PCREPATTERN(3) Library Functions Manual PCREPATTERN(3)
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.
UTF support
The original operation of PCRE was on strings of one-byte
characters. However, there is now also support for UTF-8 strings
in the original library, an extra library that 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.
Unicode property support
Another special sequence that may appear at the start of a
pattern is (*UCP). This has the same effect as setting the
PCRE_UCP option: it causes sequences such as \d and \w to use
Unicode properties to determine character types, instead of
recognizing only characters with codes less than 128 via a lookup
table.
Disabling auto-possessification
If a pattern starts with (*NO_AUTO_POSSESS), it has the same
effect as setting the PCRE_NO_AUTO_POSSESS option at compile
time. This stops PCRE from making quantifiers possessive when
what follows cannot match the repeated item. For example, by
default a+b is treated as a++b. For more details, see the pcreapi
documentation.
Disabling start-up optimizations
If a pattern starts with (*NO_START_OPT), it has the same effect
as setting the PCRE_NO_START_OPTIMIZE option either at compile or
matching time. This disables several optimizations for quickly
reaching "no match" results. For more details, see the pcreapi
documentation.
Newline conventions
PCRE supports five different conventions for indicating line
breaks in strings: a single CR (carriage return) character, a
single LF (linefeed) character, the two-character sequence CRLF,
any of the three preceding, or any Unicode newline sequence. The
pcreapi page has further discussion about 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.
Setting match and recursion limits
The caller of pcre_exec() can set a limit on the number of times
the internal match() function is called and on the maximum depth
of recursive calls. These facilities are 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.
Non-printing characters
A second use of backslash provides a way of encoding non-printing
characters in patterns in a visible manner. There is no
restriction on the appearance of non-printing 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 \c@ encodes character code 0; after \c
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 \c? becomes either 255 (hex FF) or 95 (hex 5F).
Thus, apart from \c?, these escapes generate the same character
code values as they do in an ASCII environment, though the
meanings of the values mostly differ. For example, \cG always
generates code value 7, which is BEL in ASCII but DEL in EBCDIC.
The sequence \c? generates DEL (127, hex 7F) in an ASCII
environment, but because 127 is not a control character in
EBCDIC, Perl makes it generate the APC character. Unfortunately,
there are several variants of EBCDIC. In most of them the APC
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 \c? 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).
Constraints on character values
Characters that are specified using octal or hexadecimal numbers
are limited to certain values, as follows:
8-bit non-UTF mode less than 0x100
8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
16-bit non-UTF mode less than 0x10000
16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
32-bit non-UTF mode less than 0x100000000
32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the
so-called "surrogate" codepoints), and 0xffef.
Escape sequences in character classes
All the sequences that define a single character value can 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.
Unsupported escape sequences
In Perl, the sequences \l, \L, \u, and \U are recognized by its
string handler and used to modify the case of following
characters. By default, PCRE does not support these escape
sequences. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
\U matches a "U" character, and \u can be used to define a
character by code point, as described in the previous section.
Absolute and relative back references
The sequence \g followed by an unsigned or a negative number,
optionally enclosed in braces, is an absolute or relative back
reference. A named back reference can be coded as \g{name}. Back
references are discussed later, following the discussion of
parenthesized subpatterns.
Absolute and relative subroutine calls
For compatibility with Oniguruma, the non-Perl syntax \g followed
by a name or a number enclosed either in angle brackets or single
quotes, is an alternative syntax for referencing a subpattern as
a "subroutine". Details are 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.
Generic character types
Another use of backslash is for specifying generic character
types:
\d any decimal digit
\D any character that is not a decimal digit
\h any horizontal white space character
\H any character that is not a horizontal white space
character
\s any white space character
\S any character that is not a white space character
\v any vertical white space character
\V any character that is not a vertical white space
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.
Newline sequences
Outside a character class, by default, the escape sequence \R
matches any Unicode newline sequence. In 8-bit non-UTF-8 mode \R
is equivalent to the following:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details of which 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 abbreviation 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.
Unicode character properties
When PCRE is built with Unicode character property support, three
additional escape sequences that match characters with specific
properties are available. When in 8-bit non-UTF-8 mode, these
sequences are of course limited to testing 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).
Extended grapheme clusters
The \X escape matches any number of Unicode characters that form
an "extended grapheme cluster", and treats the sequence as an
atomic group (see below). Up to and including release 8.31, PCRE
matched an earlier, simpler definition that was equivalent to
(?>\PM\pM*)
That is, it matched a character without the "mark" property,
followed by zero or more characters with the "mark" 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 followed 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.
PCRE's additional properties
As well as the standard Unicode properties described above, PCRE
supports four more that make it possible to convert traditional
escape sequences such as \w and \s to use Unicode properties.
PCRE uses these non-standard, non-Perl properties internally when
PCRE_UCP is set. However, they may also be used explicitly. These
properties are:
Xan Any alphanumeric character
Xps Any POSIX space character
Xsp Any Perl space character
Xwd Any Perl "word" character
Xan matches characters that have either the L (letter) or the N
(number) property. Xps matches the characters tab, linefeed,
vertical tab, form feed, or carriage return, and any other
character that has the Z (separator) property. Xsp is the same
as Xps; it used to exclude vertical tab, for Perl compatibility,
but Perl changed, and so PCRE followed at release 8.34. Xwd
matches the same characters as Xan, plus underscore.
There is another non-standard property, Xuc, which matches any
character that can be represented by a Universal Character Name
in C++ and other programming languages. These are the characters
$, @, ` (grave accent), and all characters with Unicode code
points greater than or equal to U+00A0, except for the surrogates
U+D800 to U+DFFF. Note that most base (ASCII) characters are
excluded. (Universal Character Names are of the form \uHHHH or
\UHHHHHHHH where H is a hexadecimal digit. Note that the Xuc
property does not match these sequences but the characters that
they represent.)
Resetting the match start
The escape sequence \K causes any previously matched 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.
Simple assertions
The final use of backslash is for certain simple assertions. An
assertion specifies a condition that has to be met at a
particular point in a match, without consuming any 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. 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.
Recursive back references
A back reference that occurs inside the parentheses to which it
refers fails when the subpattern is first used, so, for example,
(a\1) never matches. However, such references can be useful
inside repeated subpatterns. For example, the 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.)
WARNING: If a positive assertion containing one or more capturing
subpatterns succeeds, but failure to match later in the pattern
causes backtracking over this assertion, the captures within the
assertion are reset only if no higher numbered captures are
already set. This is, unfortunately, a fundamental limitation of
the current implementation, and as PCRE1 is now in maintenance-
only status, it is unlikely ever to change.
For compatibility with Perl, assertion subpatterns may be
repeated; though it makes no sense to assert the same thing
several times, the side effect of capturing parentheses may
occasionally be useful. In practice, there only three cases:
(1) If the quantifier is {0}, the assertion is never obeyed
during matching. However, it may contain internal capturing
parenthesized groups that are called from elsewhere via the
subroutine mechanism.
(2) If quantifier is {0,n} where n is greater than zero, it is
treated as if it were {0,1}. At run time, the rest of the pattern
match is tried with and without the assertion, the order
depending on the greediness of the quantifier.
(3) If the minimum repetition is greater than zero, the
quantifier is ignored. The assertion is obeyed just once when
encountered during matching.
Lookahead assertions
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
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.
Using multiple assertions
Several assertions (of any sort) may occur in succession. For
example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999". Notice
that each of the assertions is applied independently at the same
point in the subject string. First there is a check that the
previous three characters are all digits, and then there is a
check that the same three characters are not "999". This pattern
does not match "foo" preceded by six characters, the first of
which are digits and the last three of which are not "999". For
example, it doesn't match "123abcfoo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six
characters, checking that the first three are digits, and then
the second assertion checks that the preceding three characters
are not "999".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar" which in
turn is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits
and any three characters that are not "999".
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.
Checking for a used subpattern by number
If the text between the parentheses consists of a sequence of
digits, the condition is true if a capturing subpattern of that
number has previously matched. If there is more than one
capturing subpattern with the same number (see the 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.
Checking for a used subpattern by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for
a used subpattern by name. For compatibility with earlier
versions of PCRE, which had this facility before Perl, the syntax
(?(name)...) is also recognized.
Rewriting the above example to use a named subpattern gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate, the
test is applied to all subpatterns of the same name, and is true
if any one of them has matched.
Checking for pattern recursion
If the condition is the string (R), and there is no 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.
Defining subpatterns for use by reference only
If the condition is the string (DEFINE), and there is no
subpattern with the name DEFINE, the condition is always false.
In this case, there may be only one alternative in the
subpattern. It is always skipped if control reaches this point in
the pattern; the idea of DEFINE is that it can be used to define
subroutines that can be referenced from 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.
Assertion conditions
If the condition is not in any of the above formats, it must be
an assertion. This may be a positive or negative 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.
Differences in recursion processing between PCRE and Perl
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.
Optimizations that affect backtracking verbs
PCRE contains some optimizations that are used to speed up
matching by running some checks at the start of each match
attempt. For example, it may know the minimum length of matching
subject, or that a particular character must be present. When one
of these optimizations bypasses the 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.
Verbs that act immediately
The following verbs act as soon as they are encountered. They may
not be followed by a name.
(*ACCEPT)
This verb causes the match to end successfully, skipping the
remainder of the pattern. However, when it is inside a 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).
Recording which path was taken
There is one verb whose main purpose is to track how a match was
arrived at, though it also has a secondary use in 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.
Verbs that act after backtracking
The following verbs do nothing when they are encountered.
Matching continues with what follows, but if there is no
subsequent match, causing a backtrack to the verb, a failure is
forced. That is, backtracking cannot pass to the left of 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 quantifier 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.
More than one backtracking verb
If more than one backtracking verb is present in a pattern, the
one that is backtracked onto first acts. For example, consider
this pattern, where A, B, etc. are complex pattern fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes the
entire match to fail. However, if A and B match, but C fails, the
backtrack to (*THEN) causes the next alternative (ABD) to be
tried. This behaviour is consistent, but is not always the same
as Perl's. It means that if two or more backtracking verbs appear
in succession, all the the last of them has no effect. Consider
this example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto
(*PRUNE) causes it to be triggered, and its action is taken.
There can never be a backtrack onto (*COMMIT).
Backtracking verbs in repeated groups
PCRE differs from Perl in its handling of backtracking verbs in
repeated groups. For example, consider:
/(a(*COMMIT)b)+ac/
If the subject is "abac", Perl matches, but PCRE fails because
the (*COMMIT) in the second repeat of the group acts.
Backtracking verbs in assertions
(*FAIL) in an assertion has its normal effect: it forces an
immediate backtrack.
(*ACCEPT) in a positive assertion causes the assertion to succeed
without any further 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).
Backtracking verbs in subroutines
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: 23 October 2016
Copyright (c) 1997-2016 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
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PCRE 8.40 23 October 2016 PCREPATTERN(3)
Pages that refer to this page: grep(1), pcregrep(1), pcretest(1), pcresyntax(3)
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