pcrepattern(3)
PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions that are
supported by PCRE are described in detail below. There is a
quick-reference syntax summary in the pcresyntax page. PCRE
tries to match Perl syntax and semantics as closely as it
can. PCRE also supports some alternative regular expression
syntax (which does not conflict with the Perl syntax) in
order to provide some compatibility with regular expressions
in Python, .NET, and Oniguruma. Perl's regular expressions
are described in its own documentation, and regular expres-
sions in general are covered 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 reg-
ular expressions is intended as reference material. This
document discusses the patterns that are supported by PCRE
when one its main matching functions, pcre_exec() (8-bit) or
pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has
alternative matching functions, pcre_dfa_exec() and
pcre[16|32_dfa_exec(), which match using a different algo-
rithm that is not Perl-compatible. Some of the features dis-
cussed below are not available when DFA matching is used.
The advantages and disadvantages of the alternative func-
tions, and how they differ from the normal functions, are
discussed in the pcrematching page.
SPECIAL START-OF-PATTERN ITEMS
A number of options that can be passed to pcre_compile() can
also be set by special items at the start of a pattern.
These are not Perl-compatible, but are provided to make
these options accessible to pattern writers who are not able
to change the program that processes the pattern. Any number
of these items may appear, but they must all be together
right at the start of the pattern string, and the letters
must be in upper case.
UTF support
The original operation of PCRE was on strings of one-byte
characters. However, there is now also support for UTF-8
strings in the original library, an extra library that sup-
ports 16-bit and UTF-16 character strings, and a third
library that supports 32-bit and UTF-32 character strings.
To use these features, PCRE must be built to include
appropriate support. When using UTF strings you must either
call the compiling function with the PCRE_UTF8, PCRE_UTF16,
or PCRE_UTF32 option, or the pattern must start with one of
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these special sequences:
(*UTF8)
(*UTF16)
(*UTF32)
(*UTF)
(*UTF) is a generic sequence that can be used with any of
the libraries. Starting a pattern with such a sequence is
equivalent to setting the relevant option. How setting a UTF
mode affects pattern matching is mentioned in several places
below. There is also a summary of features in the pcreun-
icode page. Some applications that allow their users to
supply patterns may wish to restrict them to non-UTF data
for security reasons. If the PCRE_NEVER_UTF option is set at
compile time, (*UTF) etc. are not allowed, and their appear-
ance causes an error.
Unicode property support
Another special sequence that may appear at the start of a
pattern is (*UCP). This has the same effect as setting the
PCRE_UCP option: it causes sequences such as \d and \w to
use Unicode properties to determine character types, instead
of recognizing only characters with codes less than 128 via
a lookup table.
Disabling auto-possessification
If a pattern starts with (*NO_AUTO_POSSESS), it has the same
effect as setting the PCRE_NO_AUTO_POSSESS option at compile
time. This stops PCRE from making quantifiers possessive
when what follows cannot match the repeated item. For exam-
ple, by default a+b is treated as a++b. For more details,
see the pcreapi documentation.
Disabling start-up optimizations
If a pattern starts with (*NO_START_OPT), it has the same
effect as setting the PCRE_NO_START_OPTIMIZE option either
at compile or matching time. This disables several optimiza-
tions for quickly reaching "no match" results. For more
details, see the pcreapi documentation.
Newline conventions
PCRE supports five different conventions for indicating line
breaks in strings: a single CR (carriage return) character,
a single LF (linefeed) character, the two-character sequence
CRLF, any of the three preceding, or any Unicode newline
sequence. The pcreapi page has further discussion about new-
lines, and shows how to set the newline convention in the
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options arguments for the compiling and matching functions.
It is also possible to specify a newline convention by
starting a pattern string with one of the following five
sequences:
(*CR) carriage return
(*LF) linefeed
(*CRLF) carriage return, followed by linefeed
(*ANYCRLF) any of the three above
(*ANY) all Unicode newline sequences
These override the default and the options given to the com-
piling function. For example, on a Unix system where LF is
the default newline sequence, the pattern
(*CR)a.b
changes the convention to CR. That pattern matches "a\nb"
because LF is no longer a newline. If more than one of these
settings is present, the last one is used. The newline con-
vention affects where the circumflex and dollar assertions
are true. It also affects the interpretation of the dot
metacharacter when PCRE_DOTALL is not set, and the behaviour
of \N. However, it does not affect what the \R escape
sequence matches. By default, this is any Unicode newline
sequence, for Perl compatibility. However, this can be
changed; see the description of \R in the section entitled
"Newline sequences" below. A change of \R setting can be
combined with a change of newline convention.
Setting match and recursion limits
The caller of pcre_exec() can set a limit on the number of
times the internal match() function is called and on the
maximum depth of recursive calls. These facilities are pro-
vided to catch runaway matches that are provoked by patterns
with huge matching trees (a typical example is a pattern
with nested unlimited repeats) and to avoid running out of
system stack by too much recursion. When one of these limits
is reached, pcre_exec() gives an error return. The limits
can also be set by items at the start of the pattern of the
form
(*LIMIT_MATCH=d)
(*LIMIT_RECURSION=d)
where d is any number of decimal digits. However, the value
of the setting must be less than the value set (or
defaulted) by the caller of pcre_exec() for it to have any
effect. In other words, the pattern writer can lower the
limits set by the programmer, but not raise them. If there
is more than one setting of one of these limits, the lower
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value is used.
EBCDIC CHARACTER CODES
PCRE can be compiled to run in an environment that uses
EBCDIC as its character code rather than ASCII or Unicode
(typically a mainframe system). In the sections below, char-
acter code values are ASCII or Unicode; in an EBCDIC
environment these characters may have different code values,
and there are no code points greater than 255.
CHARACTERS AND METACHARACTERS
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. When caseless matching is specified (the
PCRE_CASELESS option), letters are matched independently of
case. In a UTF mode, PCRE always understands the concept of
case for characters whose values are less than 128, so case-
less matching is always possible. For characters with higher
values, the concept of case is supported if PCRE is compiled
with Unicode property support, but not otherwise. If you
want to use caseless matching for characters 128 and above,
you must ensure that PCRE is compiled with Unicode property
support as well as with UTF support. 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 metacharacters, which do not stand for
themselves but instead are interpreted in some special way.
There are two different sets of metacharacters: those that
are recognized anywhere in the pattern except within square
brackets, and those that are recognized within square brack-
ets. Outside square brackets, the metacharacters are as fol-
lows:
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
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* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
Part of a pattern that is in square brackets is called a
"character class". In a character class the only metacharac-
ters are:
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (only if followed by POSIX
syntax)
] terminates the character class
The following sections describe the use of each of the meta-
characters.
BACKSLASH
The backslash character has several uses. Firstly, if it is
followed by a character that is not a number or a letter, it
takes away any special meaning that character may have. This
use of backslash as an escape character applies both inside
and outside character classes. For example, if you want to
match a * character, you write \* in the pattern. This
escaping action applies whether or not the following charac-
ter 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 \\. In a UTF mode,
only ASCII numbers and letters have any special meaning
after a backslash. All other characters (in particular,
those whose codepoints are greater than 127) are treated as
literals. If a pattern is compiled with the PCRE_EXTENDED
option, most white space in the pattern (other than in a
character class), and characters between a # outside a char-
acter class and the next newline, inclusive, are ignored. An
escaping backslash can be used to include a white space or #
character as part of the pattern. If you want to remove the
special meaning from a sequence of characters, you can do so
by putting them between \Q and \E. This is different from
Perl in that $ and @ are handled as literals in \Q...\E
sequences in PCRE, whereas in Perl, $ and @ cause variable
interpolation. Note the following examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the
contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
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\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside
character classes. An isolated \E that is not preceded by
\Q is ignored. If \Q is not followed by \E later in the pat-
tern, the literal interpretation continues to the end of the
pattern (that is, \E is assumed at the end). If the isolated
\Q is inside a character class, this causes an error,
because the character class is not terminated.
Non-printing characters
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
often easier to use one of the following escape sequences
than the binary character it represents. In an ASCII or
Unicode environment, these escapes are as follows:
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any ASCII character
\e escape (hex 1B)
\f form feed (hex 0C)
\n linefeed (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\0dd character with octal code 0dd
\ddd character with octal code ddd, or back reference
\o{ddd..} character with octal code ddd..
\xhh character with hex code hh
\x{hhh..} character with hex code hhh.. (non-JavaScript
mode)
\uhhhh character with hex code hhhh (JavaScript mode
only)
The precise effect of \cx on ASCII characters 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 \cA
to \cZ become hex 01 to hex 1A (A is 41, Z is 5A), but \c{
becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B).
If the data item (byte or 16-bit value) following \c has a
value greater than 127, a compile-time error occurs. This
locks out non-ASCII characters in all modes. When PCRE is
compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t generate
the appropriate EBCDIC code values. The \c escape is pro-
cessed as specified for Perl in the perlebcdic document. The
only characters that are allowed after \c are A-Z, a-z, or
one of @, [, \, ], ^, _, or ?. Any other character provokes
a compile-time error. The sequence \c@ encodes character
code 0; after \c the letters (in either case) encode
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characters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode
characters 27-31 (hex 1B to hex 1F), and \c? becomes either
255 (hex FF) or 95 (hex 5F). Thus, apart from \c?, these
escapes generate the same character code values as they do
in an ASCII environment, though the meanings of the values
mostly differ. For example, \cG always generates code value
7, which is BEL in ASCII but DEL in EBCDIC. The sequence
\c? generates DEL (127, hex 7F) in an ASCII environment, but
because 127 is not a control character in EBCDIC, Perl makes
it generate the APC character. Unfortunately, there are
several variants of EBCDIC. In most of them the APC charac-
ter has the value 255 (hex FF), but in the one Perl calls
POSIX-BC its value is 95 (hex 5F). If certain other charac-
ters have POSIX-BC values, PCRE makes \c? generate 95; oth-
erwise it generates 255. After \0 up to two further octal
digits are read. If there are fewer than two digits, just
those that are present are used. Thus the sequence \0\x\015
specifies two binary zeros followed by a CR character (code
value 13). Make sure you supply two digits after the initial
zero if the pattern character that follows is itself an
octal digit. The escape \o must be followed by a sequence
of octal digits, enclosed in braces. An error occurs if this
is not the case. This escape is a recent addition to Perl;
it provides way of specifying character code points as octal
numbers greater than 0777, and it also allows octal numbers
and back references to be unambiguously specified. For
greater clarity and unambiguity, it is best to avoid follow-
ing \ by a digit greater than zero. Instead, use \o{} or
\x{} to specify character numbers, and \g{} to specify back
references. The following paragraphs describe the old, ambi-
guous syntax. The handling of a backslash followed by a
digit other than 0 is complicated, and Perl has changed in
recent releases, causing PCRE also to change. Outside a
character class, PCRE reads the digit and any following
digits as a decimal number. If the number is less than 8, 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 following \ is greater than 7 and there have not been
that many capturing subpatterns, PCRE handles \8 and \9 as
the literal characters "8" and "9", and otherwise re-reads
up to three octal digits following the backslash, using them
to generate a data character. Any subsequent digits stand
for themselves. For example:
\040 is another way of writing an ASCII 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
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writing a tab
\011 is always a tab
\0113 is a tab followed by the character "3"
\113 might be a back reference, otherwise the
character with octal code 113
\377 might be a back reference, otherwise
the value 255 (decimal)
\81 is either a back reference, or the two
characters "8" and "1"
Note that octal values of 100 or greater that are specified
using this syntax must not be introduced by a leading zero,
because no more than three octal digits are ever read. By
default, after \x that is not followed by {, from zero to
two hexadecimal digits are read (letters can be in upper or
lower case). Any number of hexadecimal digits may appear
between \x{ and }. If a character other than a hexadecimal
digit appears between \x{ and }, or if there is no terminat-
ing }, an error occurs. If the PCRE_JAVASCRIPT_COMPAT
option is set, the interpretation of \x is as just described
only when it is followed by two hexadecimal digits. Other-
wise, it matches a literal "x" character. In JavaScript
mode, support for code points greater than 256 is provided
by \u, which must be followed by four hexadecimal digits;
otherwise it matches a literal "u" character. Characters
whose value is less than 256 can be defined by either of the
two syntaxes for \x (or by \u in JavaScript mode). There is
no difference in the way they are handled. For example, \xdc
is exactly the same as \x{dc} (or \u00dc in JavaScript
mode).
Constraints on character values
Characters that are specified using octal or hexadecimal
numbers are limited to certain values, as follows:
8-bit non-UTF mode less than 0x100
8-bit UTF-8 mode less than 0x10ffff and a valid
codepoint
16-bit non-UTF mode less than 0x10000
16-bit UTF-16 mode less than 0x10ffff and a valid
codepoint
32-bit non-UTF mode less than 0x100000000
32-bit UTF-32 mode less than 0x10ffff and a valid
codepoint
Invalid Unicode codepoints are the range 0xd800 to 0xdfff
(the so-called "surrogate" codepoints), and 0xffef.
Escape sequences in character classes
All the sequences that define a single character value can
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be used both inside and outside character classes. In addi-
tion, inside a character class, \b is interpreted as the
backspace character (hex 08). \N is not allowed in a char-
acter class. \B, \R, and \X are not special inside a charac-
ter class. Like other unrecognized escape sequences, they
are treated as the literal characters "B", "R", and "X" by
default, but cause an error if the PCRE_EXTRA option is set.
Outside a character class, these sequences have different
meanings.
Unsupported escape sequences
In Perl, the sequences \l, \L, \u, and \U are recognized by
its string handler and used to modify the case of following
characters. By default, PCRE does not support these escape
sequences. However, if the PCRE_JAVASCRIPT_COMPAT option is
set, \U matches a "U" character, and \u can be used to
define a character by code point, as described in the previ-
ous section.
Absolute and relative back references
The sequence \g followed by an unsigned or a negative
number, optionally enclosed in braces, is an absolute or
relative back reference. A named back reference can be coded
as \g{name}. Back references are discussed later, following
the discussion of parenthesized subpatterns.
Absolute and relative subroutine calls
For compatibility with Oniguruma, the non-Perl syntax \g
followed by a name or a number enclosed either in angle
brackets or single quotes, is an alternative syntax for
referencing a subpattern as a "subroutine". Details are dis-
cussed later. Note that \g{...} (Perl syntax) and \g<...>
(Oniguruma syntax) are not synonymous. The former is a back
reference; the latter is a subroutine call.
Generic character types
Another use of backslash is for specifying generic character
types:
\d any decimal digit
\D any character that is not a decimal digit
\h any horizontal white space character
\H any character that is not a horizontal white space
character
\s any white space character
\S any character that is not a white space character
\v any vertical white space character
\V any character that is not a vertical white space
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character
\w any "word" character
\W any "non-word" character
There is also the single sequence \N, which matches a non-
newline character. This is the same as the "." metacharac-
ter when PCRE_DOTALL is not set. Perl also uses \N to match
characters by name; PCRE does not support this. Each pair
of lower and upper case escape sequences partitions the com-
plete set of characters into two disjoint sets. Any given
character matches one, and only one, of each pair. The
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, because there is no char-
acter to match. For compatibility with Perl, \s did not
used to match the VT character (code 11), which made it dif-
ferent from the the POSIX "space" class. However, Perl added
VT at release 5.18, and PCRE followed suit at release 8.34.
The default \s characters are now HT (9), LF (10), VT (11),
FF (12), CR (13), and space (32), which are defined as white
space in the "C" locale. This list may vary if locale-
specific matching is taking place. For example, in some
locales the "non-breaking space" character (\xA0) is recog-
nized as white space, and in others the VT character is not.
A "word" character is an underscore or any character that is
a letter or digit. By default, 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 pcreapi page). For exam-
ple, in a French locale such as "fr_FR" in Unix-like sys-
tems, or "french" in Windows, some character codes greater
than 127 are used for accented letters, and these are then
matched by \w. The use of locales with Unicode is
discouraged. By default, characters whose code points are
greater than 127 never match \d, \s, or \w, and always match
\D, \S, and \W, although this may vary for characters in the
range 128-255 when locale-specific matching is happening.
These escape sequences retain their original meanings from
before Unicode support was available, mainly for efficiency
reasons. If PCRE is compiled with Unicode property support,
and the PCRE_UCP option is set, the behaviour is changed so
that Unicode properties are used to determine character
types, as follows:
\d any character that matches \p{Nd} (decimal digit)
\s any character that matches \p{Z} or \h or \v
\w any character that matches \p{L} or \p{N}, plus under-
score
The upper case escapes match the inverse sets of characters.
Note that \d matches only decimal digits, whereas \w matches
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any Unicode digit, as well as any Unicode letter, and under-
score. Note also that PCRE_UCP affects \b, and \B because
they are defined in terms of \w and \W. Matching these
sequences is noticeably slower when PCRE_UCP is set. The
sequences \h, \H, \v, and \V are features that were added to
Perl at release 5.10. In contrast to the other sequences,
which match only ASCII characters by default, these always
match certain high-valued code points, whether or not
PCRE_UCP is set. The horizontal space characters are:
U+0009 Horizontal tab (HT)
U+0020 Space
U+00A0 Non-break space
U+1680 Ogham space mark
U+180E Mongolian vowel separator
U+2000 En quad
U+2001 Em quad
U+2002 En space
U+2003 Em space
U+2004 Three-per-em space
U+2005 Four-per-em space
U+2006 Six-per-em space
U+2007 Figure space
U+2008 Punctuation space
U+2009 Thin space
U+200A Hair space
U+202F Narrow no-break space
U+205F Medium mathematical space
U+3000 Ideographic space
The vertical space characters are:
U+000A Linefeed (LF)
U+000B Vertical tab (VT)
U+000C Form feed (FF)
U+000D Carriage return (CR)
U+0085 Next line (NEL)
U+2028 Line separator
U+2029 Paragraph separator
In 8-bit, non-UTF-8 mode, only the characters with
codepoints less than 256 are relevant.
Newline sequences
Outside a character class, by default, the escape sequence
\R matches any Unicode newline sequence. In 8-bit non-UTF-8
mode \R is equivalent to the following:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details of which
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are given below. This particular group matches either the
two-character sequence CR followed by LF, or one of the sin-
gle characters LF (linefeed, U+000A), VT (vertical tab,
U+000B), FF (form feed, U+000C), CR (carriage return,
U+000D), or NEL (next line, U+0085). The two-character
sequence is treated as a single unit that cannot be split.
In other modes, two additional characters whose codepoints
are greater than 255 are added: LS (line separator, U+2028)
and PS (paragraph separator, U+2029). Unicode character
property support is not needed for these characters to be
recognized. It is possible to restrict \R to match only CR,
LF, or CRLF (instead of the complete set of Unicode line
endings) by setting the option PCRE_BSR_ANYCRLF either at
compile time or when the pattern is matched. (BSR is an
abbrevation for "backslash R".) This can be made the default
when PCRE is built; if this is the case, the other behaviour
can be requested via the PCRE_BSR_UNICODE option. It is
also possible to specify these settings by starting a pat-
tern string with one of the following sequences:
(*BSR_ANYCRLF) CR, LF, or CRLF only
(*BSR_UNICODE) any Unicode newline sequence
These override the default and the options given to the com-
piling function, but they can themselves be overridden by
options given to a matching function. Note that these spe-
cial settings, which are not Perl-compatible, are recognized
only at the very start of a pattern, and that they must be
in upper case. If more than one of them is present, the last
one is used. They can be combined with a change of newline
convention; for example, a pattern can start with:
(*ANY)(*BSR_ANYCRLF)
They can also be combined with the (*UTF8), (*UTF16),
(*UTF32), (*UTF) or (*UCP) special sequences. Inside a char-
acter class, \R is treated as an unrecognized escape
sequence, and so matches the letter "R" by default, but
causes an error if PCRE_EXTRA is set.
Unicode character properties
When PCRE is built with Unicode character property support,
three additional escape sequences that match characters with
specific properties are available. When in 8-bit non-UTF-8
mode, these sequences are of course limited to testing char-
acters whose codepoints are less than 256, but they do work
in this mode. The extra escape sequences are:
\p{xx} a character with the xx property
\P{xx} a character without the xx property
\X a Unicode extended grapheme cluster
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The property names represented by xx above are limited to
the Unicode script names, the general category properties,
"Any", which matches any character (including newline), and
some special PCRE properties (described in the next sec-
tion). Other Perl properties such as "InMusicalSymbols" are
not currently supported by PCRE. Note that \P{Any} does not
match any characters, so always causes a match failure.
Sets of Unicode characters are defined as belonging to cer-
tain scripts. A character from one of these sets can be
matched using a script name. For example:
\p{Greek}
\P{Han}
Those that are not part of an identified script are lumped
together as "Common". The current list of scripts is: Ara-
bic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak,
Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid,
Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma,
Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyril-
lic, Deseret, Devanagari, Duployan, Egyptian_Hieroglyphs,
Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha,
Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,
Hiragana, Imperial_Aramaic, Inherited,
Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese,
Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer,
Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Linear_A,
Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Man-
daic, Manichaean, Meetei_Mayek, Mende_Kikakui,
Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongo-
lian, Mro, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham,
Ol_Chiki, Old_Italic, Old_North_Arabian, Old_Permic,
Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya,
Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician,
Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra,
Sharada, Shavian, Siddham, Sinhala, Sora_Sompeng, Sundanese,
Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham,
Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifi-
nagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi. Each charac-
ter has exactly one Unicode general category property,
specified by a two-letter abbreviation. For compatibility
with Perl, negation can be specified by including a circum-
flex between the opening brace and the property name. For
example, \p{^Lu} is the same as \P{Lu}. If only one letter
is specified with \p or \P, it includes all the general
category 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{L}
\pL
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The following general category property codes are supported:
C Other
Cc Control
Cf Format
Cn Unassigned
Co Private use
Cs Surrogate
L Letter
Ll Lower case letter
Lm Modifier letter
Lo Other letter
Lt Title case letter
Lu Upper case letter
M Mark
Mc Spacing mark
Me Enclosing mark
Mn Non-spacing mark
N Number
Nd Decimal number
Nl Letter number
No Other number
P Punctuation
Pc Connector punctuation
Pd Dash punctuation
Pe Close punctuation
Pf Final punctuation
Pi Initial punctuation
Po Other punctuation
Ps Open punctuation
S Symbol
Sc Currency symbol
Sk Modifier symbol
Sm Mathematical symbol
So Other symbol
Z Separator
Zl Line separator
Zp Paragraph separator
Zs Space separator
The special property L& is also supported: it matches a
character that has the Lu, Ll, or Lt property, in other
words, a letter that is not classified as a modifier or
"other". The Cs (Surrogate) property applies only to char-
acters in the range U+D800 to U+DFFF. Such characters are
not valid in Unicode strings and so cannot be tested by
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
PCRE, unless UTF validity checking has been turned off (see
the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK
and PCRE_NO_UTF32_CHECK in the pcreapi page). Perl does not
support the Cs property. The long synonyms for property
names that Perl supports (such as \p{Letter}) are not sup-
ported by PCRE, nor is it permitted to prefix any of these
properties with "Is". No character that is in the Unicode
table has the Cn (unassigned) property. Instead, this pro-
perty is assumed for any code point that is not in the
Unicode table. Specifying caseless matching does not affect
these escape sequences. For example, \p{Lu} always matches
only upper case letters. This is different from the
behaviour of current versions of Perl. Matching characters
by Unicode property is not fast, because PCRE has to do a
multistage table lookup in order to find a character's pro-
perty. That is why the traditional escape sequences such as
\d and \w do not use Unicode properties in PCRE by default,
though you can make them do so by setting the PCRE_UCP
option or by starting the pattern with (*UCP).
Extended grapheme clusters
The \X escape matches any number of Unicode characters that
form an "extended grapheme cluster", and treats the sequence
as an atomic group (see below). Up to and including release
8.31, PCRE matched an earlier, simpler definition that was
equivalent to
(?>\PM\pM*)
That is, it matched a character without the "mark" property,
followed by zero or more characters with the "mark" pro-
perty. Characters with the "mark" property are typically
non-spacing accents that affect the preceding character.
This simple definition was extended in Unicode to include
more complicated kinds of composite character by giving each
character a grapheme breaking property, and creating rules
that use these properties to define the boundaries of
extended grapheme clusters. In releases of PCRE later than
8.31, \X matches one of these clusters. \X always matches
at least one character. Then it decides whether to add addi-
tional characters according to the following rules for end-
ing a cluster: 1. End at the end of the subject string. 2.
Do not end between CR and LF; otherwise end after any con-
trol character. 3. Do not break Hangul (a Korean script)
syllable sequences. Hangul characters are of five types: L,
V, T, LV, and LVT. An L character may be followed by an L,
V, LV, or LVT character; an LV or V character may be fol-
lowed by a V or T character; an LVT or T character may be
follwed only by a T character. 4. Do not end before extend-
ing characters or spacing marks. Characters with the "mark"
property always have the "extend" grapheme breaking
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property. 5. Do not end after prepend characters. 6. Oth-
erwise, end the cluster.
PCRE's additional properties
As well as the standard Unicode properties described above,
PCRE supports four more that make it possible to convert
traditional escape sequences such as \w and \s to use
Unicode properties. PCRE uses these non-standard, non-Perl
properties internally when PCRE_UCP is set. However, they
may also be used explicitly. These properties are:
Xan Any alphanumeric character
Xps Any POSIX space character
Xsp Any Perl space character
Xwd Any Perl "word" character
Xan matches characters that have either the L (letter) or
the N (number) property. Xps matches the characters tab,
linefeed, vertical tab, form feed, or carriage return, and
any other character that has the Z (separator) property.
Xsp is the same as Xps; it used to exclude vertical tab, for
Perl compatibility, but Perl changed, and so PCRE followed
at release 8.34. Xwd matches the same characters as Xan,
plus underscore. There is another non-standard property,
Xuc, which matches any character that can be represented by
a Universal Character Name in C++ and other programming
languages. These are the characters $, @, ` (grave accent),
and all characters with Unicode code points greater than or
equal to U+00A0, except for the surrogates U+D800 to U+DFFF.
Note that most base (ASCII) characters are excluded.
(Universal Character Names are of the form \uHHHH or
\UHHHHHHHH where H is a hexadecimal digit. Note that the Xuc
property does not match these sequences but the characters
that they represent.)
Resetting the match start
The escape sequence \K causes any previously matched charac-
ters not to be included in the final matched sequence. For
example, the pattern:
foo\Kbar
matches "foobar", but reports that it has matched "bar".
This feature is similar to a lookbehind assertion (described
below). However, in this case, the part of the subject
before the real match does not have to be of fixed length,
as lookbehind assertions do. The use of \K does not inter-
fere with the setting of captured substrings. For example,
when the pattern
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(foo)\Kbar
matches "foobar", the first substring is still set to "foo".
Perl documents that the use of \K within assertions is "not
well defined". In PCRE, \K is acted upon when it occurs
inside positive assertions, but is ignored in negative
assertions. Note that when a pattern such as (?=ab\K)
matches, the reported start of the match can be greater than
the end of the match.
Simple assertions
The final 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 charac-
ters from the subject string. The use of subpatterns for
more complicated assertions is described below. The
backslashed assertions are:
\b matches at a word boundary
\B matches when not at a word boundary
\A matches at the start of the subject
\Z matches at the end of the subject
also matches before a newline at the end of the
subject
\z matches only at the end of the subject
\G matches at the first matching position in the sub-
ject
Inside a character class, \b has a different meaning; it
matches the backspace character. If any other of these
assertions appears in a character class, by default it
matches the corresponding literal character (for example, \B
matches the letter B). However, if the PCRE_EXTRA option is
set, an "invalid escape sequence" error is generated
instead. A word boundary is a position in the subject
string where the current character and the previous charac-
ter 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. In a UTF
mode, the meanings of \w and \W can be changed by setting
the PCRE_UCP option. When this is done, it also affects \b
and \B. Neither PCRE nor Perl has a separate "start of word"
or "end of word" metasequence. However, whatever follows \b
normally determines which it is. For example, the fragment
\ba matches "a" at the start of a word. 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
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behaviour of the circumflex and dollar metacharacters. How-
ever, if the startoffset argument of pcre_exec() is non-
zero, indicating that matching is to start at a point other
than the beginning of the subject, \A can never match. The
difference between \Z and \z is that \Z matches before a
newline at the end of the string as well as at the very end,
whereas \z matches only at the end. The \G assertion is
true only when the current matching position is at the start
point of the match, as specified by the startoffset argument
of pcre_exec(). It differs from \A when the value of star-
toffset is non-zero. By calling pcre_exec() multiple times
with appropriate arguments, you can mimic Perl's /g option,
and it is in this kind of implementation where \G can be
useful. Note, however, that PCRE's interpretation of \G, as
the start of the current match, is subtly different from
Perl's, which defines it as the end of the previous match.
In Perl, these can be different when the previously matched
string was empty. Because PCRE does just one match at a
time, it cannot reproduce this behaviour. If all the alter-
natives of a pattern begin with \G, the expression is
anchored to the starting match position, and the "anchored"
flag is set in the compiled regular expression.
CIRCUMFLEX AND DOLLAR
The circumflex and dollar metacharacters are zero-width
assertions. That is, they test for a particular condition
being true without consuming any characters from the subject
string. 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 startoffset argument of pcre_exec()
is non-zero, circumflex can never match if the
PCRE_MULTILINE option is unset. Inside a character class,
circumflex has an entirely different meaning (see below).
Circumflex need not be the first character of the pattern if
a number of alternatives are involved, but it should be the
first thing in each alternative in which it appears if the
pattern is ever to match that branch. If all possible alter-
natives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is
said to be an "anchored" pattern. (There are also other con-
structs that can cause a pattern to be anchored.) The dol-
lar 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 at the end of the string (by
default). Note, however, that it does not actually match the
newline. Dollar need not be the last character of the pat-
tern 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
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
end of the string, by setting the PCRE_DOLLAR_ENDONLY option
at compile time. This does not affect the \Z assertion. The
meanings of the circumflex and dollar characters are changed
if the PCRE_MULTILINE option is set. When this is the case,
a circumflex matches immediately after internal newlines as
well as at the start of the subject string. It does not
match after a newline that ends the string. A dollar matches
before any newlines in the string, as well as at the very
end, when PCRE_MULTILINE is set. When newline is specified
as the two-character sequence CRLF, isolated CR and LF char-
acters do not indicate newlines. For example, the pattern
/^abc$/ matches the subject string "def\nabc" (where \n
represents a newline) in multiline mode, but not otherwise.
Consequently, patterns that are anchored in single line mode
because all branches start with ^ are not anchored in multi-
line mode, and a match for circumflex is possible when the
startoffset argument of pcre_exec() is non-zero. The
PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
set. Note that the sequences \A, \Z, and \z can be used to
match the start and end of the subject in both modes, and if
all branches of a pattern start with \A it is always
anchored, whether or not PCRE_MULTILINE is set.
FULL STOP (PERIOD, DOT) AND \N
Outside a character class, a dot in the pattern matches any
one character in the subject string except (by default) a
character that signifies the end of a line. When a line
ending is defined as a single character, dot never matches
that character; when the two-character sequence CRLF is
used, dot does not match CR if it is immediately followed by
LF, but otherwise it matches all characters (including iso-
lated CRs and LFs). When any Unicode line endings are being
recognized, dot does not match CR or LF or any of the other
line ending characters. The behaviour of dot with regard to
newlines can be changed. If the PCRE_DOTALL option is set, a
dot matches any one character, without exception. If the
two-character sequence CRLF is present in the subject
string, it takes two dots to match it. The handling of dot
is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve
newlines. Dot has no special meaning in a character class.
The escape sequence \N behaves like a dot, except that it is
not affected by the PCRE_DOTALL option. In other words, it
matches any character except one that signifies the end of a
line. Perl also uses \N to match characters by name; PCRE
does not support this.
MATCHING A SINGLE DATA UNIT
Outside a character class, the escape sequence \C matches
any one data unit, whether or not a UTF mode is set. In the
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8-bit library, one data unit is one byte; in the 16-bit
library it is a 16-bit unit; in the 32-bit library it is a
32-bit unit. Unlike a dot, \C always matches line-ending
characters. The feature is provided in Perl in order to
match individual bytes in UTF-8 mode, but it is unclear how
it can usefully be used. Because \C breaks up characters
into individual data units, matching one unit with \C in a
UTF mode means that the rest of the string may start with a
malformed UTF character. This has undefined results, because
PCRE assumes that it is dealing with valid UTF strings (and
by default it checks this at the start of processing unless
the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
PCRE_NO_UTF32_CHECK option is used). PCRE does not allow \C
to appear in lookbehind assertions (described below) in a
UTF mode, because this would make it impossible to calculate
the length of the lookbehind. In general, the \C escape
sequence is best avoided. However, one way of using it that
avoids the problem of malformed UTF characters is to use a
lookahead to check the length of the next character, as in
this pattern, which could be used with a UTF-8 string
(ignore white space and line breaks):
(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
A group that starts with (?| resets the capturing
parentheses numbers in each alternative (see "Duplicate Sub-
pattern Numbers" below). The assertions at the start of each
branch check the next UTF-8 character for values whose
encoding uses 1, 2, 3, or 4 bytes, respectively. The
character's individual bytes are then captured by the
appropriate number of groups.
SQUARE BRACKETS AND CHARACTER CLASSES
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 by default. However, if
the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing
square bracket causes a compile-time error. If a closing
square bracket is required as a member of the class, it
should be the first data character in the class (after an
initial circumflex, if present) or escaped with a backslash.
A character class matches a single character in the subject.
In a UTF mode, the character may be more than one data unit
long. 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,
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
ensure it is not the first character, or escape it with a
backslash. For example, the character class [aeiou] matches
any lower case vowel, while [^aeiou] matches any character
that is not a lower case vowel. Note that a circumflex is
just a convenient notation for specifying the characters
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. In UTF-8 (UTF-16, UTF-32) mode, characters with
values greater than 255 (0xffff) can be included in a class
as a literal string of data units, or by using the \x{
escaping mechanism. When caseless matching is set, any
letters in a class represent both their upper case and lower
case versions, so for example, a caseless [aeiou] matches
"A" as well as "a", and a caseless [^aeiou] does not match
"A", whereas a caseful version would. In a UTF mode, PCRE
always understands the concept of case for characters whose
values are less than 128, so caseless matching is always
possible. For characters with higher values, the concept of
case is supported if PCRE is compiled with Unicode property
support, but not otherwise. If you want to use caseless
matching in a UTF mode for characters 128 and above, you
must ensure that PCRE is compiled with Unicode property sup-
port as well as with UTF support. Characters that might
indicate line breaks are never treated in any special way
when matching character classes, whatever line-ending
sequence is in use, and whatever setting of the PCRE_DOTALL
and PCRE_MULTILINE options is used. A class such as [^a]
always matches one of these characters. 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, or immediately after a range. For example, [b-d-z]
matches letters in the range b to d, a hyphen character, or
z. 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 class containing a range followed by two other
characters. The octal or hexadecimal representation of "]"
can also be used to end a range. An error is generated if a
POSIX character class (see below) or an escape sequence
other than one that defines a single character appears at a
point where a range ending character is expected. For exam-
ple, [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are
not. Ranges operate in the collating sequence of character
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
values. They can also be used for characters specified
numerically, for example [\000-\037]. Ranges can include any
characters that are valid for the current mode. 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 a
non-UTF mode, if character tables for a French locale are in
use, [\xc8-\xcb] matches accented E characters in both
cases. In UTF modes, PCRE supports the concept of case for
characters with values greater than 128 only when it is com-
piled with Unicode property support. The character escape
sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W
may appear in a character class, and add the characters that
they match to the class. For example, [\dABCDEF] matches any
hexadecimal digit. In UTF modes, the PCRE_UCP option affects
the meanings of \d, \s, \w and their upper case partners,
just as it does when they appear outside a character class,
as described in the section entitled "Generic character
types" above. The escape sequence \b has a different meaning
inside a character class; it matches the backspace charac-
ter. The sequences \B, \N, \R, and \X are not special inside
a character class. Like any other unrecognized escape
sequences, they are treated as the literal characters "B",
"N", "R", and "X" by default, but cause an error if the
PCRE_EXTRA option is set. 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, whereas [\w] includes underscore.
A positive character class should be read as "something OR
something OR ..." and a negative class as "NOT something AND
NOT something AND NOT ...". 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, or for a special compatibility feature - see the next
two sections), and the terminating closing square bracket.
However, escaping other non-alphanumeric characters does no
harm.
POSIX CHARACTER CLASSES
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,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The sup-
ported class names are:
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
alnum letters and digits
alpha letters
ascii character codes 0 - 127
blank space or tab only
cntrl control characters
digit decimal digits (same as \d)
graph printing characters, excluding space
lower lower case letters
print printing characters, including space
punct printing characters, excluding letters and digits
and space
space white space (the same as \s from PCRE 8.34)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
The default "space" characters are HT (9), LF (10), VT (11),
FF (12), CR (13), and space (32). If locale-specific match-
ing is taking place, the list of space characters may be
different; there may be fewer or more of them. "Space" used
to be different to \s, which did not include VT, for Perl
compatibility. However, Perl changed at release 5.18, and
PCRE followed at release 8.34. "Space" and \s now match the
same set of characters. The name "word" is a Perl exten-
sion, and "blank" is a GNU extension from Perl 5.8. Another
Perl extension is negation, which is indicated by a ^ char-
acter after the colon. For example,
[12[:^digit:]]
matches "1", "2", or any non-digit. PCRE (and Perl) also
recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a
"collating element", but these are not supported, and an
error is given if they are encountered. By default, charac-
ters with values greater than 128 do not match any of the
POSIX character classes. However, if the PCRE_UCP option is
passed to pcre_compile(), some of the classes are changed so
that Unicode character properties are used. This is achieved
by replacing certain POSIX classes by other sequences, as
follows:
[:alnum:] becomes \p{Xan}
[:alpha:] becomes \p{L}
[:blank:] becomes \h
[:digit:] becomes \p{Nd}
[:lower:] becomes \p{Ll}
[:space:] becomes \p{Xps}
[:upper:] becomes \p{Lu}
[:word:] becomes \p{Xwd}
Negated versions, such as [:^alpha:] use \P instead of \p.
Three other POSIX classes are handled specially in UCP mode:
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
[:graph:] This matches characters that have glyphs that mark
the page when printed. In Unicode property terms,
it matches all characters with the L, M, N, P, S,
or Cf properties, except for:
U+061C Arabic Letter Mark
U+180E Mongolian Vowel Separator
U+2066 - U+2069 Various "isolate"s
[:print:] This matches the same characters as [:graph:] plus
space characters that are not controls, that is,
characters with the Zs property.
[:punct:] This matches all characters that have the Unicode
P (punctuation) property, plus those characters
whose code points are less than 128 that have the
S (Symbol) property. The other POSIX classes are
unchanged, and match only characters with code
points less than 128.
COMPATIBILITY FEATURE FOR WORD BOUNDARIES
In the POSIX.2 compliant library that was included in 4.4BSD
Unix, the ugly syntax [[:<:]] and [[:>:]] is used for match-
ing "start of word" and "end of word". PCRE treats these
items as follows:
[[:<:]] is converted to \b(?=\w)
[[:>:]] is converted to \b(?<=\w)
Only these exact character sequences are recognized. A
sequence such as [a[:<:]b] provokes error for an unrecog-
nized POSIX class name. This support is not compatible with
Perl. It is provided to help migrations from other environ-
ments, and is best not used in any new patterns. Note that
\b matches at the start and the end of a word (see "Simple
assertions" above), and in a Perl-style pattern the preced-
ing or following character normally shows which is wanted,
without the need for the assertions that are used above in
order to give exactly the POSIX behaviour.
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
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alternative in turn, from left to right, and the first one
that succeeds is used. If the alternatives are within a sub-
pattern (defined below), "succeeds" means matching the rest
of the main pattern as well as the alternative in the sub-
pattern.
INTERNAL OPTION SETTING
The settings of the PCRE_CASELESS, PCRE_MULTILINE,
PCRE_DOTALL, and PCRE_EXTENDED options (which are Perl-
compatible) 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 PCRE-specific options PCRE_DUPNAMES,
PCRE_UNGREEDY, and PCRE_EXTRA can be changed in the same way
as the Perl-compatible options by using the characters J, U
and X respectively. When one of these option changes occurs
at top level (that is, not inside subpattern parentheses),
the change applies to the remainder of the pattern that fol-
lows. An option change within a subpattern (see below for a
description of subpatterns) 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. Note: There are other PCRE-specific options that
can be set by the application when the compiling or matching
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functions are called. In some cases the pattern can contain
special leading sequences such as (*CRLF) to override what
the application has set or what has been defaulted. Details
are given in the section entitled "Newline sequences" above.
There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP)
leading sequences that can be used to set UTF and Unicode
property modes; they are equivalent to setting the
PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP options,
respectively. The (*UTF) sequence is a generic version that
can be used with any of the libraries. However, the applica-
tion can set the PCRE_NEVER_UTF option, which locks out the
use of the (*UTF) sequences.
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets),
which can be nested. Turning part of a pattern into a sub-
pattern does two things:
1. It localizes a set of alternatives. For example, the pat-
tern
cat(aract|erpillar|)
matches "cataract", "caterpillar", or "cat". Without the
parentheses, it would match "cataract", "erpillar" or an
empty string.
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 ovector argument of the matching
function. (This applies only to the traditional matching
functions; the DFA matching functions do not support captur-
ing.) Opening parentheses are counted from left to right
(starting from 1) to obtain numbers for the capturing sub-
patterns. For example, if the string "the red king" is
matched against the pattern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king",
and are numbered 1, 2, and 3, respectively. The fact that
plain parentheses fulfil two functions is not always help-
ful. 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 sub-
patterns. For example, if the string "the white queen" is
matched against the pattern
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the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and
are numbered 1 and 2. The maximum number of capturing sub-
patterns is 65535. As a convenient shorthand, if any option
settings are required at the start of a non-capturing sub-
pattern, 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".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a
subpattern uses the same numbers for its capturing
parentheses. Such a subpattern starts with (?| and is itself
a non-capturing subpattern. For example, consider this pat-
tern:
(?|(Sat)ur|(Sun))day
Because the two alternatives are inside a (?| group, both
sets of capturing parentheses are numbered one. Thus, when
the pattern matches, you can look at captured substring
number one, whichever alternative matched. This construct is
useful when you want to capture part, but not all, of one of
a number of alternatives. Inside a (?| group, parentheses
are numbered as usual, but the number is reset at the start
of each branch. The numbers of any capturing parentheses
that follow the subpattern start after the highest number
used in any branch. The following example is taken from the
Perl documentation. The numbers underneath show in which
buffer the captured content will be stored.
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
A back reference to a numbered subpattern uses the most
recent value that is set for that number by any subpattern.
The following pattern matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
In contrast, a subroutine call to a numbered subpattern
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always refers to the first one in the pattern with the given
number. The following pattern matches "abcabc" or "defabc":
/(?|(abc)|(def))(?1)/
If a condition test for a subpattern's having matched refers
to a non-unique number, the test is true if any of the sub-
patterns of that number have matched. An alternative
approach to using this "branch reset" feature is to use
duplicate named subpatterns, as described in the next sec-
tion.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but
it can be very hard to keep track of the numbers in compli-
cated regular expressions. Furthermore, if an expression is
modified, the numbers may change. To help with this diffi-
culty, PCRE supports the naming of subpatterns. This feature
was not added to Perl until release 5.10. Python had the
feature earlier, and PCRE introduced it at release 4.0,
using the Python syntax. PCRE now supports both the Perl and
the Python syntax. Perl allows identically numbered subpat-
terns to have different names, but PCRE does not. In PCRE,
a subpattern can be named in one of three ways: (?<name>...)
or (?'name'...) as in Perl, or (?P<name>...) as in Python.
References to capturing parentheses from other parts of the
pattern, such as back references, recursion, and conditions,
can be made by name as well as by number. Names consist of
up to 32 alphanumeric characters and underscores, but must
start with a non-digit. Named capturing parentheses are
still allocated numbers as well as names, exactly as if the
names were not present. 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. By default, a name
must be unique within a pattern, but it is possible to relax
this constraint by setting the PCRE_DUPNAMES option at com-
pile time. (Duplicate names are also always permitted for
subpatterns with the same number, set up as described in the
previous section.) Duplicate names can be useful for pat-
terns where only one instance of the named parentheses can
match. Suppose you want to match the name of a weekday,
either as a 3-letter abbreviation or as the full name, and
in both cases you want to extract the abbreviation. This
pattern (ignoring the line breaks) does the job:
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
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There are five capturing substrings, but only one is ever
set after a match. (An alternative way of solving this
problem is to use a "branch reset" subpattern, as described
in the previous section.) The convenience function for
extracting the data by name returns the substring for the
first (and in this example, the only) subpattern of that
name that matched. This saves searching to find which num-
bered subpattern it was. If you make a back reference to a
non-unique named subpattern from elsewhere in the pattern,
the subpatterns to which the name refers are checked in the
order in which they appear in the overall pattern. The first
one that is set is used for the reference. For example, this
pattern matches both "foofoo" and "barbar" but not "foobar"
or "barfoo":
(?:(?<n>foo)|(?<n>bar))\k<n>
If you make a subroutine call to a non-unique named subpat-
tern, the one that corresponds to the first occurrence of
the name is used. In the absence of duplicate numbers (see
the previous section) this is the one with the lowest
number. If you use a named reference in a condition test
(see the section about conditions below), either to check
whether a subpattern has matched, or to check for recursion,
all subpatterns with the same name are tested. If the condi-
tion is true for any one of them, the overall condition is
true. This is the same behaviour as testing by number. For
further details of the interfaces for handling named subpat-
terns, see the pcreapi documentation. Warning: You cannot
use different names to distinguish between two subpatterns
with the same number because PCRE uses only the numbers when
matching. For this reason, an error is given at compile time
if different names are given to subpatterns with the same
number. However, you can always give the same name to sub-
patterns with the same number, even when PCRE_DUPNAMES is
not set.
REPETITION
Repetition is specified by quantifiers, which can follow any
of the following items:
a literal data character
the dot metacharacter
the \C escape sequence
the \X escape sequence
the \R escape sequence
an escape such as \d or \pL that matches a single charac-
ter
a character class
a back reference (see next section)
a parenthesized subpattern (including assertions)
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a subroutine call to a subpattern (recursive or otherwise)
The general repetition quantifier specifies a minimum and
maximum number of permitted matches, by giving the two
numbers in curly brackets (braces), separated by a comma.
The numbers must be less than 65536, and the first must be
less than or equal to the second. For example:
z{2,4}
matches "zz", "zzz", or "zzzz". A closing brace on its own
is not a special character. If the second number is omitted,
but the comma is present, there is no upper limit; if the
second number and the comma are both omitted, the quantifier
specifies an exact number of required matches. Thus
[aeiou]{3,}
matches at least 3 successive vowels, but may match many
more, while
\d{8}
matches exactly 8 digits. An opening curly bracket that
appears in a position where a quantifier is not allowed, or
one that does not match the syntax of a quantifier, is taken
as a literal character. For example, {,6} is not a quantif-
ier, but a literal string of four characters. In UTF modes,
quantifiers apply to characters rather than to individual
data units. Thus, for example, \x{100}{2} matches two char-
acters, each of which is represented by a two-byte sequence
in a UTF-8 string. Similarly, \X{3} matches three Unicode
extended grapheme clusters, each of which may be several
data units long (and they may be of different lengths). The
quantifier {0} is permitted, causing the expression to
behave as if the previous item and the quantifier were not
present. This may be useful for subpatterns that are refer-
enced as subroutines from elsewhere in the pattern (but see
also the section entitled "Defining subpatterns for use by
reference only" below). Items other than subpatterns that
have a {0} quantifier are omitted from the compiled pattern.
For convenience, 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:
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(a?)*
Earlier versions of Perl and PCRE used to give an error at
compile time for such patterns. However, because there are
cases where this can be useful, such patterns are now
accepted, but if any repetition of the subpattern does in
fact match no characters, the loop is forcibly broken. By
default, the quantifiers are "greedy", that is, they match
as much as possible (up to the maximum number of permitted
times), without causing the rest of the pattern to fail. The
classic example of where this gives problems is in trying to
match comments in C programs. These appear between /* and */
and within the comment, individual * and / characters may
appear. An attempt to match C comments by applying the pat-
tern
/\*.*\*/
to the string
/* first comment */ not comment /* second comment */
fails, because it matches the entire string owing to the
greediness of the .* item. However, if a quantifier is
followed by a question mark, it ceases to be greedy, and
instead matches the minimum number of times possible, so the
pattern
/\*.*?\*/
does the right thing with the C comments. The meaning of the
various quantifiers is not otherwise changed, just the pre-
ferred number of matches. Do not confuse this use of ques-
tion mark with its use as a quantifier in its own right.
Because it has two uses, it can sometimes appear doubled, as
in
\d??\d
which matches one digit by preference, but can match two if
that is the only way the rest of the pattern matches. If
the PCRE_UNGREEDY option is set (an option that is not
available in Perl), the quantifiers are not greedy by
default, but individual ones can be made greedy by following
them with a question mark. In other words, it inverts the
default behaviour. When a parenthesized subpattern is quan-
tified with a minimum repeat count that is greater than 1 or
with a limited maximum, more memory is required for the com-
piled 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 dot to match newlines, the pattern is
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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 posi-
tion after the first. PCRE normally treats such a pattern as
though it were preceded by \A. In cases where it is known
that the subject string contains no newlines, it is worth
setting PCRE_DOTALL in order to obtain this optimization, or
alternatively using ^ to indicate anchoring explicitly.
However, there are some cases where the optimization cannot
be used. When .* is inside capturing parentheses that are
the subject of a back reference elsewhere in the pattern, a
match at the start may fail where a later one succeeds. Con-
sider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the
fourth character. For this reason, such a pattern is not
implicitly anchored. Another case where implicit anchoring
is not applied is when the leading .* is inside an atomic
group. Once again, a match at the start may fail where a
later one succeeds. Consider this pattern:
(?>.*?a)b
It matches "ab" in the subject "aab". The use of the back-
tracking control verbs (*PRUNE) and (*SKIP) also disable
this optimization. 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".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing ("greedy") and minimizing ("ungreedy"
or "lazy") repetition, failure of what follows normally
causes the repeated item to be re-evaluated to see if a dif-
ferent number of repeats allows the rest of the pattern to
match. Sometimes it is useful to prevent this, either to
change the nature of the match, or to cause it fail earlier
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than it otherwise might, when the author of the pattern
knows there is no point in carrying on. Consider, for exam-
ple, the pattern \d+foo when applied to the subject line
123456bar
After matching all 6 digits and then failing to match "foo",
the normal action of the matcher is to try again with only 5
digits matching the \d+ item, and then with 4, and so on,
before ultimately failing. "Atomic grouping" (a term taken
from Jeffrey Friedl's book) provides the means for specify-
ing that once a subpattern has matched, it is not to be re-
evaluated in this way. If we use atomic grouping for the
previous example, the matcher gives up immediately on fail-
ing to match "foo" the first time. The notation is a kind of
special parenthesis, starting with (?> as in this example:
(?>\d+)foo
This kind of parenthesis "locks up" the part of the pattern
it 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
standalone pattern would match, if anchored at the current
point in the subject string. Atomic grouping subpatterns
are not capturing subpatterns. Simple cases such as the
above example can be thought of as a maximizing repeat that
must swallow everything it can. So, while both \d+ and \d+?
are prepared to adjust the number of digits they match in
order to make the rest of the pattern match, (?>\d+) can
only match an entire sequence of digits. Atomic groups in
general can of course contain arbitrarily complicated sub-
patterns, and can be nested. However, when the subpattern
for an atomic group is just a single repeated item, as in
the example above, a simpler notation, called a "possessive
quantifier" can be used. This consists of an additional +
character following a quantifier. Using this notation, the
previous example can be rewritten as
\d++foo
Note that a possessive quantifier can be used with an entire
group, for example:
(abc|xyz){2,3}+
Possessive quantifiers are always greedy; the setting of the
PCRE_UNGREEDY option is ignored. They are a convenient nota-
tion for the simpler forms of atomic group. However, there
is no difference in the meaning of a possessive quantifier
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and the equivalent atomic group, though there may be a per-
formance difference; possessive quantifiers should be
slightly faster. The possessive quantifier syntax is an
extension to the Perl 5.8 syntax. Jeffrey Friedl originated
the idea (and the name) in the first edition of his book.
Mike McCloskey liked it, so implemented it when he built
Sun's Java package, and PCRE copied it from there. It ulti-
mately found its way into Perl at release 5.10. PCRE has an
optimization that automatically "possessifies" certain sim-
ple pattern constructs. For example, the sequence A+B is
treated as A++B because there is no point in backtracking
into a sequence of A's when B must follow. When a pattern
contains an unlimited repeat inside a subpattern that can
itself be repeated an unlimited number of times, the use of
an atomic group is the only way to avoid some failing
matches taking a very long time indeed. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either con-
sist of non-digits, or digits enclosed in <>, followed by
either ! or ?. When it matches, it runs quickly. However, if
it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is
because the string can be divided between the 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:
((?>\D+)|<\d+>)*[!?]
sequences of non-digits cannot be broken, and failure hap-
pens quickly.
BACK REFERENCES
Outside a character class, a backslash followed by a digit
greater than 0 (and possibly further digits) is a back
reference to a capturing subpattern earlier (that is, to its
left) in the pattern, provided there have been that many
previous capturing left parentheses. However, if the
decimal number following the backslash is less than 10, it
is always taken as a back reference, and causes an error
only if there are not that many capturing left parentheses
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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. A "forward back reference" of this
type can make sense when a repetition is involved and the
subpattern to the right has participated in an earlier
iteration. It is not possible to have a numerical "forward
back reference" to a subpattern whose number is 10 or more
using this syntax because a sequence such as \50 is inter-
preted as a character defined in octal. See the subsection
entitled "Non-printing characters" above for further details
of the handling of digits following a backslash. There is no
such problem when named parentheses are used. A back refer-
ence to any subpattern is possible using named parentheses
(see below). Another way of avoiding the ambiguity inherent
in the use of digits following a backslash is to use the \g
escape sequence. This escape must be followed by an unsigned
number or a negative number, optionally enclosed in braces.
These examples are all identical:
(ring), \1
(ring), \g1
(ring), \g{1}
An unsigned number specifies an absolute reference without
the ambiguity that is present in the older syntax. It is
also useful when literal digits follow the reference. A
negative number is a relative reference. Consider this exam-
ple:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the most recently
started capturing subpattern before \g, that is, is it
equivalent to \2 in this example. Similarly, \g{-2} would
be equivalent to \1. The use of relative references can be
helpful in long patterns, and also in patterns that are
created by joining together fragments that contain refer-
ences within themselves. 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
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsi-
bility", but not "sense and responsibility". If caseful
matching is in force at the time of the back reference, the
case of letters is relevant. For example,
((?i)rah)\s+\1
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matches "rah rah" and "RAH RAH", but not "RAH rah", even
though the original capturing subpattern is matched case-
lessly. There are several different ways of writing back
references to named subpatterns. The .NET syntax \k{name}
and the Perl syntax \k<name> or \k'name' are supported, as
is the Python syntax (?P=name). Perl 5.10's unified back
reference syntax, in which \g can be used for both numeric
and named references, is also supported. We could rewrite
the above example in any of the following ways:
(?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
A subpattern that is referenced by name may appear in the
pattern before or after the reference. There may be more
than one back reference to the same subpattern. If a subpat-
tern has not actually been used in a particular match, any
back references to it always fail by default. For example,
the pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc".
However, if the PCRE_JAVASCRIPT_COMPAT option is set at com-
pile time, a back reference to an unset value matches an
empty string. Because there may be many capturing
parentheses in a pattern, all digits following a 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 white space. Other-
wise, the \g{ syntax or an empty comment (see "Comments"
below) can be used.
Recursive back references
A back reference that occurs inside the parentheses to which
it refers fails when the subpattern is first used, so, for
example, (a\1) never matches. However, such references can
be useful inside repeated subpatterns. For example, the pat-
tern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At
each iteration of the subpattern, the back reference matches
the character string corresponding to the previous itera-
tion. In order for this to work, the pattern must be such
that the first iteration does not need to match the back
reference. This can be done using alternation, as in the
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example above, or by a quantifier with a minimum of zero.
Back references of this type cause the group that they
reference to be treated as an atomic group. Once the whole
group has been matched, a subsequent matching failure cannot
cause backtracking into the middle of the group.
ASSERTIONS
An assertion is a test on the characters following or
preceding the current matching point that does not actually
consume any characters. The simple assertions coded as \b,
\B, \A, \G, \Z, \z, ^ and $ are described above. More com-
plicated assertions are coded as subpatterns. There are two
kinds: those that look ahead of the current position in the
subject string, and those that look behind it. An assertion
subpattern is matched in the normal way, except that it does
not cause the current matching position to be changed.
Assertion subpatterns are not capturing subpatterns. If such
an assertion contains capturing subpatterns within it, these
are counted for the purposes of numbering the capturing sub-
patterns in the whole pattern. However, substring capturing
is carried out only for positive assertions. (Perl some-
times, but not always, does do capturing in negative asser-
tions.) WARNING: If a positive assertion containing one or
more capturing subpatterns succeeds, but failure to match
later in the pattern causes backtracking over this asser-
tion, the captures within the assertion are reset only if no
higher numbered captures are already set. This is, unfor-
tunately, a fundamental limitation of the current implemen-
tation, and as PCRE1 is now in maintenance-only status, it
is unlikely ever to change. For compatibility with Perl,
assertion subpatterns may be repeated; though it makes no
sense to assert the same thing several times, the side
effect of capturing parentheses may occasionally be useful.
In practice, there only three cases:
(1) If the quantifier is {0}, the assertion is never obeyed
during matching. However, it may contain internal capturing
parenthesized groups that are called from elsewhere via the
subroutine mechanism.
(2) If quantifier is {0,n} where n is greater than zero, it
is treated as if it were {0,1}. At run time, the rest of the
pattern match is tried with and without the assertion, the
order depending on the greediness of the quantifier.
(3) If the minimum repetition is greater than zero, the
quantifier is ignored. The assertion is obeyed just once
when encountered during matching.
Lookahead assertions
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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 the other effect. If you
want to force a matching failure at some point in a pattern,
the most convenient way to do it is with (?!) because an
empty string always matches, so an assertion that requires
there not to be an empty string must always fail. The back-
tracking control verb (*FAIL) or (*F) is a synonym for (?!).
Lookbehind assertions
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 top-level alterna-
tives, they do not all have to have the same fixed length.
Thus
(?<=bullock|donkey)
is permitted, but
(?<!dogs?|cats?)
causes an error at compile time. Branches that match dif-
ferent length strings are permitted only at the top level of
a lookbehind assertion. This is an extension compared with
Perl, which requires all branches to match the same length
of string. An assertion such as
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(?<=ab(c|de))
is not permitted, because its single top-level branch can
match two different lengths, but it is acceptable to PCRE if
rewritten to use two top-level branches:
(?<=abc|abde)
In some cases, the escape sequence \K (see above) can be
used instead of a lookbehind assertion to get round the
fixed-length restriction. The implementation of lookbehind
assertions is, for each alternative, to temporarily move the
current position back by the fixed length and then try to
match. If there are insufficient characters before the
current position, the assertion fails. In a UTF mode, PCRE
does not allow the \C escape (which matches a single data
unit even in a UTF mode) to appear in lookbehind assertions,
because it makes it impossible to calculate the length of
the lookbehind. The \X and \R escapes, which can match dif-
ferent numbers of data units, are also not permitted. "Sub-
routine" calls (see below) such as (?2) or (?&X) are permit-
ted in lookbehinds, as long as the subpattern matches a
fixed-length string. Recursion, however, is not supported.
Possessive quantifiers can be used in conjunction with look-
behind assertions to specify efficient matching of fixed-
length strings at the end of subject strings. Consider a
simple pattern such as
abcd$
when applied to a long string that does not match. Because
matching proceeds from left to right, PCRE will look for
each "a" in the subject and then see if what follows matches
the rest of the pattern. If the pattern is specified as
^.*abcd$
the initial .* matches the entire string at first, but when
this fails (because there is no following "a"), it back-
tracks to match all but the last character, then all but the
last two characters, and so on. Once again the search for
"a" covers the entire string, from right to left, so we are
no better off. However, if the pattern is written as
^.*+(?<=abcd)
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.
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
Using multiple assertions
Several assertions (of any sort) may occur in succession.
For example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999".
Notice that each of the assertions is applied independently
at the same point in the subject string. First there is a
check that the previous three characters are all digits, and
then there is a check that the same three characters are not
"999". This pattern does not match "foo" preceded by six
characters, the first of which are digits and the last three
of which are not "999". For example, it doesn't match
"123abcfoo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six
characters, checking that the first three are digits, and
then the second assertion checks that the preceding three
characters are not "999". Assertions can be nested in any
combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar"
which in turn is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three
digits and any three characters that are not "999".
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 specific capturing subpattern has already been
matched. 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. Each of the two alternatives may itself con-
tain nested subpatterns of any form, including conditional
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subpatterns; the restriction to two alternatives applies
only at the level of the condition. This pattern fragment is
an example where the alternatives are complex:
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
There are four kinds of condition: references to subpat-
terns, references to recursion, a pseudo-condition called
DEFINE, and assertions.
Checking for a used subpattern by number
If the text between the parentheses consists of a sequence
of digits, the condition is true if a capturing subpattern
of that number has previously matched. If there is more than
one capturing subpattern with the same number (see the ear-
lier section about duplicate subpattern numbers), the condi-
tion is true if any of them have matched. An alternative
notation is to precede the digits with a plus or minus sign.
In this case, the subpattern number is relative rather than
absolute. The most recently opened parentheses can be refer-
enced by (?(-1), the next most recent by (?(-2), and so on.
Inside loops it can also make sense to refer to subsequent
groups. The next parentheses to be opened can be referenced
as (?(+1), and so on. (The value zero in any of these forms
is not used; it provokes a compile-time error.) 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 dis-
cussion:
( \( )? [^()]+ (?(1) \) )
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 or not the first set of
parentheses matched. 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 you were embedding this pattern in a
larger one, you could use a relative reference:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
This makes the fragment independent of the parentheses in
the larger pattern.
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Checking for a used subpattern by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to
test for a used subpattern by name. For compatibility with
earlier versions of PCRE, which had this facility before
Perl, the syntax (?(name)...) is also recognized. Rewriting
the above example to use a named subpattern gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate,
the test is applied to all subpatterns of the same name, and
is true if any one of them has matched.
Checking for pattern recursion
If the condition is the string (R), and there is no subpat-
tern with the name R, the condition is true if a recursive
call to the whole pattern or any subpattern has been made.
If digits or a name preceded by ampersand follow the letter
R, for example:
(?(R3)...) or (?(R&name)...)
the condition is true if the most recent recursion is into a
subpattern whose number or name is given. This condition
does not check the entire recursion stack. If the name used
in a condition of this kind is a duplicate, the test is
applied to all subpatterns of the same name, and is true if
any one of them is the most recent recursion. At "top
level", all these recursion test conditions are false. The
syntax for recursive patterns is described below.
Defining subpatterns for use by reference only
If the condition is the string (DEFINE), and there is no
subpattern with the name DEFINE, the condition is always
false. In this case, there may be only one alternative in
the subpattern. It is always skipped if control reaches this
point in the pattern; the idea of DEFINE is that it can be
used to define subroutines that can be referenced from else-
where. (The use of subroutines is described below.) For
example, a pattern to match an IPv4 address such as
"192.168.23.245" could be written like this (ignore white
space and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d)
)
\b (?&byte) (\.(?&byte)){3} \b
The first part of the pattern is a DEFINE group inside which
a another group named "byte" is defined. This matches an
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individual component of an IPv4 address (a number less than
256). When matching takes place, this part of the pattern is
skipped because DEFINE acts like a false condition. The rest
of the pattern uses references to the named group to match
the four dot-separated components of an IPv4 address,
insisting on a word boundary at each end.
Assertion conditions
If the condition is not in any of the above formats, it must
be an assertion. This may be a positive or negative looka-
head or lookbehind assertion. Consider this pattern, again
containing non-significant white space, and with the two
alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches
an optional sequence of non-letters followed by a letter. In
other words, it tests for the presence of at least one
letter in the subject. If a letter is found, the subject is
matched against the first alternative; otherwise it is
matched against the second. This pattern matches strings in
one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
letters and dd are digits.
COMMENTS
There are two ways of including comments in patterns that
are processed by PCRE. In both cases, the start of the com-
ment must not be in a character class, nor in the middle of
any other sequence of related characters such as (?: or a
subpattern name or number. The characters that make up a
comment play no part in the pattern matching. The sequence
(?# marks the start of a comment that continues up to the
next closing parenthesis. Nested parentheses are not permit-
ted. If the PCRE_EXTENDED option is set, an unescaped #
character also introduces a comment, which in this case con-
tinues to immediately after the next newline character or
character sequence in the pattern. Which characters are
interpreted as newlines is controlled by the options passed
to a compiling function or by a special sequence at the
start of the pattern, as described in the section entitled
"Newline conventions" above. Note that the end of this type
of comment is a literal newline sequence in the pattern;
escape sequences that happen to represent a newline do not
count. For example, consider this pattern when PCRE_EXTENDED
is set, and the default newline convention is in force:
abc #comment \n still comment
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On encountering the # character, pcre_compile() skips along,
looking for a newline in the pattern. The sequence \n is
still literal at this stage, so it does not terminate the
comment. Only an actual character with the code value 0x0a
(the default newline) does so.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses,
allowing for unlimited nested parentheses. Without the use
of recursion, the best that can be done is to use a pattern
that matches up to some fixed depth of nesting. It is not
possible to handle an arbitrary nesting depth. For some
time, Perl has provided 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 pat-
tern using code interpolation to solve the parentheses prob-
lem can be created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and
in this case refers recursively to the pattern in which it
appears. Obviously, PCRE cannot support the interpolation
of Perl code. Instead, it supports special syntax for recur-
sion of the entire pattern, and also for individual subpat-
tern recursion. After its introduction in PCRE and Python,
this kind of recursion was subsequently introduced into Perl
at release 5.10. A special item that consists of (? fol-
lowed by a number greater than zero and a closing
parenthesis is a recursive subroutine call of the subpattern
of the given number, provided that it occurs inside that
subpattern. (If not, it is a non-recursive subroutine call,
which is described in the next section.) The special item
(?R) or (?0) is a recursive call of the entire regular
expression. This PCRE pattern solves the nested parentheses
problem (assume the PCRE_EXTENDED option is set so that
white space is ignored):
\( ( [^()]++ | (?R) )* \)
First it matches an opening parenthesis. Then it matches any
number of substrings which can either be a sequence of non-
parentheses, or a recursive match of the pattern itself
(that is, a correctly parenthesized substring). Finally
there is a closing parenthesis. Note the use of a possessive
quantifier to avoid backtracking into sequences of non-
parentheses. If this were part of a larger pattern, you
would not want to recurse the entire pattern, so instead you
could use this:
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
( \( ( [^()]++ | (?1) )* \) )
We have put the pattern into parentheses, and caused the
recursion to refer to them instead of the whole pattern. In
a larger pattern, keeping track of parenthesis numbers can
be tricky. This is made easier by the use of relative refer-
ences. Instead of (?1) in the pattern above you can write
(?-2) to refer to the second most recently opened
parentheses preceding the recursion. In other words, a nega-
tive number counts capturing parentheses leftwards from the
point at which it is encountered. It is also possible to
refer to subsequently opened parentheses, by writing refer-
ences such as (?+2). However, these cannot be recursive
because the reference is not inside the parentheses that are
referenced. They are always non-recursive subroutine calls,
as described in the next section. An alternative approach
is to use named parentheses instead. The Perl syntax for
this is (?&name); PCRE's earlier syntax (?P>name) is also
supported. We could rewrite the above example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
If there is more than one subpattern with the same name, the
earliest one is used. This particular example pattern that
we have been looking at contains nested unlimited repeats,
and so the use of a possessive quantifier for matching
strings of non-parentheses is important when applying the
pattern to strings that do not match. For example, when this
pattern is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if a possessive quan-
tifier is not used, the match runs for a very long time
indeed because there are so many different ways the + and *
repeats can carve up the subject, and all have to be tested
before failure can be reported. At the end of a match, the
values of capturing parentheses are those from the outermost
level. If you want to obtain intermediate values, a callout
function can be used (see below and the pcrecallout documen-
tation). If the pattern above is matched against
(ab(cd)ef)
the value for the inner capturing parentheses (numbered 2)
is "ef", which is the last value taken on at the top level.
If a capturing subpattern is not matched at the top level,
its final captured value is unset, even if it was (tem-
porarily) set at a deeper level during the matching process.
If there are more than 15 capturing parentheses in a pat-
tern, PCRE has to obtain extra memory to store data during a
recursion, which it does by using pcre_malloc, freeing it
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
via pcre_free afterwards. If no memory can be obtained, the
match fails with the PCRE_ERROR_NOMEMORY error. Do not con-
fuse 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.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpat-
tern, with two different alternatives for the recursive and
non-recursive cases. The (?R) item is the actual recursive
call.
Differences in recursion processing between PCRE and Perl
Recursion processing in PCRE differs from Perl in two impor-
tant ways. In PCRE (like Python, but unlike Perl), a recur-
sive subpattern call is always treated as an atomic group.
That is, once it has matched some of the subject string, it
is never re-entered, even if it contains untried alterna-
tives and there is a subsequent matching failure. This can
be illustrated by the following pattern, which purports to
match a palindromic string that contains an odd number of
characters (for example, "a", "aba", "abcba", "abcdcba"):
^(.|(.)(?1)\2)$
The idea is that it either matches a single character, or
two identical characters surrounding a sub-palindrome. In
Perl, this pattern works; in PCRE it does not if the pattern
is longer than three characters. Consider the subject string
"abcba": At the top level, the first character is matched,
but as it is not at the end of the string, the first alter-
native fails; the second alternative is taken and the recur-
sion kicks in. The recursive call to subpattern 1 success-
fully matches the next character ("b"). (Note that the
beginning and end of line tests are not part of the recur-
sion). Back at the top level, the next character ("c") is
compared with what subpattern 2 matched, which was "a". This
fails. Because the recursion is treated as an atomic group,
there are now no backtracking points, and so the entire
match fails. (Perl is able, at this point, to re-enter the
recursion and try the second alternative.) However, if the
pattern is written with the alternatives in the other order,
things are different:
^((.)(?1)\2|.)$
This time, the recursing alternative is tried first, and
continues to recurse until it runs out of characters, at
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
which point the recursion fails. But this time we do have
another alternative to try at the higher level. That is the
big difference: in the previous case the remaining alterna-
tive is at a deeper recursion level, which PCRE cannot use.
To change the pattern so that it matches all palindromic
strings, not just those with an odd number of characters, it
is tempting to change the pattern to this:
^((.)(?1)\2|.?)$
Again, this works in Perl, but not in PCRE, and for the same
reason. When a deeper recursion has matched a single charac-
ter, it cannot be entered again in order to match an empty
string. The solution is to separate the two cases, and write
out the odd and even cases as alternatives at the higher
level:
^(?:((.)(?1)\2|)|((.)(?3)\4|.))
If you want to match typical palindromic phrases, the pat-
tern has to ignore all non-word characters, which can be
done like this:
^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
If run with the PCRE_CASELESS option, this pattern matches
phrases such as "A man, a plan, a canal: Panama!" and it
works well in both PCRE and Perl. Note the use of the pos-
sessive quantifier *+ to avoid backtracking into sequences
of non-word characters. Without this, PCRE takes a great
deal longer (ten times or more) to match typical phrases,
and Perl takes so long that you think it has gone into a
loop. WARNING: The palindrome-matching patterns above work
only if the subject string does not start with a palindrome
that is shorter than the entire string. For example,
although "abcba" is correctly matched, if the subject is
"ababa", PCRE finds the palindrome "aba" at the start, then
fails at top level because the end of the string does not
follow. Once again, it cannot jump back into the recursion
to try other alternatives, so the entire match fails. The
second way in which PCRE and Perl differ in their recursion
processing is in the handling of captured values. In Perl,
when a subpattern is called recursively or as a subpattern
(see the next section), it has no access to any values that
were captured outside the recursion, whereas in PCRE these
values can be referenced. Consider this pattern:
^(.)(\1|a(?2))
In PCRE, this pattern matches "bab". The first capturing
parentheses match "b", then in the second group, when the
back reference \1 fails to match "b", the second alternative
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
matches "a" and then recurses. In the recursion, \1 does now
match "b" and so the whole match succeeds. In Perl, the pat-
tern fails to match because inside the recursive call \1
cannot access the externally set value.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern call (either by
number or by name) is used outside the parentheses to which
it refers, it operates like a subroutine in a programming
language. The called subpattern may be defined before or
after the reference. A numbered reference can be absolute or
relative, as in these examples:
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
An earlier example pointed out that the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsi-
bility", but not "sense and responsibility". If instead the
pattern
(sens|respons)e and (?1)ibility
is used, it does match "sense and responsibility" as well as
the other two strings. Another example is given in the dis-
cussion of DEFINE above. All subroutine calls, whether
recursive or not, are always treated as atomic groups. That
is, once a subroutine has matched some of the subject
string, it is never re-entered, even if it contains untried
alternatives and there is a subsequent matching failure. Any
capturing parentheses that are set during the subroutine
call revert to their previous values afterwards. Processing
options such as case-independence are fixed when a subpat-
tern is defined, so if it is used as a subroutine, such
options cannot be changed for different calls. For example,
consider this pattern:
(abc)(?i:(?-1))
It matches "abcabc". It does not match "abcABC" because the
change of processing option does not affect the called sub-
pattern.
ONIGURUMA SUBROUTINE SYNTAX
For compatibility with Oniguruma, the non-Perl syntax \g
followed by a name or a number enclosed either in angle
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brackets or single quotes, is an alternative syntax for
referencing a subpattern as a subroutine, possibly recur-
sively. Here are two of the examples used above, rewritten
using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
PCRE supports an extension to Oniguruma: if a number is pre-
ceded by a plus or a minus sign it is taken as a relative
reference. For example:
(abc)(?i:\g<-1>)
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syn-
tax) are not synonymous. The former is a back reference; the
latter is a subroutine call.
CALLOUTS
Perl has a feature whereby using the sequence (?{...})
causes arbitrary Perl code to be obeyed in the middle of
matching a regular expression. This makes it possible,
amongst other things, to extract different substrings that
match the same pair of parentheses when there is a repeti-
tion. PCRE provides a similar feature, but of course it
cannot obey arbitrary Perl code. The feature is called "cal-
lout". The caller of PCRE provides an external function by
putting its entry point in the global variable pcre_callout
(8-bit library) or pcre[16|32]_callout (16-bit or 32-bit
library). By default, this variable contains NULL, which
disables all calling out. Within a regular expression, (?C)
indicates the points at which the external function is to be
called. If you want to identify different callout points,
you can put a number less than 256 after the letter C. The
default value is zero. For example, this pattern has two
callout points:
(?C1)abc(?C2)def
If the PCRE_AUTO_CALLOUT flag is passed to a compiling func-
tion, callouts are automatically installed before each item
in the pattern. They are all numbered 255. If there is a
conditional group in the pattern whose condition is an
assertion, an additional callout is inserted just before the
condition. An explicit callout may also be set at this posi-
tion, as in this example:
(?(?C9)(?=a)abc|def)
Note that this applies only to assertion conditions, not to
other types of condition. During matching, when PCRE
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PCREPATTERN(3) C LIBRARY FUNCTIONS PCREPATTERN(3)
reaches a callout point, 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 the matching function. The callout
function may cause matching to proceed, to backtrack, or to
fail altogether. By default, PCRE implements a number of
optimizations at compile time and matching time, and one
side-effect is that sometimes callouts are skipped. If you
need all possible callouts to happen, you need to set
options that disable the relevant optimizations. More
details, and a complete description of the interface to the
callout function, are given in the pcrecallout documenta-
tion.
BACKTRACKING CONTROL
Perl 5.10 introduced a number of "Special Backtracking Con-
trol Verbs", which are still described in the Perl documen-
tation as "experimental and subject to change or removal in
a future version of Perl". It goes on to say: "Their usage
in production code should be noted to avoid problems during
upgrades." The same remarks apply to the PCRE features
described in this section. The new verbs make use of what
was previously invalid syntax: an opening parenthesis fol-
lowed by an asterisk. They are generally of the form (*VERB)
or (*VERB:NAME). Some may take either form, possibly behav-
ing differently depending on whether or not a name is
present. A name is any sequence of characters that does not
include a closing parenthesis. The maximum length of name is
255 in the 8-bit library and 65535 in the 16-bit and 32-bit
libraries. If the name is empty, that is, if the closing
parenthesis immediately follows the colon, the effect is as
if the colon were not there. Any number of these verbs may
occur in a pattern. Since these verbs are specifically
related to backtracking, most of them can be used only when
the pattern is to be matched using one of the traditional
matching functions, because these use a backtracking algo-
rithm. With the exception of (*FAIL), which behaves like a
failing negative assertion, the backtracking control verbs
cause an error if encountered by a DFA matching function.
The behaviour of these verbs in repeated groups, assertions,
and in subpatterns called as subroutines (whether or not
recursively) is documented below.
Optimizations that affect backtracking verbs
PCRE contains some optimizations that are used to speed up
matching by running some checks at the start of each match
attempt. For example, it may know the minimum length of
matching subject, or that a particular character must be
present. When one of these optimizations bypasses the run-
ning of a match, any included backtracking verbs will not,
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of course, be processed. You can suppress the start-of-match
optimizations by setting the PCRE_NO_START_OPTIMIZE option
when calling pcre_compile() or pcre_exec(), or by starting
the pattern with (*NO_START_OPT). There is more discussion
of this option in the section entitled "Option bits for
pcre_exec()" in the pcreapi documentation. Experiments with
Perl suggest that it too has similar optimizations, some-
times leading to anomalous results.
Verbs that act immediately
The following verbs act as soon as they are encountered.
They may not be followed by a name.
(*ACCEPT)
This verb causes the match to end successfully, skipping the
remainder of the pattern. However, when it is inside a sub-
pattern that is called as a subroutine, only that subpattern
is ended successfully. Matching then continues at the outer
level. If (*ACCEPT) in triggered in a positive assertion,
the assertion succeeds; in a negative assertion, the asser-
tion fails. If (*ACCEPT) is inside capturing parentheses,
the data so far is captured. For example:
A((?:A|B(*ACCEPT)|C)D)
This matches "AB", "AAD", or "ACD"; when it matches "AB",
"B" is captured by the outer parentheses.
(*FAIL) or (*F)
This verb causes a matching failure, forcing backtracking to
occur. It is equivalent to (?!) but easier to read. The Perl
documentation notes that it is probably useful only when
combined with (?{}) or (??{}). Those are, of course, Perl
features that are not present in PCRE. The nearest
equivalent is the callout feature, as for example in this
pattern:
a+(?C)(*FAIL)
A match with the string "aaaa" always fails, but the callout
is taken before each backtrack happens (in this example, 10
times).
Recording which path was taken
There is one verb whose main purpose is to track how a match
was arrived at, though it also has a secondary use in con-
junction with advancing the match starting point (see
(*SKIP) below).
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(*MARK:NAME) or (*:NAME)
A name is always required with this verb. There may be as
many instances of (*MARK) as you like in a pattern, and
their names do not have to be unique. When a match
succeeds, the name of the last-encountered (*MARK:NAME),
(*PRUNE:NAME), or (*THEN:NAME) on the matching path is
passed back to the caller as described in the section enti-
tled "Extra data for pcre_exec()" in the pcreapi documenta-
tion. Here is an example of pcretest output, where the /K
modifier requests the retrieval and outputting of (*MARK)
data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XY
0: XY
MK: A
XZ
0: XZ
MK: B
The (*MARK) name is tagged with "MK:" in this output, and in
this example it indicates which of the two alternatives
matched. This is a more efficient way of obtaining this
information than putting each alternative in its own captur-
ing parentheses. If a verb with a name is encountered in a
positive assertion that is true, the name is recorded and
passed back if it is the last-encountered. This does not
happen for negative assertions or failing positive asser-
tions. After a partial match or a failed match, the last
encountered name in the entire match process is returned.
For example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XP
No match, mark = B
Note that in this unanchored example the mark is retained
from the match attempt that started at the letter "X" in the
subject. Subsequent match attempts starting at "P" and then
with an empty string do not get as far as the (*MARK) item,
but nevertheless do not reset it. If you are interested in
(*MARK) values after failed matches, you should probably set
the PCRE_NO_START_OPTIMIZE option (see above) to ensure that
the match is always attempted.
Verbs that act after backtracking
The following verbs do nothing when they are encountered.
Matching continues with what follows, but if there is no
subsequent match, causing a backtrack to the verb, a failure
is forced. That is, backtracking cannot pass to the left of
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the verb. However, when one of these verbs appears inside an
atomic group or an assertion that is true, its effect is
confined to that group, because once the group has been
matched, there is never any backtracking into it. In this
situation, backtracking can "jump back" to the left of the
entire atomic group or assertion. (Remember also, as stated
above, that this localization also applies in subroutine
calls.) These verbs differ in exactly what kind of failure
occurs when backtracking reaches them. The behaviour
described below is what happens when the verb is not in a
subroutine or an assertion. Subsequent sections cover these
special cases.
(*COMMIT)
This verb, which may not be followed by a name, causes the
whole match to fail outright if there is a later matching
failure that causes backtracking to reach it. Even if the
pattern is unanchored, no further attempts to find a match
by advancing the starting point take place. If (*COMMIT) is
the only backtracking verb that is encountered, once it has
been passed pcre_exec() is committed to finding a match at
the current starting point, or not at all. For example:
a+(*COMMIT)b
This matches "xxaab" but not "aacaab". It can be thought of
as a kind of dynamic anchor, or "I've started, so I must
finish." The name of the most recently passed (*MARK) in the
path is passed back when (*COMMIT) forces a match failure.
If there is more than one backtracking verb in a pattern, a
different one that follows (*COMMIT) may be triggered first,
so merely passing (*COMMIT) during a match does not always
guarantee that a match must be at this starting point. Note
that (*COMMIT) at the start of a pattern is not the same as
an anchor, unless PCRE's start-of-match optimizations are
turned off, as shown in this output from pcretest:
re> /(*COMMIT)abc/
data> xyzabc
0: abc
data> xyzabc\Y
No match
For this pattern, PCRE knows that any match must start with
"a", so the optimization skips along the subject to "a"
before applying the pattern to the first set of data. The
match attempt then succeeds. In the second set of data, the
escape sequence \Y is interpreted by the pcretest program.
It causes the PCRE_NO_START_OPTIMIZE option to be set when
pcre_exec() is called. This disables the optimization that
skips along to the first character. The pattern is now
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applied starting at "x", and so the (*COMMIT) causes the
match to fail without trying any other starting points.
(*PRUNE) or (*PRUNE:NAME)
This verb causes the match to fail at the current starting
position in the subject if there is a later matching failure
that causes backtracking to reach it. If the pattern is
unanchored, the normal "bumpalong" advance to the next
starting character then happens. Backtracking can occur as
usual to the left of (*PRUNE), before it is reached, or when
matching to the right of (*PRUNE), but if there is no match
to the right, backtracking cannot cross (*PRUNE). In simple
cases, the use of (*PRUNE) is just an alternative to an
atomic group or possessive quantifier, but there are some
uses of (*PRUNE) that cannot be expressed in any other way.
In an anchored pattern (*PRUNE) has the same effect as
(*COMMIT). The behaviour of (*PRUNE:NAME) is the not the
same as (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in
that the name is remembered for passing back to the caller.
However, (*SKIP:NAME) searches only for names set with
(*MARK).
(*SKIP)
This verb, when given without a name, is like (*PRUNE),
except that if the pattern is unanchored, the "bumpalong"
advance is not to the next character, but to the position in
the subject where (*SKIP) was encountered. (*SKIP) signifies
that whatever text was matched leading up to it cannot be
part of a successful match. Consider:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt
fails (starting at the first character in the string), the
starting point skips on to start the next attempt at "c".
Note that a possessive quantifer does not have the same
effect as this example; although it would suppress back-
tracking during the first match attempt, the second attempt
would start at the second character instead of skipping on
to "c".
(*SKIP:NAME)
When (*SKIP) has an associated name, its behaviour is modi-
fied. When it is triggered, the previous path through the
pattern is searched for the most recent (*MARK) that has the
same name. If one is found, the "bumpalong" advance is to
the subject position that corresponds to that (*MARK)
instead of to where (*SKIP) was encountered. If no (*MARK)
with a matching name is found, the (*SKIP) is ignored. Note
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that (*SKIP:NAME) searches only for names set by
(*MARK:NAME). It ignores names that are set by (*PRUNE:NAME)
or (*THEN:NAME).
(*THEN) or (*THEN:NAME)
This verb causes a skip to the next innermost alternative
when backtracking reaches it. That is, it cancels any
further backtracking within the current alternative. Its
name comes from the observation that it can be used for a
pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN)
BAZ ) ...
If the COND1 pattern matches, FOO is tried (and possibly
further items after the end of the group if FOO succeeds);
on failure, the matcher skips to the second alternative and
tries COND2, without backtracking into COND1. If that
succeeds and BAR fails, COND3 is tried. If subsequently BAZ
fails, there are no more alternatives, so there is a back-
track to whatever came before the entire group. If (*THEN)
is not inside an alternation, it acts like (*PRUNE). The
behaviour of (*THEN:NAME) is the not the same as
(*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the
name is remembered for passing back to the caller. However,
(*SKIP:NAME) searches only for names set with (*MARK). A
subpattern that does not contain a | character is just a
part of the enclosing alternative; it is not a nested alter-
nation with only one alternative. The effect of (*THEN)
extends beyond such a subpattern to the enclosing alterna-
tive. Consider this pattern, where A, B, etc. are complex
pattern fragments that do not contain any | characters at
this level:
A (B(*THEN)C) | D
If A and B are matched, but there is a failure in C, match-
ing does not backtrack into A; instead it moves to the next
alternative, that is, D. However, if the subpattern con-
taining (*THEN) is given an alternative, it behaves dif-
ferently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner subpat-
tern. After a failure in C, matching moves to (*FAIL), which
causes the whole subpattern to fail because there are no
more alternatives to try. In this case, matching does now
backtrack into A. Note that a conditional subpattern is not
considered as having two alternatives, because only one is
ever used. In other words, the | character in a conditional
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subpattern has a different meaning. Ignoring white space,
consider:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. Because
.*? is ungreedy, it initially matches zero characters. The
condition (?=a) then fails, the character "b" is matched,
but "c" is not. At this point, matching does not backtrack
to .*? as might perhaps be expected from the presence of the
| character. The conditional subpattern is part of the sin-
gle alternative that comprises the whole pattern, and so the
match fails. (If there was a backtrack into .*?, allowing it
to match "b", the match would succeed.) The verbs just
described provide four different "strengths" of control when
subsequent matching fails. (*THEN) is the weakest, carrying
on the match at the next alternative. (*PRUNE) comes next,
failing the match at the current starting position, but
allowing an advance to the next character (for an unanchored
pattern). (*SKIP) is similar, except that the advance may be
more than one character. (*COMMIT) is the strongest, causing
the entire match to fail.
More than one backtracking verb
If more than one backtracking verb is present in a pattern,
the one that is backtracked onto first acts. For example,
consider this pattern, where A, B, etc. are complex pattern
fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes
the entire match to fail. However, if A and B match, but C
fails, the backtrack to (*THEN) causes the next alternative
(ABD) to be tried. This behaviour is consistent, but is not
always the same as Perl's. It means that if two or more
backtracking verbs appear in succession, all the the last of
them has no effect. Consider this example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking
onto (*PRUNE) causes it to be triggered, and its action is
taken. There can never be a backtrack onto (*COMMIT).
Backtracking verbs in repeated groups
PCRE differs from Perl in its handling of backtracking verbs
in repeated groups. For example, consider:
/(a(*COMMIT)b)+ac/
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If the subject is "abac", Perl matches, but PCRE fails
because the (*COMMIT) in the second repeat of the group
acts.
Backtracking verbs in assertions
(*FAIL) in an assertion has its normal effect: it forces an
immediate backtrack. (*ACCEPT) in a positive assertion
causes the assertion to succeed without any further process-
ing. In a negative assertion, (*ACCEPT) causes the assertion
to fail without any further processing. The other back-
tracking verbs are not treated specially if they appear in a
positive assertion. In particular, (*THEN) skips to the next
alternative in the innermost enclosing group that has alter-
nations, whether or not this is within the assertion. Nega-
tive assertions are, however, different, in order to ensure
that changing a positive assertion into a negative assertion
changes its result. Backtracking into (*COMMIT), (*SKIP), or
(*PRUNE) causes a negative assertion to be true, without
considering any further alternative branches in the asser-
tion. Backtracking into (*THEN) causes it to skip to the
next enclosing alternative within the assertion (the normal
behaviour), but if the assertion does not have such an
alternative, (*THEN) behaves like (*PRUNE).
Backtracking verbs in subroutines
These behaviours occur whether or not the subpattern is
called recursively. Perl's treatment of subroutines is dif-
ferent in some cases. (*FAIL) in a subpattern called as a
subroutine has its normal effect: it forces an immediate
backtrack. (*ACCEPT) in a subpattern called as a subroutine
causes the subroutine match to succeed without any further
processing. Matching then continues after the subroutine
call. (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern
called as a subroutine cause the subroutine match to fail.
(*THEN) skips to the next alternative in the innermost
enclosing group within the subpattern that has alternatives.
If there is no such group within the subpattern, (*THEN)
causes the subroutine match to fail.
SEE ALSO
pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3),
pcre(3), pcre16(3), pcre32(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
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REVISION
Last updated: 23 October 2016
Copyright (c) 1997-2016 University of Cambridge.
PCRE 8.40 Last change: 23 October 2016 58
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