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<!-- manual page source format generated by PolyglotMan v3.2, -->
<!-- available at http://polyglotman.sourceforge.net/ -->
<html>
<head>
<title>PCRE(3) manual page</title>
</head>
<body bgcolor='white'>
<a href='#toc'>Table of Contents</a><p>
<h2><a name='sect0' href='#toc0'>Name</a></h2>
PCRE - Perl-compatible regular expressions
<h2><a name='sect1' href='#toc1'>Pcre Regular Expression Details</a></h2>
<p>
The syntax and semantics of the regular expressions supported by PCRE are
described below. Regular expressions are also described in the Perl documentation
and in a number of books, some of which have copious examples. Jeffrey Friedl’s
"Mastering Regular Expressions", published by O’Reilly, covers regular expressions
in great detail. This description of PCRE’s regular expressions is intended
as reference material. <p>
The original operation of PCRE was on strings of
one-byte characters. However, there is now also support for UTF-8 character
strings. To use this, you must build PCRE to include UTF-8 support, and then
call <b>pcre_compile()</b> with the PCRE_UTF8 option. How this affects pattern
matching is mentioned in several places below. There is also a summary of
UTF-8 features in the section on UTF-8 support in the main <b>pcre</b> page.
<p>
A regular expression is a pattern that is matched against a subject string
from left to right. Most characters stand for themselves in a pattern, and
match the corresponding characters in the subject. As a trivial example,
the pattern <p>
The quick brown fox<br>
<p>
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 <i>metacharacters</i>, which do not stand for themselves but instead are
interpreted in some special way. <p>
There are two different sets of metacharacters:
those that are recognized anywhere in the pattern except within square
brackets, and those that are recognized in square brackets. Outside square
brackets, the metacharacters are as follows: <p>
\ general escape character
with several uses<br>
^ assert start of string (or line, in multiline mode)<br>
$ assert end of string (or line, in multiline mode)<br>
. match any character except newline (by default)<br>
[ start character class definition<br>
| start of alternative branch<br>
( start subpattern<br>
) end subpattern<br>
? extends the meaning of (<br>
also 0 or 1 quantifier<br>
also quantifier minimizer<br>
* 0 or more quantifier<br>
+ 1 or more quantifier<br>
also "possessive quantifier"<br>
{ start min/max quantifier<br>
<p>
Part of a pattern that is in square brackets is called a "character class".
In a character class the only metacharacters are: <p>
\ general escape
character<br>
^ negate the class, but only if the first character<br>
- indicates character range<br>
[ POSIX character class (only if followed by POSIX<br>
syntax)<br>
] terminates the character class<br>
<p>
The following sections describe the use of each of the metacharacters.
<h2><a name='sect2' href='#toc2'>Backslash</a></h2>
<p>
The backslash character has several uses. Firstly, if it is followed
by a non-alphanumeric 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. <p>
For example, if you want to match
a * character, you write \* in the pattern. This escaping action applies
whether or not the following character would otherwise be interpreted as
a metacharacter, so it is always safe to precede a non-alphanumeric with
backslash to specify that it stands for itself. In particular, if you want
to match a backslash, you write \\. <p>
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. <p>
If you want to remove the special meaning
from a sequence of characters, you can do so by putting them between \Q
and \E. This is different from Perl in that $ and @ are handled as literals
in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable interpolation.
Note the following examples: <p>
Pattern PCRE matches Perl matches<br>
<p>
\Qabc$xyz\E abc$xyz abc followed by the<br>
contents of $xyz<br>
\Qabc\$xyz\E abc\$xyz abc\$xyz<br>
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz<br>
<p>
The \Q...\E sequence is recognized both inside and outside character classes.
<h3><a name='sect3' href='#toc3'>Non-printing characters</a></h3>
<p>
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 characters, apart from the binary zero
that terminates a pattern, but when a pattern is being prepared by text
editing, it is usually easier to use one of the following escape sequences
than the binary character it represents: <p>
\a alarm, that is, the
BEL character (hex 07)<br>
\cx "control-x", where x is any character<br>
\e escape (hex 1B)<br>
\f formfeed (hex 0C)<br>
\n newline (hex 0A)<br>
\r carriage return (hex 0D)<br>
\t tab (hex 09)<br>
\ddd character with octal code ddd, or backreference<br>
\xhh character with hex code hh<br>
\x{hhh..} character with hex code hhh... (UTF-8 mode only)<br>
<p>
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.
<p>
After \x, from zero to two hexadecimal digits are read (letters can be in
upper or lower case). In UTF-8 mode, any number of hexadecimal digits may
appear between \x{ and }, but the value of the character code must be less
than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters
other than hexadecimal digits appear between \x{ and }, or if there is no
terminating }, this form of escape is not recognized. Instead, the initial
\x will be interpreted as a basic hexadecimal escape, with no following
digits, giving a character whose value is zero. <p>
Characters whose value is
less than 256 can be defined by either of the two syntaxes for \x when PCRE
is in UTF-8 mode. There is no difference in the way they are handled. For
example, \xdc is exactly the same as \x{dc}. <p>
After \0 up to two further octal
digits are read. In both cases, if there are fewer than two digits, just
those that are present are used. Thus the sequence \0\x\07 specifies two binary
zeros followed by a BEL character (code value 7). Make sure you supply two
digits after the initial zero if the pattern character that follows is
itself an octal digit. <p>
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 <i>back reference</i>. A description
of how this works is given later, following the discussion of parenthesized
subpatterns. <p>
Inside a character class, or if the decimal number is greater
than 9 and there have not been that many capturing subpatterns, PCRE re-reads
up to three octal digits following the backslash, and generates a single
byte from the least significant 8 bits of the value. Any subsequent digits
stand for themselves. For example: <p>
\040 is another way of writing a space<br>
\40 is the same, provided there are fewer than 40<br>
previous capturing subpatterns<br>
\7 is always a back reference<br>
\11 might be a back reference, or another way of<br>
writing a tab<br>
\011 is always a tab<br>
\0113 is a tab followed by the character "3"<br>
\113 might be a back reference, otherwise the<br>
character with octal code 113<br>
\377 might be a back reference, otherwise<br>
the byte consisting entirely of 1 bits<br>
\81 is either a back reference, or a binary zero<br>
followed by the two characters "8" and "1"<br>
<p>
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. <p>
All the sequences
that define a single byte value or a single UTF-8 character (in UTF-8 mode)
can be used both inside and outside character classes. In addition, inside
a character class, the sequence \b is interpreted as the backspace character
(hex 08), and the sequence \X is interpreted as the character "X". Outside
a character class, these sequences have different meanings (see below).
<h3><a name='sect4' href='#toc4'>Generic character types</a></h3>
<p>
The third use of backslash is for specifying
generic character types. The following are always recognized: <p>
\d any
decimal digit<br>
\D any character that is not a decimal digit<br>
\s any whitespace character<br>
\S any character that is not a whitespace character<br>
\w any "word" character<br>
\W any "non-word" character<br>
<p>
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. <p>
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. <p>
For compatibility with
Perl, \s does not match the VT character (code 11). This makes it different
from the the POSIX "space" class. The \s characters are <a href='HT.9.html'>HT (9)</a>
, LF (10),
FF (12), CR (13), and space (32). <p>
A "word" character is an underscore or
any character less than 256 that is a letter or digit. The definition of
letters and digits is controlled by PCRE’s low-valued character tables, and
may vary if locale-specific matching is taking place (see "Locale support"
in the <b>pcreapi</b> page). For example, in the "fr_FR" (French) locale, some
character codes greater than 128 are used for accented letters, and these
are matched by \w. <p>
In UTF-8 mode, characters with values greater than 128
never match \d, \s, or \w, and always match \D, \S, and \W. This is true even
when Unicode character property support is available.
<h3><a name='sect5' href='#toc5'>Unicode character
properties</a></h3>
<p>
When PCRE is built with Unicode character property support,
three additional escape sequences to match generic character types are
available when UTF-8 mode is selected. They are: <p>
\p{<i>xx</i>} a character with
the <i>xx</i> property<br>
\P{<i>xx</i>} a character without the <i>xx</i> property<br>
\X an extended Unicode sequence<br>
<p>
The property names represented by <i>xx</i> above are limited to the Unicode
general category properties. Each character has exactly one such property,
specified by a two-letter abbreviation. For compatibility with Perl, negation
can be specified by including a circumflex between the opening brace and
the property name. For example, \p{^Lu} is the same as \P{Lu}. <p>
If only one letter
is specified with \p or \P, it includes all the properties that start with
that letter. In this case, in the absence of negation, the curly brackets
in the escape sequence are optional; these two examples have the same effect:
<p>
\p{L}<br>
\pL<br>
<p>
The following property codes are supported: <p>
C Other<br>
Cc Control<br>
Cf Format<br>
Cn Unassigned<br>
Co Private use<br>
Cs Surrogate<br>
<p>
L Letter<br>
Ll Lower case letter<br>
Lm Modifier letter<br>
Lo Other letter<br>
Lt Title case letter<br>
Lu Upper case letter<br>
<p>
M Mark<br>
Mc Spacing mark<br>
Me Enclosing mark<br>
Mn Non-spacing mark<br>
<p>
N Number<br>
Nd Decimal number<br>
Nl Letter number<br>
No Other number<br>
<p>
P Punctuation<br>
Pc Connector punctuation<br>
Pd Dash punctuation<br>
Pe Close punctuation<br>
Pf Final punctuation<br>
Pi Initial punctuation<br>
Po Other punctuation<br>
Ps Open punctuation<br>
<p>
S Symbol<br>
Sc Currency symbol<br>
Sk Modifier symbol<br>
Sm Mathematical symbol<br>
So Other symbol<br>
<p>
Z Separator<br>
Zl Line separator<br>
Zp Paragraph separator<br>
Zs Space separator<br>
<p>
Extended properties such as "Greek" or "InMusicalSymbols" are not supported
by PCRE. <p>
Specifying caseless matching does not affect these escape sequences.
For example, \p{Lu} always matches only upper case letters. <p>
The \X escape
matches any number of Unicode characters that form an extended Unicode
sequence. \X is equivalent to <p>
(?>\PM\pM*)<br>
<p>
That is, it matches a character without the "mark" property, followed
by zero or more characters with the "mark" property, and treats the sequence
as an atomic group (see below). Characters with the "mark" property are
typically accents that affect the preceding character. <p>
Matching characters
by Unicode property is not fast, because PCRE has to search a structure
that contains data for over fifteen thousand characters. That is why the
traditional escape sequences such as \d and \w do not use Unicode properties
in PCRE.
<h3><a name='sect6' href='#toc6'>Simple assertions</a></h3>
<p>
The fourth use of backslash is for certain
simple assertions. 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: <p>
\b matches at
a word boundary<br>
\B matches when not at a word boundary<br>
\A matches at start of subject<br>
\Z matches at end of subject or before newline at end<br>
\z matches at end of subject<br>
\G matches at first matching position in subject<br>
<p>
These assertions may not appear in character classes (but note that \b
has a different meaning, namely the backspace character, inside a character
class). <p>
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. <p>
The \A, \Z, and \z assertions differ
from the traditional circumflex and dollar (described in the next section)
in that they only ever match at the very start and end of the subject string,
whatever options are set. Thus, they are independent of multiline mode. These
three assertions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options,
which affect only the behaviour of the circumflex and dollar metacharacters.
However, if the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero, indicating
that matching is to start at a point other than the beginning of the subject,
\A can never match. The difference between \Z and \z is that \Z matches before
a newline that is the last character of the string as well as at the end
of the string, whereas \z matches only at the end. <p>
The \G assertion is true
only when the current matching position is at the start point of the match,
as specified by the <i>startoffset</i> argument of <b>pcre_exec()</b>. It differs from
\A when the value of <i>startoffset</i> is non-zero. By calling <b>pcre_exec()</b> multiple
times with appropriate arguments, you can mimic Perl’s /g option, and it
is in this kind of implementation where \G can be useful. <p>
Note, however,
that PCRE’s interpretation of \G, as the start of the current match, is subtly
different from Perl’s, which defines it as the end of the previous match.
In Perl, these can be different when the previously matched string was
empty. Because PCRE does just one match at a time, it cannot reproduce this
behaviour. <p>
If all the alternatives of a pattern begin with \G, the expression
is anchored to the starting match position, and the "anchored" flag is
set in the compiled regular expression.
<h2><a name='sect7' href='#toc7'>Circumflex and Dollar</a></h2>
<p>
Outside
a character class, in the default matching mode, the circumflex character
is an assertion that is true only if the current matching point is at the
start of the subject string. If the <i>startoffset</i> argument of <b>pcre_exec()</b>
is non-zero, circumflex can never match if the PCRE_MULTILINE option is
unset. Inside a character class, circumflex has an entirely different meaning
(see below). <p>
Circumflex need not be the first character of the pattern
if a number of alternatives are involved, but it should be the first thing
in each alternative in which it appears if the pattern is ever to match
that branch. If all possible alternatives start with a circumflex, that
is, if the pattern is constrained to match only at the start of the subject,
it is said to be an "anchored" pattern. (There are also other constructs
that can cause a pattern to be anchored.) <p>
A dollar character is an assertion
that 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. <p>
The meaning of dollar can be changed so that it matches
only at the very end of the string, by setting the PCRE_DOLLAR_ENDONLY
option at compile time. This does not affect the \Z assertion. <p>
The meanings
of the circumflex and dollar characters are changed if the PCRE_MULTILINE
option is set. When this is the case, they match immediately after and immediately
before an internal newline character, respectively, in addition to matching
at the start and end of the subject string. For example, the pattern /^abc$/
matches the subject string "def\nabc" (where \n represents a newline character)
in multiline mode, but not otherwise. Consequently, patterns that are anchored
in single line mode because all branches start with ^ are not anchored in
multiline mode, and a match for circumflex is possible when the <i>startoffset</i>
argument of <b>pcre_exec()</b> is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored
if PCRE_MULTILINE is set. <p>
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.
<h2><a name='sect8' href='#toc8'>Full Stop (period, Dot)</a></h2>
<p>
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. In UTF-8 mode, a dot matches any
UTF-8 character, which might be more than one byte long, except (by default)
newline. If the PCRE_DOTALL option is set, dots match newlines as well. The
handling of dot is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve newline characters.
Dot has no special meaning in a character class.
<h2><a name='sect9' href='#toc9'>Matching a Single Byte</a></h2>
<p>
Outside a character class, the escape sequence \C matches any one byte,
both in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature
is provided in Perl in order to match individual bytes in UTF-8 mode. Because
it breaks up UTF-8 characters into individual bytes, what remains in the
string may be a malformed UTF-8 string. For this reason, the \C escape sequence
is best avoided. <p>
PCRE does not allow \C to appear in lookbehind assertions
(described below), because in UTF-8 mode this would make it impossible
to calculate the length of the lookbehind.
<h2><a name='sect10' href='#toc10'>Square Brackets and Character
Classes</a></h2>
<p>
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. <p>
A character class matches a single
character in the subject. In UTF-8 mode, the character may occupy more than
one byte. A matched character must be in the set of characters defined by
the class, unless the first character in the class definition is a circumflex,
in which case the subject character must not be in the set defined by the
class. If a circumflex is actually required as a member of the class, ensure
it is not the first character, or escape it with a backslash. <p>
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 that are in
the class by enumerating those that are not. A class that starts with a
circumflex is not an assertion: it still consumes a character from the
subject string, and therefore it fails if the current pointer is at the
end of the string. <p>
In UTF-8 mode, characters with values greater than 255
can be included in a class as a literal string of bytes, or by using the
\x{ escaping mechanism. <p>
When caseless matching is set, any letters in a class
represent both their upper case and lower case versions, so for example,
a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does
not match "A", whereas a caseful version would. When running in UTF-8 mode,
PCRE supports the concept of case for characters with values greater than
128 only when it is compiled with Unicode property support. <p>
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. <p>
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. <p>
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 class containing a range followed by two
other characters. The octal or hexadecimal representation of "]" can also
be used to end a range. <p>
Ranges operate in the collating sequence of character
values. They can also be used for characters specified numerically, for
example [\000-\037]. In UTF-8 mode, ranges can include characters whose values
are greater than 255, for example [\x{100}-\x{2ff}]. <p>
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 in non-UTF-8 mode, if character tables for the "fr_FR" locale are in
use, [\xc8-\xcb] matches accented E characters in both cases. In UTF-8 mode,
PCRE supports the concept of case for characters with values greater than
128 only when it is compiled with Unicode property support. <p>
The character
types \d, \D, \p, \P, \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. <p>
The only metacharacters that are recognized
in character classes are backslash, hyphen (only where it can be interpreted
as specifying a range), circumflex (only at the start), opening square
bracket (only when it can be interpreted as introducing a POSIX class name
- see the next section), and the terminating closing square bracket. However,
escaping other non-alphanumeric characters does no harm.
<h2><a name='sect11' href='#toc11'>Posix Character
Classes</a></h2>
<p>
Perl supports the POSIX notation for character classes. This uses
names enclosed by [: and :] within the enclosing square brackets. PCRE also
supports this notation. For example, <p>
[01[:alpha:]%]<br>
<p>
matches "0", "1", any alphabetic character, or "%". The supported class
names are <p>
alnum letters and digits<br>
alpha letters<br>
ascii character codes 0 - 127<br>
blank space or tab only<br>
cntrl control characters<br>
digit decimal digits (same as \d)<br>
graph printing characters, excluding space<br>
lower lower case letters<br>
print printing characters, including space<br>
punct printing characters, excluding letters and digits<br>
space white space (not quite the same as \s)<br>
upper upper case letters<br>
word "word" characters (same as \w)<br>
xdigit hexadecimal digits<br>
<p>
The "space" characters are <a href='HT.9.html'>HT (9)</a>
, LF (10), VT (11), FF (12), CR (13),
and space (32). Notice that this list includes the VT character (code 11).
This makes "space" different to \s, which does not include VT (for Perl
compatibility). <p>
The name "word" is a Perl extension, and "blank" is a GNU
extension from Perl 5.8. Another Perl extension is negation, which is indicated
by a ^ character after the colon. For example, <p>
[12[:^digit:]]<br>
<p>
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. <p>
In UTF-8 mode,
characters with values greater than 128 do not match any of the POSIX character
classes.
<h2><a name='sect12' href='#toc12'>Vertical Bar</a></h2>
<p>
Vertical bar characters are used to separate alternative
patterns. For example, the pattern <p>
gilbert|sullivan<br>
<p>
matches either "gilbert" or "sullivan". Any number of alternatives may
appear, and an empty alternative is permitted (matching the empty string).
The matching process tries each alternative in turn, from left to right,
and the first one that succeeds is used. If the alternatives are within
a subpattern (defined below), "succeeds" means matching the rest of
the main pattern as well as the alternative in the subpattern.
<h2><a name='sect13' href='#toc13'>Internal
Option Setting</a></h2>
<p>
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL,
and PCRE_EXTENDED options can be changed from within the pattern by a sequence
of Perl option letters enclosed between "(?" and ")". The option letters
are <p>
i for PCRE_CASELESS<br>
m for PCRE_MULTILINE<br>
s for PCRE_DOTALL<br>
x for PCRE_EXTENDED<br>
<p>
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. <p>
When an option
change occurs at top level (that is, not inside subpattern parentheses),
the change applies to the remainder of the pattern that follows. If the
change is placed right at the start of a pattern, PCRE extracts it into
the global options (and it will therefore show up in data extracted by
the <b>pcre_fullinfo()</b> function). <p>
An option change within a subpattern affects
only that part of the current pattern that follows it, so <p>
(a(?i)b)c<br>
<p>
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, <p>
(a(?i)b|c)<br>
<p>
matches "ab", "aB", "c", and "C", even though when matching "C" the first
branch is abandoned before the option setting. This is because the effects
of option settings happen at compile time. There would be some very weird
behaviour otherwise. <p>
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 to put it
at the start.
<h2><a name='sect14' href='#toc14'>Subpatterns</a></h2>
<p>
Subpatterns are delimited by parentheses (round
brackets), which can be nested. Turning part of a pattern into a subpattern
does two things: <p>
1. It localizes a set of alternatives. For example, the
pattern <p>
cat(aract|erpillar|)<br>
<p>
matches one of the words "cat", "cataract", or "caterpillar". Without the
parentheses, it would match "cataract", "erpillar" or the empty string.
<p>
2. It sets up the subpattern as a capturing subpattern. This means that,
when the whole pattern matches, that portion of the subject string that
matched the subpattern is passed back to the caller via the <i>ovector</i> argument
of <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
from 1) to obtain numbers for the capturing subpatterns. <p>
For example, if
the string "the red king" is matched against the pattern <p>
the ((red|white)
(king|queen))<br>
<p>
the captured substrings are "red king", "red", and "king", and are numbered
1, 2, and 3, respectively. <p>
The fact that plain parentheses fulfil two functions
is not always helpful. There are often times when a grouping subpattern
is required without a capturing requirement. If an opening parenthesis is
followed by a question mark and a colon, the subpattern does not do any
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 pattern <p>
the ((?:red|white) (king|queen))<br>
<p>
the captured substrings are "white queen" and "queen", and are numbered
1 and 2. The maximum number of capturing subpatterns is 65535, and the maximum
depth of nesting of all subpatterns, both capturing and non-capturing, is
200. <p>
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 <p>
(?i:saturday|sunday)<br>
(?:(?i)saturday|sunday)<br>
<p>
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".
<h2><a name='sect15' href='#toc15'>Named Subpatterns</a></h2>
<p>
Identifying capturing parentheses by number is simple,
but it can be very hard to keep track of the numbers in complicated regular
expressions. Furthermore, if an expression is modified, the numbers may
change. To help with this difficulty, PCRE supports the naming of subpatterns,
something that Perl does not provide. The Python syntax (?P<name>...) is used.
Names consist of alphanumeric characters and underscores, and must be unique
within a pattern. <p>
Named capturing parentheses are still allocated numbers
as well as names. The PCRE API provides function calls for extracting the
name-to-number translation table from a compiled pattern. There is also a
convenience function for extracting a captured substring by name. For further
details see the <b>pcreapi</b> documentation.
<h2><a name='sect16' href='#toc16'>Repetition</a></h2>
<p>
Repetition is specified
by quantifiers, which can follow any of the following items: <p>
a literal
data character<br>
the . metacharacter<br>
the \C escape sequence<br>
the \X escape sequence (in UTF-8 mode with Unicode properties)<br>
an escape such as \d that matches a single character<br>
a character class<br>
a back reference (see next section)<br>
a parenthesized subpattern (unless it is an assertion)<br>
<p>
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: <p>
z{2,4}<br>
<p>
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 <p>
[aeiou]{3,}<br>
<p>
matches at least 3 successive vowels, but may match many more, while <p>
\d{8}<br>
<p>
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. <p>
In UTF-8 mode, quantifiers
apply to UTF-8 characters rather than to individual bytes. Thus, for example,
\x{100}{2} matches two UTF-8 characters, each of which is represented by
a two-byte sequence. Similarly, when Unicode property support is available,
\X{3} matches three Unicode extended sequences, each of which may be several
bytes long (and they may be of different lengths). <p>
The quantifier {0} is
permitted, causing the expression to behave as if the previous item and
the quantifier were not present. <p>
For convenience (and historical compatibility)
the three most common quantifiers have single-character abbreviations: <p>
* is equivalent to {0,}<br>
+ is equivalent to {1,}<br>
? is equivalent to {0,1}<br>
<p>
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: <p>
(a?)*<br>
<p>
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. <p>
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 /* and */
and within the comment, individual * and / characters may appear. An attempt
to match C comments by applying the pattern <p>
/\*.*\*/<br>
<p>
to the string <p>
/* first comment */ not comment /* second comment */<br>
<p>
fails, because it matches the entire string owing to the greediness of
the .* item. <p>
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 <p>
/\*.*?\*/<br>
<p>
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 <p>
\d??\d<br>
<p>
which matches one digit by preference, but can match two if that is the
only way the rest of the pattern matches. <p>
If the PCRE_UNGREEDY option is
set (an option which is not available in Perl), the quantifiers are not
greedy by default, but individual ones can be made greedy by following
them with a question mark. In other words, it inverts the default behaviour.
<p>
When a parenthesized subpattern is quantified with a minimum repeat count
that is greater than 1 or with a limited maximum, more memory is required
for the compiled pattern, in proportion to the size of the minimum or maximum.
<p>
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 character position in the subject string, so there is no point in
retrying the overall match at any position after the first. PCRE normally
treats such a pattern as though it were preceded by \A. <p>
In cases where it
is known that the subject string contains no newlines, it is worth setting
PCRE_DOTALL in order to obtain this optimization, or alternatively using
^ to indicate anchoring explicitly. <p>
However, there is one situation where
the optimization cannot be used. When .* is inside capturing parentheses
that are the subject of a backreference elsewhere in the pattern, a match
at the start may fail, and a later one succeed. Consider, for example: <p>
(.*)abc\1<br>
<p>
If the subject is "xyz123abc123" the match point is the fourth character.
For this reason, such a pattern is not implicitly anchored. <p>
When a capturing
subpattern is repeated, the value captured is the substring that matched
the final iteration. For example, after <p>
(tweedle[dume]{3}\s*)+<br>
<p>
has matched "tweedledum tweedledee" the value of the captured substring
is "tweedledee". However, if there are nested capturing subpatterns, the
corresponding captured values may have been set in previous iterations.
For example, after <p>
/(a|(b))+/<br>
<p>
matches "aba" the value of the second captured substring is "b".
<h2><a name='sect17' href='#toc17'>Atomic
Grouping and Possessive Quantifiers</a></h2>
<p>
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. <p>
Consider, for example, the pattern \d+foo when applied to
the subject line <p>
123456bar<br>
<p>
After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the \d+
item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
(a term taken from Jeffrey Friedl’s book) provides the means for specifying
that once a subpattern has matched, it is not to be re-evaluated in this
way. <p>
If we use atomic grouping for the previous example, the matcher would
give up immediately on failing to match "foo" the first time. The notation
is a kind of special parenthesis, starting with (?> as in this example:
<p>
(?>\d+)foo<br>
<p>
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. Backtracking past it to previous items, however,
works as normal. <p>
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. <p>
Atomic
grouping subpatterns are not capturing subpatterns. Simple cases such as
the above example can be thought of as a maximizing repeat that must swallow
everything it can. So, while both \d+ and \d+? are prepared to adjust the
number of digits they match in order to make the rest of the pattern match,
(?>\d+) can only match an entire sequence of digits. <p>
Atomic groups in general
can of course contain arbitrarily complicated subpatterns, and can be nested.
However, when the subpattern for an atomic group is just a single repeated
item, as in the example above, a simpler notation, called a "possessive
quantifier" can be used. This consists of an additional + character following
a quantifier. Using this notation, the previous example can be rewritten
as <p>
\d++foo<br>
<p>
Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
option is ignored. They are a convenient notation for the simpler forms
of atomic group. However, there is no difference in the meaning or processing
of a possessive quantifier and the equivalent atomic group. <p>
The possessive
quantifier syntax is an extension to the Perl syntax. It originates in Sun’s
Java package. <p>
When a pattern contains an unlimited repeat inside a subpattern
that can itself be repeated an unlimited number of times, the use of an
atomic group is the only way to avoid some failing matches taking a very
long time indeed. The pattern <p>
(\D+|<\d+>)*[!?]<br>
<p>
matches an unlimited number of substrings that either consist of non-digits,
or digits enclosed in <>, followed by either ! or ?. When it matches, it runs
quickly. However, if it is applied to <p>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa<br>
<p>
it takes a long time before reporting failure. This is because the string
can be divided between the internal \D+ repeat and the external * repeat
in a large number of ways, and all have to be tried. (The example uses [!?]
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
so that it uses an atomic group, like this: <p>
((?>\D+)|<\d+>)*[!?]<br>
<p>
sequences of non-digits cannot be broken, and failure happens quickly.
<h2><a name='sect18' href='#toc18'>Back References</a></h2>
<p>
Outside a character class, a backslash followed by a
digit greater than 0 (and possibly further digits) is a back reference
to a capturing subpattern earlier (that is, to its left) in the pattern,
provided there have been that many previous capturing left parentheses.
<p>
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 subsection entitled "Non-printing
characters" above for further details of the handling of digits following
a backslash. <p>
A back reference matches whatever actually matched the capturing
subpattern in the current subject string, rather than anything matching
the subpattern itself (see "Subpatterns as subroutines" below for a
way of doing that). So the pattern <p>
(sens|respons)e and \1ibility<br>
<p>
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If caseful matching is in force at the time
of the back reference, the case of letters is relevant. For example, <p>
((?i)rah)\s+\1<br>
<p>
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
capturing subpattern is matched caselessly. <p>
Back references to named subpatterns
use the Python syntax (?P=name). We could rewrite the above example as follows:
<p>
(?<p1>(?i)rah)\s+(?P=p1)<br>
<p>
There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back references
to it always fail. For example, the pattern <p>
(a|(bc))\2<br>
<p>
always fails if it starts to match "a" rather than "bc". Because there
may be many capturing parentheses in a pattern, all digits following the
backslash are taken as part of a potential back reference number. If the
pattern continues with a digit character, some delimiter must be used to
terminate the back reference. If the PCRE_EXTENDED option is set, this can
be whitespace. Otherwise an empty comment (see "Comments" below) can
be used. <p>
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 pattern <p>
(a|b\1)+<br>
<p>
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.
<h2><a name='sect19' href='#toc19'>Assertions</a></h2>
<p>
An assertion is a test on the characters
following or preceding the current matching point that does not actually
consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z, \z,
^ and $ are described above. <p>
More complicated assertions are coded as
subpatterns. There are two kinds: those that look ahead of the current position
in the subject string, and those that look behind it. An assertion subpattern
is matched in the normal way, except that it does not cause the current
matching position to be changed. <p>
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.
<h3><a name='sect20' href='#toc20'>Lookahead assertions</a></h3>
<p>
Lookahead assertions start
with (?= for positive assertions and (?! for negative assertions. For example,
<p>
\w+(?=;)<br>
<p>
matches a word followed by a semicolon, but does not include the semicolon
in the match, and <p>
foo(?!bar)<br>
<p>
matches any occurrence of "foo" that is not followed by "bar". Note that
the apparently similar pattern <p>
(?!foo)bar<br>
<p>
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 the other effect. <p>
If you want to force a
matching failure at some point in a pattern, the most convenient way to
do it is with (?!) because an empty string always matches, so an assertion
that requires there not to be an empty string must always fail.
<h3><a name='sect21' href='#toc21'>Lookbehind
assertions</a></h3>
<p>
Lookbehind assertions start with (?<= for positive assertions
and (?<! for negative assertions. For example, <p>
(?<!foo)bar<br>
<p>
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 <p>
(?<=bullock|donkey)<br>
<p>
is permitted, but <p>
(?<!dogs?|cats?)<br>
<p>
causes an error at compile time. Branches that match different length strings
are permitted only at the top level of a lookbehind assertion. This is an
extension compared with Perl (at least for 5.8), which requires all branches
to match the same length of string. An assertion such as <p>
(?<=ab(c|de))<br>
<p>
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:
<p>
(?<=abc|abde)<br>
<p>
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. <p>
PCRE does not allow the \C escape (which matches
a single byte in UTF-8 mode) to appear in lookbehind assertions, because
it makes it impossible to calculate the length of the lookbehind. The \X
escape, which can match different numbers of bytes, is also not permitted.
<p>
Atomic groups can be used in conjunction with lookbehind assertions to
specify efficient matching at the end of the subject string. Consider a
simple pattern such as <p>
abcd$<br>
<p>
when applied to a long string that does not match. Because matching proceeds
from left to right, PCRE will look for each "a" in the subject and then
see if what follows matches the rest of the pattern. If the pattern is specified
as <p>
^.*abcd$<br>
<p>
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 <p>
^(?>.*)(?<=abcd)<br>
<p>
or, equivalently, using the possessive quantifier syntax, <p>
^.*+(?<=abcd)<br>
<p>
there can be no backtracking for the .* item; it can match only the entire
string. The subsequent lookbehind assertion does a single test on the last
four characters. If it fails, the match fails immediately. For long strings,
this approach makes a significant difference to the processing time.
<h3><a name='sect22' href='#toc22'>Using
multiple assertions</a></h3>
<p>
Several assertions (of any sort) may occur in succession.
For example, <p>
(?<=\d{3})(?<!999)foo<br>
<p>
matches "foo" preceded by three digits that are not "999". Notice that
each of the assertions is applied independently at the same point in the
subject string. First there is a check that the previous three characters
are all digits, and then there is a check that the same three characters
are not "999". This pattern does <i>not</i> 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 <p>
(?<=\d{3}...)(?<!999)foo<br>
<p>
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". <p>
Assertions can be nested in
any combination. For example, <p>
(?<=(?<!foo)bar)baz<br>
<p>
matches an occurrence of "baz" that is preceded by "bar" which in turn
is not preceded by "foo", while <p>
(?<=\d{3}(?!999)...)foo<br>
<p>
is another pattern that matches "foo" preceded by three digits and any
three characters that are not "999".
<h2><a name='sect23' href='#toc23'>Conditional Subpatterns</a></h2>
<p>
It is possible
to cause the matching process to obey a subpattern 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 <p>
(?(condition)yes-pattern)<br>
(?(condition)yes-pattern|no-pattern)<br>
<p>
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 compile-time error occurs. <p>
There are three kinds of condition. If the text
between the parentheses consists of a sequence of digits, the condition
is satisfied if the capturing subpattern of that number has previously
matched. The number must be greater than zero. Consider the following pattern,
which contains non-significant white space to make it more readable (assume
the PCRE_EXTENDED option) and to divide it into three parts for ease of
discussion: <p>
( \( )? [^()]+ (?(1) \) )<br>
<p>
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. <p>
If the condition is the string (R),
it is satisfied if a recursive call to the pattern or subpattern has been
made. At "top level", the condition is false. This is a PCRE extension. Recursive
patterns are described in the next section. <p>
If the condition is not a sequence
of digits or (R), it must be an assertion. This may be a positive or negative
lookahead or lookbehind assertion. Consider this pattern, again containing
non-significant white space, and with the two alternatives on the second
line: <p>
(?(?=[^a-z]*[a-z])<br>
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )<br>
<p>
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.
<h2><a name='sect24' href='#toc24'>Comments</a></h2>
<p>
The sequence (?# marks the start of a comment that continues up to the
next closing parenthesis. Nested parentheses are not permitted. The characters
that make up a comment play no part in the pattern matching at all. <p>
If the
PCRE_EXTENDED option is set, an unescaped # character outside a character
class introduces a comment that continues up to the next newline character
in the pattern.
<h2><a name='sect25' href='#toc25'>Recursive Patterns</a></h2>
<p>
Consider the problem of matching a
string in parentheses, allowing for unlimited nested parentheses. Without
the use of recursion, the best that can be done is to use a pattern that
matches up to some fixed depth of nesting. It is not possible to handle
an arbitrary nesting depth. Perl provides a facility that allows regular
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: <p>
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;<br>
<p>
The (?p{...}) item interpolates Perl code at run time, and in this case refers
recursively to the pattern in which it appears. Obviously, PCRE cannot support
the interpolation of Perl code. Instead, it supports some special syntax
for recursion of the entire pattern, and also for individual subpattern
recursion. <p>
The special item that consists of (? followed by a number greater
than zero and a closing parenthesis is a recursive call of the subpattern
of the given number, provided that it occurs inside that subpattern. (If
not, it is a "subroutine" call, which is described in the next section.)
The special item (?R) is a recursive call of the entire regular expression.
<p>
For example, this PCRE pattern solves the nested parentheses problem (assume
the PCRE_EXTENDED option is set so that white space is ignored): <p>
\( (
(?>[^()]+) | (?R) )* \)<br>
<p>
First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a recursive
match of the pattern itself (that is a correctly parenthesized substring).
Finally there is a closing parenthesis. <p>
If this were part of a larger pattern,
you would not want to recurse the entire pattern, so instead you could
use this: <p>
( \( ( (?>[^()]+) | (?1) )* \) )<br>
<p>
We have put the pattern into parentheses, and caused the recursion to
refer to them instead of the whole pattern. In a larger pattern, keeping
track of parenthesis numbers can be tricky. It may be more convenient to
use named parentheses instead. For this, PCRE uses (?P>name), which is an
extension to the Python syntax that PCRE uses for named parentheses (Perl
does not provide named parentheses). We could rewrite the above example
as follows: <p>
(?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )<br>
<p>
This particular example pattern contains nested unlimited repeats, and
so the use of atomic grouping for matching strings of non-parentheses is
important when applying the pattern to strings that do not match. For example,
when this pattern is applied to <p>
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()<br>
<p>
it yields "no match" quickly. However, if atomic grouping is not used,
the match runs for a very long time indeed because there are so many different
ways the + and * repeats can carve up the subject, and all have to be tested
before failure can be reported. <p>
At the end of a match, the values set for
any capturing subpatterns are those from the outermost level of the recursion
at which the subpattern value is set. If you want to obtain intermediate
values, a callout function can be used (see the next section and the <b>pcrecallout</b>
documentation). If the pattern above is matched against <p>
(ab(cd)ef)<br>
<p>
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 <p>
\( ( ( (?>[^()]+) | (?R) )* ) \)<br>
^ ^<br>
^ ^<br>
<p>
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 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory
can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. <p>
Do
not confuse the (?R) item with the condition (R), which tests for recursion.
Consider this pattern, which matches text in angle brackets, allowing for
arbitrary nesting. Only digits are allowed in nested brackets (that is,
when recursing), whereas any characters are permitted at the outer level.
<p>
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * ><br>
<p>
In this pattern, (?(R) is the start of a conditional subpattern, with
two different alternatives for the recursive and non-recursive cases. The
(?R) item is the actual recursive call.
<h2><a name='sect26' href='#toc26'>Subpatterns As Subroutines</a></h2>
<p>
If
the syntax for a recursive subpattern reference (either by number or by
name) is used outside the parentheses to which it refers, it operates like
a subroutine in a programming language. An earlier example pointed out that
the pattern <p>
(sens|respons)e and \1ibility<br>
<p>
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If instead the pattern <p>
(sens|respons)e
and (?1)ibility<br>
<p>
is used, it does match "sense and responsibility" as well as the other
two strings. Such references must, however, follow the subpattern to which
they refer.
<h2><a name='sect27' href='#toc27'>Callouts</a></h2>
<p>
Perl has a feature whereby using the sequence (?{...})
causes arbitrary Perl code to be obeyed in the middle of matching a regular
expression. This makes it possible, amongst other things, to extract different
substrings that match the same pair of parentheses when there is a repetition.
<p>
PCRE provides a similar feature, but of course it cannot obey arbitrary
Perl code. The feature is called "callout". The caller of PCRE provides an
external function by putting its entry point in the global variable <i>pcre_callout</i>.
By default, this variable contains NULL, which disables all calling out.
<p>
Within a regular expression, (?C) indicates the points at which the external
function is to be called. If you want to identify different callout points,
you can put a number less than 256 after the letter C. The default value
is zero. For example, this pattern has two callout points: <p>
(?C1)dabc(?C2)def<br>
<p>
If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
automatically installed before each item in the pattern. They are all numbered
255. <p>
During matching, when PCRE reaches a callout point (and <i>pcre_callout</i>
is set), the external function is called. It is provided with the number
of the callout, the position in the pattern, and, optionally, one item
of data originally supplied by the caller of <b>pcre_exec()</b>. The callout function
may cause matching to proceed, to backtrack, or to fail altogether. A complete
description of the interface to the callout function is given in the <b>pcrecallout</b>
documentation. <p>
Last updated: 09 September 2004 <br>
Copyright (c) 1997-2004 University of Cambridge. <p>
<hr><p>
<a name='toc'><b>Table of Contents</b></a><p>
<ul>
<li><a name='toc0' href='#sect0'>Name</a></li>
<li><a name='toc1' href='#sect1'>Pcre Regular Expression Details</a></li>
<li><a name='toc2' href='#sect2'>Backslash</a></li>
<ul>
<li><a name='toc3' href='#sect3'>Non-printing characters</a></li>
<li><a name='toc4' href='#sect4'>Generic character types</a></li>
<li><a name='toc5' href='#sect5'>Unicode character properties</a></li>
<li><a name='toc6' href='#sect6'>Simple assertions</a></li>
</ul>
<li><a name='toc7' href='#sect7'>Circumflex and Dollar</a></li>
<li><a name='toc8' href='#sect8'>Full Stop (period, Dot)</a></li>
<li><a name='toc9' href='#sect9'>Matching a Single Byte</a></li>
<li><a name='toc10' href='#sect10'>Square Brackets and Character Classes</a></li>
<li><a name='toc11' href='#sect11'>Posix Character Classes</a></li>
<li><a name='toc12' href='#sect12'>Vertical Bar</a></li>
<li><a name='toc13' href='#sect13'>Internal Option Setting</a></li>
<li><a name='toc14' href='#sect14'>Subpatterns</a></li>
<li><a name='toc15' href='#sect15'>Named Subpatterns</a></li>
<li><a name='toc16' href='#sect16'>Repetition</a></li>
<li><a name='toc17' href='#sect17'>Atomic Grouping and Possessive Quantifiers</a></li>
<li><a name='toc18' href='#sect18'>Back References</a></li>
<li><a name='toc19' href='#sect19'>Assertions</a></li>
<ul>
<li><a name='toc20' href='#sect20'>Lookahead assertions</a></li>
<li><a name='toc21' href='#sect21'>Lookbehind assertions</a></li>
<li><a name='toc22' href='#sect22'>Using multiple assertions</a></li>
</ul>
<li><a name='toc23' href='#sect23'>Conditional Subpatterns</a></li>
<li><a name='toc24' href='#sect24'>Comments</a></li>
<li><a name='toc25' href='#sect25'>Recursive Patterns</a></li>
<li><a name='toc26' href='#sect26'>Subpatterns As Subroutines</a></li>
<li><a name='toc27' href='#sect27'>Callouts</a></li>
</ul>
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