PCREPATTERN(3) Library Functions Manual PCREPATTERN(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions that are supported
by PCRE are described in detail below. There is a quick-reference syntax
summary in the pcresyntax page. PCRE tries to match Perl syntax and se-
mantics as closely as it can. PCRE also supports some alternative regu-
lar 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 Expres-
sions", published by O'Reilly, covers regular expressions in great de-
tail. This description of PCRE's regular expressions is intended as ref-
erence 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.
SPECIAL START-OF-PATTERN ITEMS
A number of options that can be passed to pcre_compile() can also be set
by special items at the start of a pattern. These are not Perl-compati-
ble, but are provided to make these options accessible to pattern writ-
ers 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 up-
per 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 li-
brary, 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 appropri-
ate 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 men-
tioned 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 al-
lowed, 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 deter-
mine 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 set-
ting 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 op-
tions arguments for the compiling and matching functions.
It is also possible to specify a newline convention by starting a pat-
tern 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 func-
tion. For example, on a Unix system where LF is the default newline se-
quence, 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 asser-
tions are true. It also affects the interpretation of the dot metachar-
acter 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 in-
ternal 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 sys-
tem 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 set-
ting 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.
EBCDIC CHARACTER CODES
PCRE can be compiled to run in an environment that uses EBCDIC as its
character code rather than ASCII or Unicode (typically a mainframe sys-
tem). In the sections below, character code values are ASCII or Unicode;
in an EBCDIC environment these characters may have different code val-
ues, and there are no code points greater than 255.
CHARACTERS AND METACHARACTERS
A regular expression is a pattern that is matched against a subject
string from left to right. Most characters stand for themselves in a
pattern, and match the corresponding 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 prop-
erty 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 al-
ternatives and repetitions in the pattern. These are encoded in the pat-
tern 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 recog-
nized 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.
BACKSLASH
The backslash character has several uses. Firstly, if it is followed by
a character that is not a number or a letter, it takes away any special
meaning that character may have. This use of backslash as an escape
character applies both inside and outside character classes.
For example, if you want to match a * character, you write \* in the
pattern. This escaping action applies whether or not the following
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 ig-
nored. 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 con-
tinues 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, be-
cause the character class is not terminated.
Non-printing characters
A second use of backslash provides a way of encoding non-printing char-
acters 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 edit-
ing, it is often easier to use one of the following escape sequences
than the binary character it represents. In an ASCII or Unicode envi-
ronment, 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 gener-
ate the appropriate EBCDIC code values. The \c escape is processed as
specified for Perl in the perlebcdic document. The only characters that
are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. Any
other character provokes a compile-time error. The sequence \@ encodes
character code 0; the letters (in either case) encode characters 1-26
(hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 (hex 1B to
hex 1F), and \? becomes either 255 (hex FF) or 95 (hex 5F).
Thus, apart from \?, these escapes generate the same character code val-
ues as they do in an ASCII environment, though the meanings of the val-
ues mostly differ. For example, \G always generates code value 7, which
is BEL in ASCII but DEL in EBCDIC.
The sequence \? generates DEL (127, hex 7F) in an ASCII environment, but
because 127 is not a control character in EBCDIC, Perl makes it generate
the APC character. Unfortunately, there are several variants of EBCDIC.
In most of them the APC character has the value 255 (hex FF), but in the
one Perl calls POSIX-BC its value is 95 (hex 5F). If certain other char-
acters have POSIX-BC values, PCRE makes \? generate 95; otherwise it
generates 255.
After \0 up to two further octal digits are read. If there are fewer
than two digits, just those that are present are used. Thus the sequence
\0\x\015 specifies two binary zeros followed by a CR character (code
value 13). Make sure you supply two digits after the initial zero if the
pattern character that follows is itself an octal digit.
The escape \o must be followed by a sequence of octal digits, enclosed
in braces. An error occurs if this is not the case. This escape is a re-
cent addition to Perl; it provides way of specifying character code
points as octal numbers greater than 0777, and it also allows octal num-
bers 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 charac-
ter numbers, and \g{} to specify back references. The following para-
graphs describe the old, ambiguous syntax.
The handling of a backslash followed by a digit other than 0 is compli-
cated, and Perl has changed in recent releases, causing PCRE also to
change. Outside a character class, PCRE reads the digit and any follow-
ing 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 other-
wise 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 hexa-
decimal digits are read (letters can be in upper or lower case). Any
number of hexadecimal digits may appear between \x{ and }. If a charac-
ter 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, sup-
port 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 differ-
ence 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 ref-
erence; 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 char-
acter. 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 com-
plete set of characters into two disjoint sets. Any given character
matches one, and only one, of each pair. The sequences can appear both
inside and outside character classes. They each match one character of
the appropriate type. If the current matching point is at the end of the
subject string, all of them fail, because there is no 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 re-
lease 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 rea-
sons. If PCRE is compiled with Unicode property support, and the
PCRE_UCP option is set, the behaviour is changed so that Unicode proper-
ties 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 af-
fects \b, and \B because they are defined in terms of \w and \W. Match-
ing 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 be-
low. 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 sepa-
rator, U+2029). Unicode character property support is not needed for
these characters to be recognized.
It is possible to restrict \R to match only CR, LF, or CRLF (instead of
the complete set of Unicode line endings) by setting the option
PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
(BSR is an abbrevation for "backslash R".) This can be made the default
when PCRE is built; if this is the case, the other behaviour can be re-
quested 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 func-
tion, but they can themselves be overridden by options given to a match-
ing function. Note that these special settings, which are not Perl-com-
patible, 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 conven-
tion; 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 de-
fault, but causes an error if PCRE_EXTRA is set.
Unicode character properties
When PCRE is built with Unicode character property support, three addi-
tional 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 (de-
scribed in the next section). Other Perl properties such as "InMusical-
Symbols" 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, Gu-
jarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Ara-
maic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, Ja-
vanese, 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, Mon-
golian, Mro, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki,
Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian, Old_South_Ara-
bian, 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, speci-
fied 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 gen-
eral 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{Let-
ter}) 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) prop-
erty. 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 differ-
ent 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 "ex-
tended 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 pre-
ceding character.
This simple definition was extended in Unicode to include more compli-
cated kinds of composite character by giving each character a grapheme
breaking property, and creating rules that use these properties to de-
fine 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 charac-
ter.
3. Do not break Hangul (a Korean script) syllable sequences. Hangul
characters are of five types: L, V, T, LV, and LVT. An L character may
be followed by an L, V, LV, or LVT character; an LV or V character may
be followed by a V or T character; an LVT or T character may be follwed
only by a T character.
4. Do not end before extending characters or spacing marks. Characters
with the "mark" property always have the "extend" grapheme breaking
property.
5. Do not end after prepend characters.
6. Otherwise, end the cluster.
PCRE's additional properties
As well as the standard Unicode properties described above, PCRE sup-
ports four more that make it possible to convert traditional escape se-
quences such as \w and \s to use Unicode properties. PCRE uses these
non-standard, non-Perl properties internally when PCRE_UCP is set. How-
ever, 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 (sepa-
rator) 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 in-
terfere 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 de-
fined". In PCRE, \K is acted upon when it occurs inside positive asser-
tions, 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 asser-
tion 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 char-
acter class, by default it matches the corresponding literal character
(for example, \B matches the letter B). However, if the PCRE_EXTRA op-
tion 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. How-
ever, 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 pre-
viously 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.
CIRCUMFLEX AND DOLLAR
The circumflex and dollar metacharacters are zero-width assertions. That
is, they test for a particular condition being true without consuming
any characters from the subject string.
Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching
point is at the start of the subject string. If the startoffset argument
of pcre_exec() is non-zero, circumflex can never match if the PCRE_MUL-
TILINE option is unset. Inside a character class, circumflex has an en-
tirely 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 al-
ternative 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 be-
fore 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.
FULL STOP (PERIOD, DOT) AND \N
Outside a character class, a dot in the pattern matches any one charac-
ter 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 Uni-
code 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 ex-
ception. 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 circum-
flex and dollar, the only relationship being that they both involve new-
lines. Dot has no special meaning in a character class.
The escape sequence \N behaves like a dot, except that it is not af-
fected by the PCRE_DOTALL option. In other words, it matches any charac-
ter except one that signifies the end of a line. Perl also uses \N to
match characters by name; PCRE does not support this.
MATCHING A SINGLE DATA UNIT
Outside a character class, the escape sequence \C matches any one data
unit, whether or not a UTF mode is set. In the 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 use-
fully 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 be-
low) 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 asser-
tions 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 char-
acter's individual bytes are then captured by the appropriate number of
groups.
SQUARE BRACKETS AND CHARACTER CLASSES
An opening square bracket introduces a character class, terminated by a
closing square bracket. A closing square bracket on its own is not spe-
cial 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 char-
acter 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 con-
sumes 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 case-
less matching in a UTF mode for characters 128 and above, you must en-
sure 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 spe-
cial 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 char-
acters.
The minus (hyphen) character can be used to specify a range of charac-
ters 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 char-
acter 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 charac-
ter 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 inter-
preted as a class containing a range followed by two other characters.
The octal or hexadecimal representation of "]" can also be used to end a
range.
An error is generated if a POSIX character class (see below) or an es-
cape 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 char-
acter tables for a French locale are in use, [\xc8-\xcb] matches ac-
cented E characters in both cases. In UTF modes, PCRE supports the con-
cept 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 out-
side 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 se-
quences \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 ter-
minating closing square bracket. However, escaping other non-alphanu-
meric characters does no harm.
POSIX CHARACTER CLASSES
Perl supports the POSIX notation for character classes. This uses names
enclosed by [: and :] within the enclosing square brackets. PCRE also
supports this notation. For example,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The 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 char-
acter 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 char-
acters 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 char-
acters that are not controls, that is, characters with the Zs
property.
[:punct:] This matches all characters that have the Unicode P (punctua-
tion) property, plus those characters whose code points are
less than 128 that have the S (Symbol) property.
The other POSIX classes are unchanged, and match only characters with
code points less than 128.
COMPATIBILITY FEATURE FOR WORD BOUNDARIES
In the POSIX.2 compliant library that was included in 4.4BSD Unix, the
ugly syntax [[:<:]] and [[:>:]] is used for 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 sup-
port 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
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.
INTERNAL OPTION SETTING
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
PCRE_EXTENDED options (which are Perl-compatible) can be changed from
within the pattern by a sequence of Perl option letters enclosed between
"(?" and ")". The option letters are
i for PCRE_CASELESS
m for PCRE_MULTILINE
s for PCRE_DOTALL
x for PCRE_EXTENDED
For example, (?im) sets caseless, multiline matching. It is also possi-
ble to unset these options by preceding the letter with a hyphen, and a
combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
LESS 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 in-
side subpattern parentheses), the change applies to the remainder of the
pattern that follows. If the change is placed right at the start of a
pattern, PCRE extracts it into the global options (and it will therefore
show up in data extracted by the pcre_fullinfo() function).
An option change within a subpattern (see below for a description of
subpatterns) affects only that part of the subpattern that follows it,
so
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
used). By this means, options can be made to have different settings in
different parts of the pattern. Any changes made in one alternative do
carry on into subsequent branches within the same subpattern. For exam-
ple,
(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 ap-
plication 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. De-
tails 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 equiva-
lent 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
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 ar-
gument of the matching function. (This applies only to the traditional
matching functions; the DFA matching functions do not support captur-
ing.)
Opening parentheses are counted from left to right (starting from 1) to
obtain numbers for the capturing 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 num-
bered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not always help-
ful. There are often times when a grouping subpattern is required with-
out 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 be-
tween 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 sub-
sequent branches, so the above patterns match "SUNDAY" as well as "Sat-
urday".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a subpattern
uses the same numbers for its capturing parentheses. Such a subpattern
starts with (?| and is itself a non-capturing subpattern. For example,
consider this pattern:
(?|(Sat)ur|(Sun))day
Because the two alternatives are inside a (?| group, both sets of cap-
turing 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 subpat-
tern 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 pat-
tern 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.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated regular expres-
sions. Furthermore, if an expression is modified, the numbers may
change. To help with this difficulty, PCRE supports the naming of sub-
patterns. 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 syn-
tax. 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 ref-
erences, 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 allo-
cated 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 sub-
pattern it was.
If you make a back reference to a non-unique named subpattern from else-
where 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 pat-
tern 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 ab-
sence 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 de-
tails of the interfaces for handling named subpatterns, see the pcreapi
documentation.
Warning: You cannot use different names to distinguish between two sub-
patterns with the same number because PCRE uses only the numbers when
matching. For this reason, an error is given at compile time if differ-
ent 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
Repetition is specified by quantifiers, which can follow any of the fol-
lowing 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 spe-
cial 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 po-
sition 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. Simi-
larly, \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 use-
ful 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 bro-
ken.
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 be-
tween /* 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 min-
imum or maximum.
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiva-
lent 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 charac-
ter. 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 con-
trol verbs (*PRUNE) and (*SKIP) also disable this optimization.
When a capturing subpattern is repeated, the value captured is the sub-
string 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".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
repetition, failure of what follows normally causes the repeated item to
be re-evaluated to see if a 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, how-
ever, 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_UN-
GREEDY 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 sim-
ple pattern constructs. For example, the sequence A+B is treated as A++B
because there is no point in backtracking into a sequence of A's when B
must follow.
When a pattern contains an unlimited repeat inside a subpattern that can
itself be repeated an unlimited number of times, the use of an atomic
group is the only way to avoid some failing matches taking a very long
time indeed. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either 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 re-
quired 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.
BACK REFERENCES
Outside a character class, a backslash followed by a digit greater than
0 (and possibly further digits) is a back reference to a capturing sub-
pattern 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 pat-
tern. 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 ref-
erence" 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 se-
quence such as \50 is interpreted as a character defined in octal. See
the subsection entitled "Non-printing characters" above for further de-
tails of the handling of digits following a backslash. There is no such
problem when named parentheses are used. A back reference to any subpat-
tern is possible using named parentheses (see below).
Another way of avoiding the ambiguity inherent in the use of digits fol-
lowing 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 dig-
its 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 captur-
ing subpattern before \g, that is, is it equivalent to \2 in this exam-
ple. 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 subpat-
tern 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 exam-
ple,
((?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 ex-
ample 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 op-
tion 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 subpat-
terns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each itera-
tion 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.
ASSERTIONS
An assertion is a test on the characters following or preceding the cur-
rent 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 asser-
tion 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 asser-
tions. (Perl sometimes, but not always, does do capturing in negative
assertions.)
For compatibility with Perl, assertion subpatterns may be repeated;
though it makes no sense to assert the same thing several times, the
side effect of capturing parentheses may occasionally be useful. In
practice, there only three cases:
(1) If the quantifier is {0}, the assertion is never obeyed during
matching. However, it may contain internal capturing parenthesized
groups that are called from elsewhere via the subroutine mechanism.
(2) If quantifier is {0,n} where n is greater than zero, it is treated
as if it were {0,1}. At run time, the rest of the pattern match is tried
with and without the assertion, the order depending on the greediness of
the quantifier.
(3) If the minimum repetition is greater than zero, the quantifier is
ignored. The assertion is obeyed just once when encountered during
matching.
Lookahead assertions
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 semi-
colon 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 as-
sertion (?!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 con-
tents 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 dif-
ferent 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, be-
cause 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 asser-
tions 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 pro-
ceeds 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 en-
tire 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 pro-
cessing 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 charac-
ters are all digits, and then there is a check that the same three char-
acters are not "999". This pattern does not match "foo" preceded by six
characters, the first of which are digits and the last three of which
are not "999". For example, it doesn't match "123abcfoo". A pattern to
do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters,
checking that the first three are digits, and then the second assertion
checks that the preceding three characters are not "999".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar" which in turn
is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits and any
three characters that are not "999".
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern condi-
tionally 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 subpat-
tern 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 alterna-
tives may itself contain nested subpatterns of any form, including con-
ditional 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 pre-
viously matched. If there is more than one capturing subpattern with the
same number (see the earlier section about duplicate subpattern num-
bers), 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 re-
fer to subsequent groups. The next parentheses to be opened can be ref-
erenced 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 sec-
ond 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. Other-
wise, since no-pattern is not present, the subpattern matches nothing.
In other words, this pattern matches a sequence of non-parentheses, op-
tionally enclosed in parentheses.
If you were embedding this pattern in a larger one, you could use a rel-
ative 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 amper-
sand 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 du-
plicate, 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 syn-
tax 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 con-
trol 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 as-
sertion. This may be a positive or negative lookahead or lookbehind as-
sertion. Consider this pattern, again containing non-significant white
space, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an optional
sequence of non-letters followed by a letter. In other words, it tests
for the presence of at least one letter in the subject. If a letter is
found, the subject is matched against the first alternative; otherwise
it is matched against the second. This pattern matches strings in one of
the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are
digits.
COMMENTS
There are two ways of including comments in patterns that are processed
by PCRE. In both cases, the start of the comment must not be in a char-
acter class, nor in the middle of any other sequence of related charac-
ters 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 new-
line 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.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best
that can be done is to use a pattern that matches up to some fixed depth
of nesting. It is not possible to handle an arbitrary nesting depth.
For some time, Perl has provided a facility that allows regular expres-
sions 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 sub-
pattern. (If not, it is a non-recursive subroutine call, which is de-
scribed in the next section.) The special item (?R) or (?0) is a recur-
sive 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 re-
cursive 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-paren-
theses.
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 re-
cently 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 be-
cause 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 pat-
tern to strings that do not match. For example, when this pattern is ap-
plied 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 documenta-
tion). 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 sub-
pattern is not matched at the top level, its final captured value is un-
set, 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 re-
cursion. Consider this pattern, which matches text in angle brackets,
allowing for arbitrary nesting. Only digits are allowed in nested brack-
ets (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 al-
ways 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 il-
lustrated by the following pattern, which purports to match a palin-
dromic 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. Con-
sider 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 alterna-
tive is taken and the recursion kicks in. The recursive call to subpat-
tern 1 successfully matches the next character ("b"). (Note that the be-
ginning 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 en-
tered again in order to match an empty string. The solution is to sepa-
rate 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 ig-
nore 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 sub-
ject 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 process-
ing 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.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern call (either by number or by
name) is used outside the parentheses to which it refers, it operates
like a subroutine in a programming language. The called subpattern may
be defined before or after the reference. A numbered reference can be
absolute or relative, as in these examples:
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
An earlier example pointed out that the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and 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 sub-
ject string, it is never re-entered, even if it contains untried alter-
natives and there is a subsequent matching failure. Any capturing paren-
theses 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.
ONIGURUMA SUBROUTINE SYNTAX
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, pos-
sibly 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.
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary
Perl code to be obeyed in the middle of matching a regular expression.
This makes it possible, amongst other things, to extract different sub-
strings that match the same pair of parentheses when there is a repeti-
tion.
PCRE provides a similar feature, but of course it cannot obey arbitrary
Perl code. The feature is called "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 ex-
ternal function is to be called. If you want to identify different call-
out 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, call-
outs are automatically installed before each item in the pattern. They
are all numbered 255. If there is a conditional group in the pattern
whose condition is an assertion, an additional callout is inserted just
before the condition. An explicit callout may also be set at this posi-
tion, as in this example:
(?(?C9)(?=a)abc|def)
Note that this applies only to assertion conditions, not to other types
of condition.
During matching, when PCRE reaches a callout point, the external func-
tion is called. It is provided with the number of the callout, the posi-
tion in the pattern, and, optionally, one item of data originally sup-
plied 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.
BACKTRACKING CONTROL
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 clos-
ing 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 tra-
ditional matching functions, because these use a backtracking algorithm.
With the exception of (*FAIL), which behaves like a failing negative as-
sertion, 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 sub-
patterns called as subroutines (whether or not recursively) is docu-
mented 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 set-
ting 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 posi-
tive assertion, the assertion succeeds; in a negative assertion, the as-
sertion fails.
If (*ACCEPT) is inside capturing parentheses, the data so far is cap-
tured. For example:
A((?:A|B(*ACCEPT)|C)D)
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
tured 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 pat-
tern:
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 out-
putting 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 exam-
ple it indicates which of the two alternatives matched. This is a more
efficient way of obtaining this information than putting each alterna-
tive 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-encoun-
tered. 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 con-
tinues 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 con-
fined 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. (Remem-
ber also, as stated above, that this localization also applies in sub-
routine calls.)
These verbs differ in exactly what kind of failure occurs when back-
tracking reaches them. The behaviour described below is what happens
when the verb is not in a subroutine or an assertion. Subsequent sec-
tions 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 back-
tracking to reach it. Even if the pattern is unanchored, no further at-
tempts 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 an-
chor, 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 pro-
gram. 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 backtrack-
ing 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 quanti-
fier, 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 re-
membered 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 encoun-
tered. (*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 quan-
tifer 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 "bumpa-
long" 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 back-
tracking 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 subse-
quently BAZ fails, there are no more alternatives, so there is a back-
track to whatever came before the entire group. If (*THEN) is not inside
an alternation, it acts like (*PRUNE).
The behaviour of (*THEN:NAME) is the not the same as
(*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is remem-
bered 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 al-
ternative. The effect of (*THEN) extends beyond such a subpattern to the
enclosing alternative. Consider this pattern, where A, B, etc. are com-
plex 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 subpat-
tern 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 al-
ternatives, because only one is ever used. In other words, the | charac-
ter 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 un-
greedy, 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 char-
acter (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 with-
out 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 alterna-
tive 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 re-
sult. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a nega-
tive 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 be-
haviour), 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 recur-
sively. 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.
SEE ALSO
pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3),
pcre16(3), pcre32(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 14 June 2015
Copyright (c) 1997-2015 University of Cambridge.
PCRE 8.38 14 June 2015 PCREPATTERN(3)
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