PCRE - Perl-compatible regular expressions
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
expressions 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 regular 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 algorithm that is not Perl-compatible. Some of
the features discussed below are not available when DFA matching is used. The
advantages and disadvantages of the alternative functions, and how they differ
from the normal functions, are discussed in the
pcrematching page.
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.
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 supports 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 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
pcreunicode 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 appearance causes an
error.
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.
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
example, by default a+b is treated as a++b. For more details, see the
pcreapi documentation.
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 optimizations for quickly reaching "no match"
results. For more details, see the
pcreapi documentation.
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
newlines, and shows how to set the newline convention in the
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 compiling 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 convention 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.
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 provided 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 value is used.
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, character 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.
A regular expression is a pattern that is matched against a subject string from
left to right. Most characters stand for themselves in a pattern, and match
the corresponding characters in the subject. As a trivial example, the pattern
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 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 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 brackets. Outside square brackets, the metacharacters
are as follows:
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
Part of a pattern that is in square brackets is called a "character
class". In a character class the only metacharacters 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 metacharacters.
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 character would
otherwise be interpreted as a metacharacter, so it is always safe to precede a
non-alphanumeric with backslash to specify that it stands for itself. In
particular, if you want to match a backslash, you write \\.
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 character 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 $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$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 pattern, 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.
A second use of backslash provides a way of encoding non-printing characters in
patterns in a visible manner. There is no restriction on the appearance of
non-printing characters, apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is 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 processed 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 \@ encodes character code 0; the
letters (in either case) encode characters 1-26 (hex 01 to hex 1A); [, \, ],
^, and _ encode characters 27-31 (hex 1B to hex 1F), and \? becomes either 255
(hex FF) or 95 (hex 5F).
Thus, apart from \?, 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, \G always generates code value 7, which is BEL in ASCII
but DEL in EBCDIC.
The sequence \? 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 character has the value 255 (hex FF), but in the one Perl calls
POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC
values, PCRE makes \? generate 95; otherwise 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 following \ 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,
ambiguous 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
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 terminating },
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. Otherwise, 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).
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.
All the sequences that define a single character value can be used both inside
and outside character classes. In addition, inside a character class, \b is
interpreted as the backspace character (hex 08).
\N is not allowed in a character class. \B, \R, and \X are not special inside a
character 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.
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 previous section.
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.
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
discussed 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.
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 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 "." metacharacter 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 complete 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 character to match.
For compatibility with Perl, \s did not used to match the VT character (code
11), which made it different 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 recognized 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 example, in a French locale such as "fr_FR" in Unix-like
systems, 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 underscore
The upper case escapes match the inverse sets of characters. Note that \d
matches only decimal digits, whereas \w matches any Unicode digit, as well as
any Unicode letter, and underscore. 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.
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 are given
below. This particular group matches either the two-character sequence CR
followed by LF, or one of the single 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 pattern 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 compiling function, but
they can themselves be overridden by options given to a matching function.
Note that these special 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 character 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.
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
characters 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
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 section). 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 certain 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:
Arabic, 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, Cyrillic, 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, Mandaic,
Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive,
Meroitic_Hieroglyphs, Miao, Modi, Mongolian, 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, Tifinagh, Tirhuta,
Ugaritic, Vai, Warang_Citi, Yi.
Each character has exactly one Unicode general category property, specified by a
two-letter abbreviation. For compatibility with Perl, negation can be
specified by including a circumflex between the opening brace and the property
name. For example, \p{^Lu} is the same as \P{Lu}.
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
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 characters in the range U+D800 to
U+DFFF. Such characters are not valid in Unicode strings and so cannot be
tested by 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 supported 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 property 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 property. 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).
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" property. 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
additional characters according to the following rules for ending a cluster:
1. End at the end of the subject string.
2. Do not end between CR and LF; otherwise end after any control 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 followed by a V or T
character; an LVT or T character may be follwed only by a T character.
4. Do not end before extending characters or spacing marks. Characters with the
"mark" property always have the "extend" grapheme breaking
property.
5. Do not end after prepend characters.
6. Otherwise, end the cluster.
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.)
The escape sequence \K causes any previously matched characters 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
interfere with the setting of captured substrings. For example, when the
pattern
(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.
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 characters from the subject string. The use of
subpatterns for more complicated assertions is described below. The
backslashed assertions are:
\b matches at a word boundary
\B matches when not at a word boundary
\A matches at 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 subject
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 character do not both match \w or \W (i.e. one matches \w and
the other matches \W), or the start or end of the string if the first or last
character matches \w, respectively. 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 behaviour of the
circumflex and dollar metacharacters. However, 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
startoffset is
non-zero. By calling
pcre_exec() multiple times with appropriate
arguments, you can mimic Perl's /g option, and it is in this kind of
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 alternatives of a pattern begin with \G, the expression is anchored
to the starting match position, and the "anchored" flag is set in
the compiled regular expression.
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 alternatives start with a circumflex, that is, if the pattern
is constrained to match only at the start of the subject, it is said to be an
"anchored" pattern. (There are also other constructs that can cause
a pattern to be anchored.)
The dollar character is an assertion that is true only if the current matching
point is at the end of the subject string, or immediately before a newline 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 pattern if a
number of alternatives are involved, but it should be the last item in any
branch in which it appears. Dollar has no special meaning in a character
class.
The meaning of dollar can be changed so that it matches only at the very end of
the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
does not affect the \Z assertion.
The meanings of the circumflex and dollar characters are changed if the
PCRE_MULTILINE option is set. When this is the case, 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 characters 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 multiline mode, and a match for
circumflex is possible when the
startoffset argument of
pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
PCRE_MULTILINE is set.
Note that the sequences \A, \Z, and \z can be used to match the start and end of
the subject in both modes, and if all branches of a pattern start with \A it
is always anchored, whether or not PCRE_MULTILINE is set.
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 isolated 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.
Outside a character class, the escape sequence \C matches any one data unit,
whether or not a UTF mode is set. In the 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 Subpattern 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.
An opening square bracket introduces a character class, terminated 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, 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 support 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 "-") followed by a literal string
"46]", so it would match "W46]" or "-46]".
However, if the "]" is escaped with a backslash it is interpreted as
the end of range, so [W-\]46] is interpreted as a class containing a range
followed by two other characters. The octal or hexadecimal representation of
"]" can also be used to end a range.
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 example, [z-\xff] is valid,
but [A-\d] and [A-[:digit:]] are not.
Ranges operate in the collating sequence of character 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 compiled 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 character. 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.
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 supported class names are:
alnum letters and digits
alpha letters
ascii character codes 0 - 127
blank space or tab only
cntrl control characters
digit decimal digits (same as \d)
graph printing characters, excluding space
lower lower case letters
print printing characters, including space
punct printing characters, excluding letters and digits 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 matching 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 extension, and "blank" is a GNU
extension from Perl 5.8. Another Perl extension is negation, which is
indicated by a ^ character after the colon. For example,
[12[:^digit:]]
matches "1", "2", or any non-digit. PCRE (and Perl) also
recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a
"collating element", but these are not supported, and an error is
given if they are encountered.
By default, characters 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:
- [: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.
In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
syntax [[:<:]] and [[:>:]] is used for matching "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 unrecognized POSIX class name. This support
is not compatible with Perl. It is provided to help migrations from other
environments, 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 preceding 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 characters are used to separate alternative patterns. For example,
the pattern
gilbert|sullivan
matches either "gilbert" or "sullivan". Any number of
alternatives may appear, and an empty alternative is permitted (matching the
empty string). The matching process tries each alternative in turn, from left
to right, and the first one that succeeds is used. If the alternatives are
within a subpattern (defined below), "succeeds" means matching the
rest of the main pattern as well as the alternative in the subpattern.
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 follows. If the change is placed right at the start of a pattern, PCRE
extracts it into the global options (and it will therefore show up in data
extracted by the
pcre_fullinfo() function).
An option change within a subpattern (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 otherwise.
Note: There are other PCRE-specific options that can be set by the
application when the compiling or matching 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 application can set the PCRE_NEVER_UTF option, which
locks out the use of the (*UTF) sequences.
Subpatterns are delimited by parentheses (round brackets), which can be nested.
Turning part of a pattern into a subpattern does two things:
1. It localizes a set of alternatives. For example, the pattern
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 capturing.)
Opening parentheses are counted from left to right (starting from 1) to obtain
numbers for the capturing subpatterns. For example, if the string "the
red king" is matched against the pattern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and
"king", and are numbered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not always helpful.
There are often times when a grouping subpattern is required without a
capturing requirement. If an opening parenthesis is followed by a question
mark and a colon, the subpattern does not do any capturing, and is not counted
when computing the number of any subsequent capturing subpatterns. For
example, if the string "the white queen" is matched against the
pattern
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and
are numbered 1 and 2. The maximum number of capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the start of a
non-capturing subpattern, the option letters may appear between the
"?" and the ":". Thus the two patterns
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative branches are tried
from left to right, and options are not reset until the end of the subpattern
is reached, an option setting in one branch does affect subsequent branches,
so the above patterns match "SUNDAY" as well as
"Saturday".
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 pattern:
(?|(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 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 subpatterns 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 section.
Identifying capturing parentheses by number is simple, but it can be very hard
to keep track of the numbers in complicated regular expressions. Furthermore,
if an expression is modified, the numbers may change. To help with this
difficulty, PCRE supports the naming of subpatterns. 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 subpatterns
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 compile 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 patterns 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)?
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 numbered 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 subpattern, 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
condition 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 subpatterns, 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 subpatterns with the same number, even when
PCRE_DUPNAMES is not set.
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 character
a character class
a back reference (see next section)
a parenthesized subpattern (including assertions)
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
quantifier, 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 characters, 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 referenced 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:
(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 pattern
/\*.*\*/
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 preferred number of matches. Do not confuse
this use of question mark with its use as a quantifier in its own right.
Because it has two uses, it can sometimes appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that is the only way
the rest of the pattern matches.
If the PCRE_UNGREEDY option is set (an option 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 quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more memory is required for the
compiled pattern, in proportion to the size of the minimum or maximum.
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
implicitly anchored, because whatever follows will be tried against every
character position in the subject string, so there is no point in retrying the
overall match at any position after the first. PCRE normally treats such a
pattern as though it were preceded by \A.
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. Consider, 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
backtracking 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 captured
substring is "tweedledee". However, if there are nested capturing
subpatterns, the corresponding captured values may have been set in previous
iterations. For example, after
/(a|(b))+/
matches "aba" the value of the second captured substring is
"b".
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 different number of repeats
allows the rest of the pattern to match. Sometimes it is useful to prevent
this, either to change the nature of the match, or to cause it fail earlier
than it otherwise might, when the author of the pattern knows there is no
point in carrying on.
Consider, for example, the pattern \d+foo when applied to the subject line
123456bar
After matching all 6 digits and then failing to match "foo", the
normal action of the matcher is to try again with only 5 digits matching the
\d+ item, and then with 4, and so on, before ultimately failing. "Atomic
grouping" (a term taken from Jeffrey Friedl's book) provides the means
for specifying 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 failing 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. Backtracking past it to previous items,
however, works as normal.
An alternative description is that a subpattern of this type matches the string
of characters that an identical standalone pattern would match, if anchored at
the current point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
the above example can be thought of as a maximizing repeat that must swallow
everything it can. So, while both \d+ and \d+? are prepared to adjust the
number of digits they match in order to make the rest of the pattern match,
(?>\d+) can only match an entire sequence of digits.
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an atomic
group is just a single repeated item, as in the example above, a simpler
notation, called a "possessive quantifier" can be used. This
consists of an additional + character following a quantifier. Using this
notation, the previous example can be rewritten as
\d++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 notation for the simpler forms of
atomic group. However, there is no difference in the meaning of a possessive
quantifier and the equivalent atomic group, though there may be a performance
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 ultimately found its way into Perl at
release 5.10.
PCRE has an optimization that automatically "possessifies" certain
simple 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 consist 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 happens quickly.
Outside a character class, a backslash followed by a digit greater than 0 (and
possibly further digits) is a back reference to a capturing subpattern earlier
(that is, to its left) in the pattern, provided there have been that many
previous capturing left parentheses.
However, if the decimal number following the backslash is less than 10, it is
always taken as a back reference, and causes an error only if there are not
that many capturing left parentheses in the entire pattern. In other words,
the parentheses that are referenced need not be to the left of the reference
for numbers less than 10. 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 interpreted 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 reference 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 example:
(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 references 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
responsibility", but not "sense and responsibility". If caseful
matching is in force at the time of the back reference, the case of letters is
relevant. For example,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH
rah", even though the original capturing subpattern is matched
caselessly.
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
subpattern 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 compile 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. Otherwise, the \g{ syntax or an empty comment (see
"Comments" below) can be used.
A back reference that occurs inside the parentheses to which it refers fails
when the subpattern is first used, so, for example, (a\1) never matches.
However, such references can be useful inside repeated subpatterns. For
example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba",
"ababbaa" etc. At each iteration of the subpattern, the back
reference matches the character string corresponding to the previous
iteration. In order for this to work, the pattern must be such that the first
iteration does not need to match the back reference. This can be done using
alternation, as in the example above, or by a quantifier with a minimum of
zero.
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.
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 complicated assertions are coded as subpatterns. There are two kinds: those
that look ahead of the current position in the subject string, and those that
look behind it. An assertion subpattern is matched in the normal way, except
that it does not cause the current matching position to be changed.
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 subpatterns in the whole pattern. However,
substring capturing is carried out only for positive assertions. (Perl
sometimes, but not always, does do capturing in negative assertions.)
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 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 backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
Lookbehind assertions start with (?<= for positive assertions 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 alternatives, they do not all have to have the same
fixed length. Thus
(?<=bullock|donkey)
is permitted, but
(?<!dogs?|cats?)
causes an error at compile time. Branches that match different 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
(?<=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 different numbers of data units, are also not
permitted.
"Subroutine" calls (see below) such as (?2) or (?&X) are permitted
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 lookbehind 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 backtracks to match all but the last
character, then all but the last two characters, and so on. Once again the
search for "a" covers the entire string, from right to left, so we
are no better off. However, if the pattern is written as
^.*+(?<=abcd)
there can be no backtracking for the .*+ item; it can match only the entire
string. The subsequent lookbehind assertion does a single test on the last
four characters. If it fails, the match fails immediately. For long strings,
this approach makes a significant difference to the processing time.
Several assertions (of any sort) may occur in succession. For example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999".
Notice that each of the assertions is applied independently at the same point
in the subject string. First there is a check that the previous three
characters are all digits, and then there is a check that the same three
characters are not "999". This pattern does
not match
"foo" preceded by six characters, the first of which are digits and
the last three of which are not "999". For example, it doesn't match
"123abcfoo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters, checking
that the first three are digits, and then the second assertion checks that the
preceding three characters are not "999".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar"
which in turn is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits and any
three characters that are not "999".
It is possible to cause the matching process to obey a subpattern conditionally
or to choose between two alternative subpatterns, depending on the result of
an assertion, or whether a 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; otherwise 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 contain nested subpatterns of any form, including conditional
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 subpatterns, references to
recursion, a pseudo-condition called DEFINE, and assertions.
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 earlier section about duplicate subpattern numbers), the condition 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
referenced 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 discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that character is
present, sets it as the first captured substring. The second part matches one
or more characters that are not parentheses. The third part is a conditional
subpattern that tests whether 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.
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.
If the condition is the string (R), and there is no subpattern 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.
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 elsewhere. (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 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.
If the condition is not in any of the above formats, it must be an assertion.
This may be a positive or negative lookahead or lookbehind assertion. Consider
this pattern, again containing non-significant white space, and with the two
alternatives on the second line:
(?(?=[^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.
There are two ways of including comments in patterns that are processed by PCRE.
In both cases, the start of the comment 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 permitted. If the
PCRE_EXTENDED option is set, an unescaped # character also introduces a
comment, which in this case continues 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
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.
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 pattern using code interpolation to solve the parentheses problem can be
created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and in this case refers
recursively to the pattern in which it appears.
Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
supports special syntax for recursion of the entire pattern, and also for
individual subpattern 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 (? followed 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:
( \( ( [^()]++ | (?1) )* \) )
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 references. 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 negative 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
references 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 quantifier 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 documentation). 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 (temporarily) set at a deeper level during the matching process.
If there are more than 15 capturing parentheses in a pattern, PCRE has to obtain
extra memory to store data during a recursion, which it does by using
pcre_malloc, freeing it via
pcre_free afterwards. If no memory
can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
Do not confuse the (?R) item with the condition (R), which tests for recursion.
Consider this pattern, which matches text in angle brackets, allowing for
arbitrary nesting. Only digits are allowed in nested brackets (that is, when
recursing), whereas any characters are permitted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpattern, with two
different alternatives for the recursive and non-recursive cases. The (?R)
item is the actual recursive call.
Recursion processing in PCRE differs from Perl in two important ways. In PCRE
(like Python, but unlike Perl), a recursive 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 alternatives 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 alternative fails; the second alternative is taken and
the recursion kicks in. The recursive call to subpattern 1 successfully
matches the next character ("b"). (Note that the beginning and end
of line tests are not part of the recursion).
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 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 alternative 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 character, 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 pattern 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 possessive 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 matches "a" and
then recurses. In the recursion, \1 does now match "b" and so the
whole match succeeds. In Perl, the pattern fails to match because inside the
recursive call \1 cannot access the externally set value.
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
responsibility", 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 discussion 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 subpattern 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 subpattern.
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, possibly recursively.
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 preceded 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 syntax) are
not synonymous. The former is a back reference; the latter is a
subroutine call.
Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
code to be obeyed in the middle of matching a regular expression. This makes
it possible, amongst other things, to extract different substrings that match
the same pair of parentheses when there is a repetition.
PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
code. The feature is called "callout". The caller of PCRE provides
an external function by putting its entry point in the global variable
pcre_callout (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 function, 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 position, 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 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
documentation.
Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
which are still described in the Perl documentation 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 followed by an asterisk. They are generally of the form (*VERB) or
(*VERB:NAME). Some may take either form, possibly behaving 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 algorithm. 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.
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 running of a match, any
included backtracking verbs will not, 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, sometimes
leading to anomalous results.
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 subpattern 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 assertion
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).
There is one verb whose main purpose is to track how a match was arrived at,
though it also has a secondary use in conjunction with advancing the match
starting point (see (*SKIP) below).
(*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 entitled "Extra data for
pcre_exec()" in the
pcreapi documentation. 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 capturing 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 assertions.
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.
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 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 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
backtracking 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 modified. 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 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 backtrack 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 alternation with only one alternative. The
effect of (*THEN) extends beyond such a subpattern to the enclosing
alternative. 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, matching does not backtrack
into A; instead it moves to the next alternative, that is, D. However, if the
subpattern containing (*THEN) is given an alternative, it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner subpattern. 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 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 single 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.
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).
PCRE differs from Perl in its handling of backtracking verbs in repeated groups.
For example, consider:
/(a(*COMMIT)b)+ac/
If the subject is "abac", Perl matches, but PCRE fails because the
(*COMMIT) in the second repeat of the group acts.
(*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 processing. In a negative assertion, (*ACCEPT) causes the assertion to
fail without any further processing.
The other backtracking 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 alternations, whether or not this is
within the assertion.
Negative 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 assertion.
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).
These behaviours occur whether or not the subpattern is called recursively.
Perl's treatment of subroutines is different 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.
pcreapi(3),
pcrecallout(3),
pcrematching(3),
pcresyntax(3),
pcre(3),
pcre16(3),
pcre32(3).
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 14 June 2015
Copyright (c) 1997-2015 University of Cambridge.