points to a textual error 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. The library contains a default set of tables
which is created in the default C locale when PCRE is com-
piled. This is used when the final argument of
pcre_compile() is NULL, and is sufficient for many applica-
tions.
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
The pcre_info() function returns information about a com-
piled pattern. Its yield is the number of capturing subpat-
terns, 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. 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 set if the form of the pattern implies
that it can match only at the start of a subject string.
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. If there is a fixed
first character, e.g. from a pattern such as
(cat|cow|coyote), then it is returned in the integer pointed
to by firstcharptr. 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),
then -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.
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 argument. Otherwise this
must be NULL.
The PCRE_ANCHORED option can be passed in the options argu-
ment, 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 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 this option, but it makes a
special case of a pattern match of the empty string within
its split() function, or when using the /g modifier. Using
PCRE_NOTEMPTY it is possible to emulate this behaviour.
The subject string is passed as a pointer in subject, a
length in length, and a starting offset in startoffset.
Unlike the pattern string, it may contain binary zero char-
acters. 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 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.
EXTRACTING CAPTURED SUBSTRINGS
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. 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 ovec-
tor, then 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 store 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.
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. The maximum
number of capturing subpatterns is 99. The maximum number
of all parenthesized subpatterns, including capturing sub-
patterns, 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.
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 char-
acters. The \v escape that was in the Perl documentation for
a long time was never in fact recognized. However, the char-
acter 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
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.
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 pat-
tern 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})
construction.
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) get set.
In Perl 5.004 $2 is set in both cases, and that is also true
of PCRE. If in the future Perl changes to a consistent state
that is different, PCRE may change to follow.
9. Another as yet unresolved discrepancy is that in Perl
5.005_02 the pattern /^(a)?(?(1)a|b)+$/ 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.
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 (ISBN 1-56592-257-3), covers them in great detail.
The description here is intended as reference documentation.
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 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 follow-
ing 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 particu-
lar, 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.
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)
\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 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 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 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 can be
used both inside and outside character classes. In addition,
inside a character class, the sequence "\b" is interpreted
as the backspace character (hex 08). Outside a character
class it has a different meaning (see below).
The third use of backslash is for specifying generic charac-
ter types:
\d any decimal digit
\D any character that is not a decimal digit
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 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"
above). For example, in the "fr" (French) locale, some char-
acter 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 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 argu-
ment 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 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 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 addition
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 mul-
tiline 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 is it 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. If the PCRE_DOTALL
option is set, then dots match newlines as well. The han-
dling of dot is entirely independent of the handling of cir-
cumflex 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, 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;
the character must be in the set of characters defined by
the class, unless the first character in the class is a cir-
cumflex, 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.
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.
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 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 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.
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 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_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.
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" set-
tings apply to the whole pattern (unless there are other
changes inside subpatterns). If there is more than one set-
ting 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 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.
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 "?:", the subpattern does
not do any capturing, and is not counted when computing the
number of any subsequent capturing subpatterns. For example,
if the string "the white queen" is matched against the pat-
tern
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 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.
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 due to the
greediness of the .* item.
However, if a quantifier is followed by a question mark,
then 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) then 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, then 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 treats such a pattern as though it were preceded by \A.
In cases where it is known that the subject string contains
no newlines, it is worth setting PCRE_DOTALL when the pat-
tern 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 iteration. 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 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. 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, then
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, then 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, then 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", "ababaa" 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, \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 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.
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 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 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. Lookbehinds in conjunction with
once-only subpatterns can be particularly useful for match-
ing 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 independently
at the same point in the subject string. First there is a
check that the previous three characters are all digits,
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.
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 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. Once-only subpatterns 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 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.
Once-only subpatterns are not capturing subpatterns. Simple
cases such as the above example can be thought of as a max-
imizing 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.
This construction can of course contain arbitrarily compli-
cated subpatterns, and it can be nested.
Once-only subpatterns can be used in conjunction with look-
behind 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 it.
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$
then the initial .* matches the entire string at first, but
when this fails, 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 pat-
tern is written as
^(?>.*)(?<=abcd)
then 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 process-
ing time.
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 two kinds of condition. If the text between the
parentheses consists of a sequence of digits, then the con-
dition is satisfied if the capturing subpattern of that
number has previously matched. Consider the following pat-
tern, 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 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 contain-
ing non-significant white space, and with the two alterna-
tives 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.
PERFORMANCE
Certain items that may appear in 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 discussion about optimizing
regular expressions for efficient performance.
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. 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 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 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.
AUTHOR
Philip Hazel <ph10@cam.ac.uk>
University Computing Service,
New Museums Site,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714
Last updated: 29 July 1999
Copyright (c) 1997-1999 University of Cambridge.