Provided by: libpcre3_8.31-2ubuntu2.3_amd64 
NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions that are supported by PCRE are described in detail
below. There is a quick-reference syntax summary in the pcresyntax page. PCRE tries to match Perl syntax
and semantics as closely as it can. PCRE also supports some alternative regular expression syntax (which
does not conflict with the Perl syntax) in order to provide some compatibility with regular expressions
in Python, .NET, and Oniguruma.
Perl's regular expressions are described in its own documentation, and regular expressions in general are
covered in a number of books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular
Expressions", published by O'Reilly, covers regular expressions in great detail. This description of
PCRE's regular expressions is intended as reference material.
The original operation of PCRE was on strings of one-byte characters. However, there is now also support
for UTF-8 strings in the original library, and a second library that supports 16-bit and UTF-16 character
strings. To use these features, PCRE must be built to include appropriate support. When using UTF strings
you must either call the compiling function with the PCRE_UTF8 or PCRE_UTF16 option, or the pattern must
start with one of these special sequences:
(*UTF8)
(*UTF16)
Starting a pattern with such a sequence is equivalent to setting the relevant option. This feature is not
Perl-compatible. How setting a UTF mode affects pattern matching is mentioned in several places below.
There is also a summary of features in the pcreunicode page.
Another special sequence that may appear at the start of a pattern or in combination with (*UTF8) or
(*UTF16) is:
(*UCP)
This has the same effect as setting the PCRE_UCP option: it causes sequences such as \d and \w to use
Unicode properties to determine character types, instead of recognizing only characters with codes less
than 128 via a lookup table.
If a pattern starts with (*NO_START_OPT), it has the same effect as setting the PCRE_NO_START_OPTIMIZE
option either at compile or matching time. There are also some more of these special sequences that are
concerned with the handling of newlines; they are described below.
The remainder of this document discusses the patterns that are supported by PCRE when one its main
matching functions, pcre_exec() (8-bit) or pcre16_exec() (16-bit), is used. PCRE also has alternative
matching functions, pcre_dfa_exec() and pcre16_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.
NEWLINE CONVENTIONS
PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage
return) character, a single LF (linefeed) character, the two-character sequence CRLF, any of the three
preceding, or any Unicode newline sequence. The pcreapi page has further discussion about newlines, and
shows how to set the newline convention in the options arguments for the compiling and matching
functions.
It is also possible to specify a newline convention by starting a pattern string with one of the
following five sequences:
(*CR) carriage return
(*LF) linefeed
(*CRLF) carriage return, followed by linefeed
(*ANYCRLF) any of the three above
(*ANY) all Unicode newline sequences
These override the default and the options given to the compiling function. For example, on a Unix system
where LF is the default newline sequence, the pattern
(*CR)a.b
changes the convention to CR. That pattern matches "a\nb" because LF is no longer a newline. Note that
these special settings, which are not Perl-compatible, are recognized only at the very start of a
pattern, and that they must be in upper case. If more than one of them is present, the last one is used.
The newline convention affects the interpretation of the dot metacharacter when PCRE_DOTALL is not set,
and also 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.
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 property support, but not otherwise. If you want to use caseless matching for characters
128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF
support.
The power of regular expressions comes from the ability to include alternatives and repetitions in the
pattern. These are encoded in the pattern by the use of metacharacters, which do not stand for themselves
but instead are interpreted in some special way.
There are two different sets of metacharacters: those that are recognized anywhere in the pattern except
within square brackets, and those that are recognized within square brackets. Outside square brackets,
the metacharacters are as follows:
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
Part of a pattern that is in square brackets is called a "character class". In a character class the only
metacharacters are:
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (only if followed by POSIX
syntax)
] terminates the character class
The following sections describe the use of each of the metacharacters.
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, white space in the pattern (other than in a
character class) and characters between a # outside a character class and the next newline are ignored.
An escaping backslash can be used to include a white space or # character as part of the pattern.
If you want to remove the special meaning from a sequence of characters, you can do so by putting them
between \Q and \E. This is different from Perl in that $ and @ are handled as literals in \Q...\E
sequences in PCRE, whereas in Perl, $ and @ cause variable interpolation. Note the following examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the
contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character classes. An isolated \E that is not
preceded by \Q is ignored. If \Q is not followed by \E later in the pattern, the literal interpretation
continues to the end of the pattern (that is, \E is assumed at the end). If the isolated \Q is inside a
character class, this causes an error, because the character class is not terminated.
Non-printing characters
A second use of backslash provides a way of encoding non-printing characters in patterns in a visible
manner. There is no restriction on the appearance of non-printing characters, apart from the binary zero
that terminates a pattern, but when a pattern is being prepared by text editing, it is often easier to
use one of the following escape sequences than the binary character it represents:
\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)
\ddd character with octal code ddd, or back reference
\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 is as follows: if x is a lower case letter, it is converted to upper case. Then
bit 6 of the character (hex 40) is inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B
({ is 7B), while \c; becomes hex 7B (; is 3B). If the byte 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, all byte values are valid. A lower case letter is converted to upper case, and then the 0xc0
bits are flipped.)
By default, after \x, from zero to two hexadecimal digits are read (letters can be in upper or lower
case). Any number of hexadecimal digits may appear between \x{ and }, but the character code is
constrained 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
Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called "surrogate" codepoints).
If characters other than hexadecimal digits appear between \x{ and }, or if there is no terminating },
this form of escape is not recognized. Instead, the initial \x will be interpreted as a basic hexadecimal
escape, with no following digits, giving a character whose value is zero.
If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is as just described only when it
is followed by two hexadecimal digits. Otherwise, it matches a literal "x" character. In JavaScript
mode, support for code points greater than 256 is provided by \u, which must be followed by four
hexadecimal digits; otherwise it matches a literal "u" character. Character codes specified by \u in
JavaScript mode are constrained in the same was as those specified by \x in non-JavaScript mode.
Characters whose value is less than 256 can be defined by either of the two syntaxes for \x (or by \u in
JavaScript mode). There is no difference in the way they are handled. For example, \xdc is exactly the
same as \x{dc} (or \u00dc in JavaScript mode).
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\07 specifies two binary zeros followed by a BEL character (code
value 7). Make sure you supply two digits after the initial zero if the pattern character that follows is
itself an octal digit.
The handling of a backslash followed by a digit other than 0 is complicated. Outside a character class,
PCRE reads it and any following digits as a decimal number. If the number is less than 10, or if there
have been at least that many previous capturing left parentheses in the expression, the entire sequence
is taken as a 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 is greater than 9 and there have not been that many
capturing subpatterns, PCRE re-reads up to three octal digits following the backslash, and uses them to
generate a data character. Any subsequent digits stand for themselves. The value of the character is
constrained in the same way as characters specified in hexadecimal. For example:
\040 is another way of writing a 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 a binary zero
followed by the two characters "8" and "1"
Note that octal values of 100 or greater must not be introduced by a leading zero, because no more than
three octal digits are ever read.
All the sequences that define a single character value can be used both inside and outside character
classes. In addition, inside a character class, \b is interpreted as the backspace character (hex 08).
\N is not allowed in a character class. \B, \R, and \X are not special inside a character class. Like
other unrecognized escape sequences, they are treated as the literal characters "B", "R", and "X" by
default, but cause an error if the PCRE_EXTRA option is set. Outside a character class, these sequences
have different meanings.
Unsupported escape sequences
In Perl, the sequences \l, \L, \u, and \U are recognized by its string handler and used to modify the
case of following characters. By default, PCRE does not support these escape sequences. However, if the
PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and \u can be used to define a
character by code point, as described in the previous section.
Absolute and relative back references
The sequence \g followed by an unsigned or a negative number, optionally enclosed in braces, is an
absolute or relative back reference. A named back reference can be coded as \g{name}. Back references are
discussed later, following the discussion of parenthesized subpatterns.
Absolute and relative subroutine calls
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either
in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a
"subroutine". Details are discussed later. Note that \g{...} (Perl syntax) and \g<...> (Oniguruma
syntax) are not synonymous. The former is a back reference; the latter is a subroutine call.
Generic character types
Another use of backslash is for specifying generic character types:
\d any decimal digit
\D any character that is not a decimal digit
\h any horizontal white space character
\H any character that is not a horizontal white space character
\s any white space character
\S any character that is not a white space character
\v any vertical white space character
\V any character that is not a vertical white space character
\w any "word" character
\W any "non-word" character
There is also the single sequence \N, which matches a non-newline character. This is the same as the "."
metacharacter when PCRE_DOTALL is not set. Perl also uses \N to match characters by name; PCRE does not
support this.
Each pair of lower and upper case escape sequences partitions the complete set of characters into two
disjoint sets. Any given character matches one, and only one, of each pair. The sequences can appear both
inside and outside character classes. They each match one character of the appropriate type. If the
current matching point is at the end of the subject string, all of them fail, because there is no
character to match.
For compatibility with Perl, \s does not match the VT character (code 11). This makes it different from
the the POSIX "space" class. The \s characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
"use locale;" is included in a Perl script, \s may match the VT character. In PCRE, it never does.
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 128 are used for accented letters, and these are then matched by \w. The use of locales with Unicode
is discouraged.
By default, in a UTF mode, characters with values greater than 128 never match \d, \s, or \w, and always
match \D, \S, and \W. These sequences retain their original meanings from before UTF support was
available, mainly for efficiency reasons. However, if PCRE is compiled with Unicode property support, and
the PCRE_UCP option is set, the behaviour is changed so that Unicode properties are used to determine
character types, as follows:
\d any character that \p{Nd} matches (decimal digit)
\s any character that \p{Z} matches, plus HT, LF, FF, CR
\w any character that \p{L} or \p{N} matches, plus underscore
The upper case escapes match the inverse sets of characters. Note that \d matches only decimal digits,
whereas \w matches any Unicode digit, as well as any Unicode letter, and underscore. Note also that
PCRE_UCP affects \b, and \B because they are defined in terms of \w and \W. Matching these sequences is
noticeably slower when PCRE_UCP is set.
The sequences \h, \H, \v, and \V are features that were added to Perl at release 5.10. In contrast to the
other sequences, which match only ASCII characters by default, these always match certain high-valued
codepoints, whether or not PCRE_UCP is set. The horizontal space characters are:
U+0009 Horizontal tab
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
U+000B Vertical tab
U+000C Form feed
U+000D Carriage return
U+0085 Next line
U+2028 Line separator
U+2029 Paragraph separator
In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are relevant.
Newline sequences
Outside a character class, by default, the escape sequence \R matches any Unicode newline sequence. In
8-bit non-UTF-8 mode \R is equivalent to the following:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details of which are given below. This particular group matches
either the two-character sequence CR followed by LF, or one of the single characters LF (linefeed,
U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
line, U+0085). The two-character sequence is treated as a single unit that cannot be split.
In other modes, two additional characters whose codepoints are greater than 255 are added: LS (line
separator, U+2028) and PS (paragraph separator, U+2029). Unicode character property support is not
needed for these characters to be recognized.
It is possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line
endings) by setting the option PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
(BSR is an abbrevation for "backslash R".) This can be made the default when PCRE is built; if this is
the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. It is also possible to
specify these settings by starting a pattern string with one of the following sequences:
(*BSR_ANYCRLF) CR, LF, or CRLF only
(*BSR_UNICODE) any Unicode newline sequence
These override the default and the options given to the compiling function, but they can themselves be
overridden by options given to a matching function. Note that these special settings, which are not Perl-
compatible, are recognized only at the very start of a pattern, and that they must be in upper case. If
more than one of them is present, the last one is used. They can be combined with a change of newline
convention; for example, a pattern can start with:
(*ANY)(*BSR_ANYCRLF)
They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special sequences. Inside a character
class, \R is treated as an unrecognized escape sequence, and so matches the letter "R" by default, but
causes an error if PCRE_EXTRA is set.
Unicode character properties
When PCRE is built with Unicode character property support, three additional escape sequences that match
characters with specific properties are available. When in 8-bit non-UTF-8 mode, these sequences are of
course limited to testing characters whose codepoints are less than 256, but they do work in this mode.
The extra escape sequences are:
\p{xx} a character with the xx property
\P{xx} a character without the xx property
\X an extended Unicode sequence
The property names represented by xx above are limited to the Unicode script names, the general category
properties, "Any", which matches any character (including newline), and some special PCRE properties
(described in the next section). Other Perl properties such as "InMusicalSymbols" are not currently
supported by PCRE. Note that \P{Any} does not match any characters, so always causes a match failure.
Sets of Unicode characters are defined as belonging to certain scripts. A character from one of these
sets can be matched using a script name. For example:
\p{Greek}
\P{Han}
Those that are not part of an identified script are lumped together as "Common". The current list of
scripts is:
Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid,
Canadian_Aboriginal, Carian, Chakma, Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic,
Deseret, Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati,
Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi,
Inscriptional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao, Latin,
Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive,
Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, Old_Persian,
Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samaritan,
Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, Yi.
Each character has exactly one Unicode general category property, specified by a two-letter abbreviation.
For compatibility with Perl, negation can be specified by including a circumflex between the opening
brace and the property name. For example, \p{^Lu} is the same as \P{Lu}.
If only one letter is specified with \p or \P, it includes all the general category properties that start
with that letter. In this case, in the absence of negation, the curly brackets in the escape sequence are
optional; these two examples have the same effect:
\p{L}
\pL
The following general category property codes are supported:
C Other
Cc Control
Cf Format
Cn Unassigned
Co Private use
Cs Surrogate
L Letter
Ll Lower case letter
Lm Modifier letter
Lo Other letter
Lt Title case letter
Lu Upper case letter
M Mark
Mc Spacing mark
Me Enclosing mark
Mn Non-spacing mark
N Number
Nd Decimal number
Nl Letter number
No Other number
P Punctuation
Pc Connector punctuation
Pd Dash punctuation
Pe Close punctuation
Pf Final punctuation
Pi Initial punctuation
Po Other punctuation
Ps Open punctuation
S Symbol
Sc Currency symbol
Sk Modifier symbol
Sm Mathematical symbol
So Other symbol
Z Separator
Zl Line separator
Zp Paragraph separator
Zs Space separator
The special property L& is also supported: it matches a character that has the Lu, Ll, or Lt property, in
other words, a letter that is not classified as a modifier or "other".
The Cs (Surrogate) property applies only to characters in the range U+D800 to U+DFFF. Such characters are
not valid in Unicode strings and so cannot be tested by PCRE, unless UTF validity checking has been
turned off (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the pcreapi page). Perl
does not support the Cs property.
The long synonyms for property names that Perl supports (such as \p{Letter}) are not supported by PCRE,
nor is it permitted to prefix any of these properties with "Is".
No character that is in the Unicode table has the Cn (unassigned) property. Instead, this property is
assumed for any code point that is not in the Unicode table.
Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} always matches
only upper case letters.
The \X escape matches any number of Unicode characters that form an extended Unicode sequence. \X is
equivalent to
(?>\PM\pM*)
That is, it matches a character without the "mark" property, followed by zero or more characters with the
"mark" property, and treats the sequence as an atomic group (see below). Characters with the "mark"
property are typically accents that affect the preceding character. None of them have codepoints less
than 256, so in 8-bit non-UTF-8 mode \X matches any one character.
Note that recent versions of Perl have changed \X to match what Unicode calls an "extended grapheme
cluster", which has a more complicated definition.
Matching characters by Unicode property is not fast, because PCRE has to search a structure that contains
data for over fifteen thousand characters. That is why the traditional escape sequences such as \d and \w
do not use Unicode properties in PCRE by default, though you can make them do so by setting the PCRE_UCP
option or by starting the pattern with (*UCP).
PCRE's additional properties
As well as the standard Unicode properties described in the previous section, PCRE supports four more
that make it possible to convert traditional escape sequences such as \w and \s and POSIX character
classes to use Unicode properties. PCRE uses these non-standard, non-Perl properties internally when
PCRE_UCP is set. They are:
Xan Any alphanumeric character
Xps Any POSIX space character
Xsp Any Perl space character
Xwd Any Perl "word" character
Xan matches characters that have either the L (letter) or the N (number) property. Xps matches the
characters tab, linefeed, vertical tab, form feed, or carriage return, and any other character that has
the Z (separator) property. Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches
the same characters as Xan, plus underscore.
Resetting the match start
The escape sequence \K causes any previously matched characters not to be included in the final matched
sequence. For example, the pattern:
foo\Kbar
matches "foobar", but reports that it has matched "bar". This feature is similar to a lookbehind
assertion (described below). However, in this case, the part of the subject before the real match does
not have to be of fixed length, as lookbehind assertions do. The use of \K does not interfere with the
setting of captured substrings. For example, when the pattern
(foo)\Kbar
matches "foobar", the first substring is still set to "foo".
Perl documents that the use of \K within assertions is "not well defined". In PCRE, \K is acted upon when
it occurs inside positive assertions, but is ignored in negative assertions.
Simple assertions
The final use of backslash is for certain simple assertions. An assertion specifies a condition that has
to be met at a particular point in a match, without consuming any characters from the subject string. The
use of subpatterns for more complicated assertions is described below. The backslashed assertions are:
\b matches at a word boundary
\B matches when not at a word boundary
\A matches at the start of the subject
\Z matches at the end of the subject
also matches before a newline at the end of the subject
\z matches only at the end of the subject
\G matches at the first matching position in the subject
Inside a character class, \b has a different meaning; it matches the backspace character. If any other of
these assertions appears in a character class, by default it matches the corresponding literal character
(for example, \B matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid escape
sequence" error is generated instead.
A word boundary is a position in the subject string where the current character and the previous
character do not both match \w or \W (i.e. one matches \w and the other matches \W), or the start or end
of the string if the first or last character matches \w, respectively. In a UTF mode, the meanings of \w
and \W can be changed by setting the PCRE_UCP option. When this is done, it also affects \b and \B.
Neither PCRE nor Perl has a separate "start of word" or "end of word" metasequence. However, whatever
follows \b normally determines which it is. For example, the fragment \ba matches "a" at the start of a
word.
The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described in the next
section) in that they only ever match at the very start and end of the subject string, whatever options
are set. Thus, they are independent of multiline mode. These three assertions are not affected by the
PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the circumflex and dollar
metacharacters. However, if the startoffset argument of pcre_exec() is non-zero, indicating that matching
is to start at a point other than the beginning of the subject, \A can never match. The difference
between \Z and \z is that \Z matches before a newline at the end of the string as well as at the very
end, whereas \z matches only at the end.
The \G assertion is true only when the current matching position is at the start point of the match, as
specified by the startoffset argument of pcre_exec(). It differs from \A when the value of startoffset is
non-zero. By calling pcre_exec() multiple times with appropriate arguments, you can mimic Perl's /g
option, and it is in this kind of implementation where \G can be useful.
Note, however, that PCRE's interpretation of \G, as the start of the current match, is subtly different
from Perl's, which defines it as the end of the previous match. In Perl, these can be different when the
previously matched string was empty. Because PCRE does just one match at a time, it cannot reproduce this
behaviour.
If all the alternatives of a pattern begin with \G, the expression is anchored to the starting match
position, and the "anchored" flag is set in the compiled regular expression.
CIRCUMFLEX AND DOLLAR
Outside a character class, in the default matching mode, the circumflex character is an assertion that is
true only if the current matching point is at the start of the subject string. If the startoffset
argument of pcre_exec() is non-zero, circumflex can never match if the PCRE_MULTILINE option is unset.
Inside a character class, circumflex has an entirely different meaning (see below).
Circumflex need not be the first character of the pattern if a number of alternatives are involved, but
it should be the first thing in each alternative in which it appears if the pattern is ever to match that
branch. If all possible alternatives start with a circumflex, that is, if the pattern is constrained to
match only at the start of the subject, it is said to be an "anchored" pattern. (There are also other
constructs that can cause a pattern to be anchored.)
A dollar character is an assertion that is true only if the current matching point is at the end of the
subject string, or immediately before a newline at the end of the string (by default). Dollar need not be
the last character of the pattern if a number of alternatives are involved, but it should be the last
item in any branch in which it appears. Dollar has no special meaning in a character class.
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 character in the subject string except
(by default) a character that signifies the end of a line.
When a line ending is defined as a single character, dot never matches that character; when the two-
character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, but otherwise
it matches all characters (including isolated CRs and LFs). When any Unicode line endings are being
recognized, dot does not match CR or LF or any of the other line ending characters.
The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL option is set, a dot
matches any one character, without exception. If the two-character sequence CRLF is present in the
subject string, it takes two dots to match it.
The handling of dot is entirely independent of the handling of circumflex and dollar, the only
relationship being that they both involve newlines. Dot has no special meaning in a character class.
The escape sequence \N behaves like a dot, except that it is not affected by the PCRE_DOTALL option. In
other words, it matches any character except one that signifies the end of a line. Perl also uses \N to
match characters by name; PCRE does not support this.
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. Unlike a
dot, \C always matches line-ending characters. The feature is provided in Perl in order to match
individual bytes in UTF-8 mode, but it is unclear how it can usefully be used. Because \C breaks up
characters into individual data units, matching one unit with \C in a UTF mode means that the rest of the
string may start with a malformed UTF character. This has undefined results, because PCRE assumes that it
is dealing with valid UTF strings (and by default it checks this at the start of processing unless the
PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option is used).
PCRE does not allow \C to appear in lookbehind assertions (described below) in a UTF mode, because this
would make it impossible to calculate the length of the lookbehind.
In general, the \C escape sequence is best avoided. However, one way of using it that avoids the problem
of malformed UTF characters is to use a lookahead to check the length of the next character, as in this
pattern, which could be used with a UTF-8 string (ignore white space and line breaks):
(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
A group that starts with (?| resets the capturing parentheses numbers in each alternative (see "Duplicate
Subpattern Numbers" below). The assertions at the start of each branch check the next UTF-8 character for
values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The character's individual bytes are then
captured by the appropriate number of groups.
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 special by default. However, if the PCRE_JAVASCRIPT_COMPAT option is
set, a lone closing square bracket causes a compile-time error. If a closing square bracket is required
as a member of the class, it should be the first data character in the class (after an initial
circumflex, if present) or escaped with a backslash.
A character class matches a single character in the subject. In a UTF mode, the character may be more
than one data unit long. A matched character must be in the set of characters defined by the class,
unless the first character in the class definition is a circumflex, in which case the subject character
must not be in the set defined by the class. If a circumflex is actually required as a member of the
class, ensure it is not the first character, or escape it with a backslash.
For example, the character class [aeiou] matches any lower case vowel, while [^aeiou] matches any
character that is not a lower case vowel. Note that a circumflex is just a convenient notation for
specifying the characters that are in the class by enumerating those that are not. A class that starts
with a circumflex is not an assertion; it still consumes a character from the subject string, and
therefore it fails if the current pointer is at the end of the string.
In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be included in a class as a
literal string of data units, or by using the \x{ escaping mechanism.
When caseless matching is set, any letters in a class represent both their upper case and lower case
versions, so for example, a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
match "A", whereas a caseful version would. In a UTF mode, PCRE always understands the concept of case
for characters whose values are less than 128, so caseless matching is always possible. For characters
with higher values, the concept of case is supported if PCRE is compiled with Unicode property support,
but not otherwise. If you want to use caseless matching in a UTF mode for characters 128 and above, you
must ensure that PCRE is compiled with Unicode property support as well as with UTF support.
Characters that might indicate line breaks are never treated in any special way when matching character
classes, whatever line-ending sequence is in use, and whatever setting of the PCRE_DOTALL and
PCRE_MULTILINE options is used. A class such as [^a] always matches one of these characters.
The minus (hyphen) character can be used to specify a range of characters in a character class. For
example, [d-m] matches any letter between d and m, inclusive. If a minus character is required in a
class, it must be escaped with a backslash or appear in a position where it cannot be interpreted as
indicating a range, typically as the first or last character in the class.
It is not possible to have the literal character "]" as the end character of a range. A pattern such as
[W-]46] is interpreted as a class of two characters ("W" and "-") followed by a literal string "46]", so
it would match "W46]" or "-46]". However, if the "]" is escaped with a backslash it is interpreted as the
end of range, so [W-\]46] is interpreted as a class containing a range followed by two other characters.
The octal or hexadecimal representation of "]" can also be used to end a range.
Ranges operate in the collating sequence of character values. They can also be used for characters
specified numerically, for example [\000-\037]. Ranges can include any characters that are valid for the
current mode.
If a range that includes letters is used when caseless matching is set, it matches the letters in either
case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
character tables for a French locale are in use, [\xc8-\xcb] matches accented E characters in both cases.
In UTF modes, PCRE supports the concept of case for characters with values greater than 128 only when it
is compiled with Unicode property support.
The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W may appear in a
character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any
hexadecimal digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w and their upper
case partners, just as it does when they appear outside a character class, as described in the section
entitled "Generic character types" above. The escape sequence \b has a different meaning inside a
character class; it matches the backspace character. The sequences \B, \N, \R, and \X are not special
inside a character class. Like any other unrecognized escape sequences, they are treated as the literal
characters "B", "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is set.
A circumflex can conveniently be used with the upper case character types to specify a more restricted
set of characters than the matching lower case type. For example, the class [^\W_] matches any letter or
digit, but not underscore, whereas [\w] includes underscore. A positive character class should be read as
"something OR something OR ..." and a negative class as "NOT something AND NOT something AND NOT ...".
The only metacharacters that are recognized in character classes are backslash, hyphen (only where it can
be interpreted as specifying a range), circumflex (only at the start), opening square bracket (only when
it can be interpreted as introducing a POSIX class name - see the next section), and the terminating
closing square bracket. However, escaping other non-alphanumeric characters does no harm.
POSIX CHARACTER CLASSES
Perl supports the POSIX notation for character classes. This uses names enclosed by [: and :] within the
enclosing square brackets. PCRE also supports this notation. For example,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The 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 (not quite the same as \s)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32). Notice that this
list includes the VT character (code 11). This makes "space" different to \s, which does not include VT
(for Perl compatibility).
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, in UTF modes, characters with values greater than 128 do not match any of the POSIX character
classes. However, if the PCRE_UCP option is passed to pcre_compile(), some of the classes are changed so
that Unicode character properties are used. This is achieved by replacing the 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. The other POSIX classes are unchanged, and
match only characters with code points less than 128.
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 possible to unset these options by
preceding the letter with a hyphen, and a combined setting and unsetting such as (?im-sx), which sets
PCRE_CASELESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also permitted. If a
letter appears both before and after the hyphen, the option is unset.
The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be changed in the same way as
the Perl-compatible options by using the characters J, U and X respectively.
When one of these option changes occurs at top level (that is, not inside subpattern parentheses), the
change applies to the remainder of the pattern that follows. If the change is placed right at the start
of a pattern, PCRE extracts it into the global options (and it will therefore show up in data extracted
by the pcre_fullinfo() function).
An option change within a subpattern (see below for a description of subpatterns) affects only that part
of the subpattern that follows it, so
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). By this means, options
can be made to have different settings in different parts of the pattern. Any changes made in one
alternative do carry on into subsequent branches within the same subpattern. For example,
(a(?i)b|c)
matches "ab", "aB", "c", and "C", even though when matching "C" the first branch is abandoned before the
option setting. This is because the effects of option settings happen at compile time. There would be
some very weird behaviour otherwise.
Note: There are other PCRE-specific options that can be set by the application when the compiling or
matching functions are called. In some cases the pattern can contain special leading sequences such as
(*CRLF) to override what the application has set or what has been defaulted. Details are given in the
section entitled "Newline sequences" above. There are also the (*UTF8), (*UTF16), and (*UCP) leading
sequences that can be used to set UTF and Unicode property modes; they are equivalent to setting the
PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
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 argument of the matching function. (This applies only to the traditional matching functions; the
DFA matching functions do not support capturing.)
Opening parentheses are counted from left to right (starting from 1) to obtain numbers for the capturing
subpatterns. For example, if the string "the red king" is matched against the pattern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king", and are numbered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not always helpful. There are often times when a
grouping subpattern is required without a capturing requirement. If an opening parenthesis is followed by
a question mark and a colon, the subpattern does not do any capturing, and is not counted when computing
the number of any subsequent capturing subpatterns. For example, if the string "the white queen" is
matched against the pattern
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and are numbered 1 and 2. The maximum number of
capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the start of a non-capturing
subpattern, the option letters may appear between the "?" and the ":". Thus the two patterns
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative branches are tried from left to right, and
options are not reset until the end of the subpattern is reached, an option setting in one branch does
affect subsequent branches, so the above patterns match "SUNDAY" as well as "Saturday".
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 capturing parentheses are numbered one.
Thus, when the pattern matches, you can look at captured substring number one, whichever alternative
matched. This construct is useful when you want to capture part, but not all, of one of a number of
alternatives. Inside a (?| group, parentheses are numbered as usual, but the number is reset at the start
of each branch. The numbers of any capturing parentheses that follow the subpattern start after the
highest number used in any branch. The following example is taken from the Perl documentation. The
numbers underneath show in which buffer the captured content will be stored.
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
A back reference to a numbered subpattern uses the most recent value that is set for that number by any
subpattern. The following pattern matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
In contrast, a subroutine call to a numbered subpattern always refers to the first one in the pattern
with the given number. The following pattern matches "abcabc" or "defabc":
/(?|(abc)|(def))(?1)/
If a condition test for a subpattern's having matched refers to a non-unique number, the test is true if
any of the subpatterns of that number have matched.
An alternative approach to using this "branch reset" feature is to use duplicate named subpatterns, as
described in the next section.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be very hard to keep track of the
numbers in complicated regular expressions. Furthermore, if an expression is modified, the numbers may
change. To help with this difficulty, PCRE supports the naming of subpatterns. This feature was not added
to Perl until release 5.10. Python had the feature earlier, and PCRE introduced it at release 4.0, using
the Python syntax. PCRE now supports both the Perl and the Python syntax. Perl allows identically
numbered subpatterns to have different names, but PCRE does not.
In PCRE, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as in Perl, or
(?P<name>...) as in Python. References to capturing parentheses from other parts of the pattern, such as
back references, recursion, and conditions, can be made by name as well as by number.
Names consist of up to 32 alphanumeric characters and underscores. Named capturing parentheses are still
allocated numbers as well as names, exactly as if the names were not present. The PCRE API provides
function calls for extracting the name-to-number translation table from a compiled pattern. There is also
a convenience function for extracting a captured substring by name.
By default, a name must be unique within a pattern, but it is possible to relax this constraint by
setting the PCRE_DUPNAMES option at compile time. (Duplicate names are also always permitted for
subpatterns with the same number, set up as described in the previous section.) Duplicate names can be
useful for patterns where only one instance of the named parentheses can match. Suppose you want to match
the name of a weekday, either as a 3-letter abbreviation or as the full name, and in both cases you want
to extract the abbreviation. This pattern (ignoring the line breaks) does the job:
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
There are five capturing substrings, but only one is ever set after a match. (An alternative way of
solving this problem is to use a "branch reset" subpattern, as described in the previous section.)
The convenience function for extracting the data by name returns the substring for the first (and in this
example, the only) subpattern of that name that matched. This saves searching to find which numbered
subpattern it was.
If you make a back reference to a non-unique named subpattern from elsewhere in the pattern, the one that
corresponds to the first occurrence of the name is used. In the absence of duplicate numbers (see the
previous section) this is the one with the lowest number. If you use a named reference in a condition
test (see the section about conditions below), either to check whether a subpattern has matched, or to
check for recursion, all subpatterns with the same name are tested. If the condition is true for any one
of them, the overall condition is true. This is the same behaviour as testing by number. For further
details of the interfaces for handling named subpatterns, see the pcreapi documentation.
Warning: You cannot use different names to distinguish between two subpatterns with the same number
because PCRE uses only the numbers when matching. For this reason, an error is given at compile time if
different names are given to subpatterns with the same number. However, you can 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 following items:
a literal data character
the dot metacharacter
the \C escape sequence
the \X escape sequence
the \R escape sequence
an escape such as \d or \pL that matches a single character
a character class
a back reference (see next section)
a parenthesized subpattern (including assertions)
a subroutine call to a subpattern (recursive or otherwise)
The general repetition quantifier specifies a minimum and maximum number of permitted matches, by giving
the two numbers in curly brackets (braces), separated by a comma. The numbers must be less than 65536,
and the first must be less than or equal to the second. For example:
z{2,4}
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special character. If the second
number is omitted, but the comma is present, there is no upper limit; if the second number and the comma
are both omitted, the quantifier specifies an exact number of required matches. Thus
[aeiou]{3,}
matches at least 3 successive vowels, but may match many more, while
\d{8}
matches exactly 8 digits. An opening curly bracket that appears in a position where a quantifier is not
allowed, or one that does not match the syntax of a quantifier, is taken as a literal character. For
example, {,6} is not a quantifier, but a literal string of four characters.
In UTF modes, quantifiers apply to characters rather than to individual data units. Thus, for example,
\x{100}{2} matches two characters, each of which is represented by a two-byte sequence in a UTF-8 string.
Similarly, \X{3} matches three Unicode extended sequences, each of which may be several data units long
(and they may be of different lengths).
The quantifier {0} is permitted, causing the expression to behave as if the previous item and the
quantifier were not present. This may be useful for subpatterns that are referenced as subroutines from
elsewhere in the pattern (but see also the section entitled "Defining subpatterns for use by reference
only" below). Items other than subpatterns that have a {0} quantifier are omitted from the compiled
pattern.
For convenience, the three most common quantifiers have single-character abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
It is possible to construct infinite loops by following a subpattern that can match no characters with a
quantifier that has no upper limit, for example:
(a?)*
Earlier versions of Perl and PCRE used to give an error at compile time for such patterns. However,
because there are cases where this can be useful, such patterns are now accepted, but if any repetition
of the subpattern does in fact match no characters, the loop is forcibly broken.
By default, the quantifiers are "greedy", that is, they match as much as possible (up to the maximum
number of permitted times), without causing the rest of the pattern to fail. The classic example of where
this gives problems is in trying to match comments in C programs. These appear between /* and */ and
within the comment, individual * and / characters may appear. An attempt to match C comments by applying
the pattern
/\*.*\*/
to the string
/* first comment */ not comment /* second comment */
fails, because it matches the entire string owing to the greediness of the .* item.
However, if a quantifier is followed by a question mark, it ceases to be greedy, and instead matches the
minimum number of times possible, so the pattern
/\*.*?\*/
does the right thing with the C comments. The meaning of the various quantifiers is not otherwise
changed, just the preferred number of matches. Do not confuse this use of question mark with its use as
a quantifier in its own right. Because it has two uses, it can sometimes appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that is the only way the rest of the pattern
matches.
If the PCRE_UNGREEDY option is set (an option that is not available in Perl), the quantifiers are not
greedy by default, but individual ones can be made greedy by following them with a question mark. In
other words, it inverts the default behaviour.
When a parenthesized subpattern is quantified with a minimum repeat count that is greater than 1 or with
a limited maximum, more memory is required for the compiled pattern, in proportion to the size of the
minimum or maximum.
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent to Perl's /s) is set, thus
allowing the dot to match newlines, the pattern is implicitly anchored, because whatever follows will be
tried against every character position in the subject string, so there is no point in retrying the
overall match at any position after the first. PCRE normally treats such a pattern as though it were
preceded by \A.
In cases where it is known that the subject string contains no newlines, it is worth setting PCRE_DOTALL
in order to obtain this optimization, or alternatively using ^ to indicate anchoring explicitly.
However, there is one situation where the optimization cannot be used. When .* is inside capturing
parentheses that are the subject of a back reference elsewhere in the pattern, a match at the start may
fail where a later one succeeds. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the fourth character. For this reason, such a pattern
is not implicitly anchored.
When a capturing subpattern is repeated, the value captured is the substring that matched the final
iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the captured substring is "tweedledee". However, if
there are nested capturing subpatterns, the corresponding captured values may have been set in previous
iterations. For example, after
/(a|(b))+/
matches "aba" the value of the second captured substring is "b".
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, however, works as normal.
An alternative description is that a subpattern of this type matches the string of characters that an
identical standalone pattern would match, if anchored at the current point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as the above example can be
thought of as a maximizing repeat that must swallow everything it can. So, while both \d+ and \d+? are
prepared to adjust the number of digits they match in order to make the rest of the pattern match,
(?>\d+) can only match an entire sequence of digits.
Atomic groups in general can of course contain arbitrarily complicated subpatterns, and can be nested.
However, when the subpattern for an atomic group is just a single repeated item, as in the example above,
a simpler notation, called a "possessive quantifier" can be used. This consists of an additional +
character following a quantifier. Using this notation, the previous example can be rewritten as
\d++foo
Note that a possessive quantifier can be used with an entire group, for example:
(abc|xyz){2,3}+
Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY option is ignored. They are a
convenient notation for the simpler forms of atomic group. However, there is no difference in the meaning
of a possessive quantifier and the equivalent atomic group, though there may be a performance difference;
possessive quantifiers should be slightly faster.
The possessive quantifier syntax is an extension to the Perl 5.8 syntax. Jeffrey Friedl originated the
idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he
built Sun's Java package, and PCRE copied it from there. It ultimately found its way into Perl at release
5.10.
PCRE has an optimization that automatically "possessifies" certain simple pattern constructs. For
example, the sequence A+B is treated as A++B because there is no point in backtracking into a sequence of
A's when B must follow.
When a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited
number of times, the use of an atomic group is the only way to avoid some failing matches taking a very
long time indeed. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of non-digits, or digits enclosed in <>,
followed by either ! or ?. When it matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because the string can be divided between the
internal \D+ repeat and the external * repeat in a large number of ways, and all have to be tried. (The
example uses [!?] rather than a single character at the end, because both PCRE and Perl have an
optimization that allows for fast failure when a single character is used. They remember the last single
character that is required for a match, and fail early if it is not present in the string.) If the
pattern is changed so that it uses an atomic group, like this:
((?>\D+)|<\d+>)*[!?]
sequences of non-digits cannot be broken, and failure happens quickly.
BACK REFERENCES
Outside a character class, a backslash followed by a digit greater than 0 (and possibly further digits)
is a back reference to a capturing subpattern earlier (that is, to its left) in the pattern, provided
there have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is less than 10, it is always taken as a back
reference, and causes an error only if there are not that many capturing left parentheses in the entire
pattern. In other words, the parentheses that are referenced need not be to the left of the reference for
numbers less than 10. A "forward back reference" of this type can make sense when a repetition is
involved and the subpattern to the right has participated in an earlier iteration.
It is not possible to have a numerical "forward back reference" to a subpattern whose number is 10 or
more using this syntax because a sequence such as \50 is interpreted as a character defined in octal. See
the subsection entitled "Non-printing characters" above for further details of the handling of digits
following a backslash. There is no such problem when named parentheses are used. A back reference to any
subpattern is possible using named parentheses (see below).
Another way of avoiding the ambiguity inherent in the use of digits following a backslash is to use the
\g escape sequence. This escape must be followed by an unsigned number or a negative number, optionally
enclosed in braces. These examples are all identical:
(ring), \1
(ring), \g1
(ring), \g{1}
An unsigned number specifies an absolute reference without the ambiguity that is present in the older
syntax. It is also useful when literal digits follow the reference. A negative number is a relative
reference. Consider this example:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the most recently started capturing subpattern before \g, that is,
is it equivalent to \2 in this example. Similarly, \g{-2} would be equivalent to \1. The use of relative
references can be helpful in long patterns, and also in patterns that are created by joining together
fragments that contain references within themselves.
A back reference matches whatever actually matched the capturing subpattern in the current subject
string, rather than anything matching the subpattern itself (see "Subpatterns as subroutines" below for a
way of doing that). So the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If
caseful matching is in force at the time of the back reference, the case of letters is relevant. For
example,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original capturing subpattern is
matched caselessly.
There are several different ways of writing back references to named subpatterns. The .NET syntax
\k{name} and the Perl syntax \k<name> or \k'name' are supported, as is the Python syntax (?P=name). Perl
5.10's unified back reference syntax, in which \g can be used for both numeric and named references, is
also supported. We could rewrite the above example in any of the following ways:
(?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
A subpattern that is referenced by name may appear in the pattern before or after the reference.
There may be more than one back reference to the same subpattern. If a subpattern has not actually been
used in a particular match, any back references to it always fail by default. For example, the pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc". However, if the PCRE_JAVASCRIPT_COMPAT option is
set at compile time, a back reference to an unset value matches an empty string.
Because there may be many capturing parentheses in a pattern, all digits following a backslash are taken
as part of a potential back reference number. If the pattern continues with a digit character, some
delimiter must be used to terminate the back reference. If the PCRE_EXTENDED option is set, this can be
white space. Otherwise, the \g{ syntax or an empty comment (see "Comments" below) can be used.
Recursive back references
A back reference that occurs inside the parentheses to which it refers fails when the subpattern is first
used, so, for example, (a\1) never matches. However, such references can be useful inside repeated
subpatterns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of the subpattern, the back
reference matches the character string corresponding to the previous iteration. In order for this to
work, the pattern must be such that the first iteration does not need to match the back reference. This
can be done using alternation, as in the example above, or by a quantifier with a minimum of zero.
Back references of this type cause the group that they reference to be treated as an atomic group. Once
the whole group has been matched, a subsequent matching failure cannot cause backtracking into the middle
of the group.
ASSERTIONS
An assertion is a test on the characters following or preceding the current matching point that does not
actually consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
described above.
More complicated assertions are coded as subpatterns. There are two kinds: those that look ahead of the
current position in the subject string, and those that look behind it. An assertion subpattern is matched
in the normal way, except that it does not cause the current matching position to be changed.
Assertion subpatterns are not capturing subpatterns. If such an assertion contains capturing subpatterns
within it, these are counted for the purposes of numbering the capturing subpatterns in the whole
pattern. However, substring capturing is carried out only for positive assertions, because it does not
make sense for 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 semicolon in the match, and
foo(?!bar)
matches any occurrence of "foo" that is not followed by "bar". Note that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something other than "foo"; it finds any
occurrence of "bar" whatsoever, because the assertion (?!foo) is always true when the next three
characters are "bar". A lookbehind assertion is needed to achieve the other effect.
If you want to force a matching failure at some point in a pattern, the most convenient way to do it is
with (?!) because an empty string always matches, so an assertion that requires there not to be an empty
string must always fail. The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
Lookbehind assertions
Lookbehind assertions start with (?<= for positive assertions and (?<! for negative assertions. For
example,
(?<!foo)bar
does find an occurrence of "bar" that is not preceded by "foo". The contents of a lookbehind assertion
are restricted such that all the strings it matches must have a fixed length. However, if there are
several top-level alternatives, they do not all have to have the same fixed length. Thus
(?<=bullock|donkey)
is permitted, but
(?<!dogs?|cats?)
causes an error at compile time. Branches that match different length strings are permitted only at the
top level of a lookbehind assertion. This is an extension compared with Perl, which requires all branches
to match the same length of string. An assertion such as
(?<=ab(c|de))
is not permitted, because its single top-level branch can match two different lengths, but it is
acceptable to PCRE if rewritten to use two top-level branches:
(?<=abc|abde)
In some cases, the escape sequence \K (see above) can be used instead of a lookbehind assertion to get
round the fixed-length restriction.
The implementation of lookbehind assertions is, for each alternative, to temporarily move the current
position back by the fixed length and then try to match. If there are insufficient characters before the
current position, the assertion fails.
In a UTF mode, PCRE does not allow the \C escape (which matches a single data unit even in a UTF mode) to
appear in lookbehind assertions, because it makes it impossible to calculate the length of the
lookbehind. The \X and \R escapes, which can match different numbers of data units, are also not
permitted.
"Subroutine" calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long as the
subpattern matches a fixed-length string. Recursion, however, is not supported.
Possessive quantifiers can be used in conjunction with lookbehind assertions to specify efficient
matching of fixed-length strings at the end of subject strings. Consider a simple pattern such as
abcd$
when applied to a long string that does not match. Because matching proceeds from left to right, PCRE
will look for each "a" in the subject and then see if what follows matches the rest of the pattern. If
the pattern is specified as
^.*abcd$
the initial .* matches the entire string at first, but when this fails (because there is no following
"a"), it backtracks to match all but the last character, then all but the last two characters, and so on.
Once again the search for "a" covers the entire string, from right to left, so we are no better off.
However, if the pattern is written as
^.*+(?<=abcd)
there can be no backtracking for the .*+ item; it can match only the entire string. The subsequent
lookbehind assertion does a single test on the last four characters. If it fails, the match fails
immediately. For long strings, this approach makes a significant difference to the processing time.
Using multiple assertions
Several assertions (of any sort) may occur in succession. For example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999". Notice that each of the assertions is applied
independently at the same point in the subject string. First there is a check that the previous three
characters are all digits, and then there is a check that the same three characters are not "999". This
pattern does not match "foo" preceded by six characters, the first of which are digits and the last three
of which are not "999". For example, it doesn't match "123abcfoo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters, checking that the first three are
digits, and then the second assertion checks that the preceding three characters are not "999".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar" which in turn is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits and any three characters that are not
"999".
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern conditionally or to choose between two
alternative subpatterns, depending on the result of an assertion, or whether a specific capturing
subpattern has already been matched. The two possible forms of conditional subpattern are:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used; otherwise the no-pattern (if present) is used. If
there are more than two alternatives in the subpattern, a compile-time error occurs. Each of the two
alternatives may itself contain nested subpatterns of any form, including conditional subpatterns; the
restriction to two alternatives applies only at the level of the condition. This pattern fragment is an
example where the alternatives are complex:
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
There are four kinds of condition: references to subpatterns, references to recursion, a pseudo-condition
called DEFINE, and assertions.
Checking for a used subpattern by number
If the text between the parentheses consists of a sequence of digits, the condition is true if a
capturing subpattern of that number has previously matched. If there is more than one capturing
subpattern with the same number (see the earlier section about duplicate subpattern numbers), the
condition is true if any of them have matched. An alternative notation is to precede the digits with a
plus or minus sign. In this case, the subpattern number is relative rather than absolute. The most
recently opened parentheses can be referenced by (?(-1), the next most recent by (?(-2), and so on.
Inside loops it can also make sense to refer to subsequent groups. The next parentheses to be opened can
be referenced as (?(+1), and so on. (The value zero in any of these forms is not used; it provokes a
compile-time error.)
Consider the following pattern, which contains non-significant white space to make it more readable
(assume the PCRE_EXTENDED option) and to divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that character is present, sets it as the
first captured substring. The second part matches one or more characters that are not parentheses. The
third part is a conditional subpattern that tests whether or not the first set of parentheses matched. If
they did, that is, if subject started with an opening parenthesis, the condition is true, and so the yes-
pattern is executed and a closing parenthesis is required. Otherwise, since no-pattern is not present,
the subpattern matches nothing. In other words, this pattern matches a sequence of non-parentheses,
optionally enclosed in parentheses.
If you were embedding this pattern in a larger one, you could use a relative reference:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
This makes the fragment independent of the parentheses in the larger pattern.
Checking for a used subpattern by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used subpattern by name. For
compatibility with earlier versions of PCRE, which had this facility before Perl, the syntax (?(name)...)
is also recognized. However, there is a possible ambiguity with this syntax, because subpattern names may
consist entirely of digits. PCRE looks first for a named subpattern; if it cannot find one and the name
consists entirely of digits, PCRE looks for a subpattern of that number, which must be greater than zero.
Using subpattern names that consist entirely of digits is not recommended.
Rewriting the above example to use a named subpattern gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of
the same name, and is true if any one of them has matched.
Checking for pattern recursion
If the condition is the string (R), and there is no subpattern with the name R, the condition is true if
a recursive call to the whole pattern or any subpattern has been made. If digits or a name preceded by
ampersand follow the letter R, for example:
(?(R3)...) or (?(R&name)...)
the condition is true if the most recent recursion is into a subpattern whose number or name is given.
This condition does not check the entire recursion stack. If the name used in a condition of this kind is
a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them is
the most recent recursion.
At "top level", all these recursion test conditions are false. The syntax for recursive patterns is
described below.
Defining subpatterns for use by reference only
If the condition is the string (DEFINE), and there is no subpattern with the name DEFINE, the condition
is always false. In this case, there may be only one alternative in the subpattern. It is always skipped
if control reaches this point in the pattern; the idea of DEFINE is that it can be used to define
subroutines that can be referenced from elsewhere. (The use of subroutines is described below.) For
example, a pattern to match an IPv4 address such as "192.168.23.245" could be written like this (ignore
white space and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
The first part of the pattern is a DEFINE group inside which a another group named "byte" is defined.
This matches an individual component of an IPv4 address (a number less than 256). When matching takes
place, this part of the pattern is skipped because DEFINE acts like a false condition. The rest of the
pattern uses references to the named group to match the four dot-separated components of an IPv4 address,
insisting on a word boundary at each end.
Assertion conditions
If the condition is not in any of the above formats, it must be an assertion. This may be a positive or
negative lookahead or lookbehind assertion. Consider this pattern, again containing non-significant white
space, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed
by a letter. In other words, it tests for the presence of at least one letter in the subject. If a letter
is found, the subject is matched against the first alternative; otherwise it is matched against the
second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters
and dd are digits.
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 character class, nor in the middle of any other sequence of related
characters such as (?: or a subpattern name or number. The characters that make up a comment play no part
in the pattern matching.
The sequence (?# marks the start of a comment that continues up to the next closing parenthesis. Nested
parentheses are not permitted. If the PCRE_EXTENDED option is set, an unescaped # character also
introduces a comment, which in this case continues to immediately after the next newline character or
character sequence in the pattern. Which characters are interpreted as newlines is controlled by the
options passed to a compiling function or by a special sequence at the start of the pattern, as described
in the section entitled "Newline conventions" above. Note that the end of this type of comment is a
literal newline sequence in the pattern; escape sequences that happen to represent a newline do not
count. For example, consider this pattern when PCRE_EXTENDED is set, and the default newline convention
is in force:
abc #comment \n still comment
On encountering the # character, pcre_compile() skips along, looking for a newline in the pattern. The
sequence \n is still literal at this stage, so it does not terminate the comment. Only an actual
character with the code value 0x0a (the default newline) does so.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses.
Without the use of recursion, the best that can be done is to use a pattern that matches up to some fixed
depth of nesting. It is not possible to handle an arbitrary nesting depth.
For some time, Perl has provided a facility that allows regular expressions to recurse (amongst other
things). It does this by interpolating Perl code in the expression at run time, and the code can refer to
the expression itself. A Perl pattern using code interpolation to solve the parentheses problem can be
created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively to the pattern
in which it appears.
Obviously, PCRE cannot support the interpolation of Perl code. Instead, it supports special syntax for
recursion of the entire pattern, and also for individual subpattern recursion. After its introduction in
PCRE and Python, this kind of recursion was subsequently introduced into Perl at release 5.10.
A special item that consists of (? followed by a number greater than zero and a closing parenthesis is a
recursive subroutine call of the subpattern of the given number, provided that it occurs inside that
subpattern. (If not, it is a non-recursive subroutine call, which is described in the next section.) The
special item (?R) or (?0) is a recursive call of the entire regular expression.
This PCRE pattern solves the nested parentheses problem (assume the PCRE_EXTENDED option is set so that
white space is ignored):
\( ( [^()]++ | (?R) )* \)
First it matches an opening parenthesis. Then it matches any number of substrings which can either be a
sequence of non-parentheses, or a recursive match of the pattern itself (that is, a correctly
parenthesized substring). Finally there is a closing parenthesis. Note the use of a possessive
quantifier to avoid backtracking into sequences of non-parentheses.
If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you
could use this:
( \( ( [^()]++ | (?1) )* \) )
We have put the pattern into parentheses, and caused the recursion to refer to them instead of the whole
pattern.
In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made easier by the use
of relative references. Instead of (?1) in the pattern above you can write (?-2) to refer to the second
most recently opened parentheses preceding the recursion. In other words, a negative number counts
capturing parentheses leftwards from the point at which it is encountered.
It is also possible to refer to subsequently opened parentheses, by writing references such as (?+2).
However, these cannot be recursive because the reference is not inside the parentheses that are
referenced. They are always non-recursive subroutine calls, as described in the next section.
An alternative approach is to use named parentheses instead. The Perl syntax for this is (?&name); PCRE's
earlier syntax (?P>name) is also supported. We could rewrite the above example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
If there is more than one subpattern with the same name, the earliest one is used.
This particular example pattern that we have been looking at contains nested unlimited repeats, and so
the use of a possessive quantifier for matching strings of non-parentheses is important when applying the
pattern to strings that do not match. For example, when this pattern is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if a possessive quantifier is not used, the match runs for a very
long time indeed because there are so many different ways the + and * repeats can carve up the subject,
and all have to be tested before failure can be reported.
At the end of a match, the values of capturing parentheses are those from the outermost level. If you
want to obtain intermediate values, a callout function can be used (see below and the pcrecallout
documentation). If the pattern above is matched against
(ab(cd)ef)
the value for the inner capturing parentheses (numbered 2) is "ef", which is the last value taken on at
the top level. If a capturing subpattern is not matched at the top level, its final captured value is
unset, even if it was (temporarily) set at a deeper level during the matching process.
If there are more than 15 capturing parentheses in a pattern, PCRE has to obtain extra memory to store
data during a recursion, which it does by using pcre_malloc, freeing it via pcre_free afterwards. If no
memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
Do not confuse the (?R) item with the condition (R), which tests for recursion. Consider this pattern,
which matches text in angle brackets, allowing for arbitrary nesting. Only digits are allowed in nested
brackets (that is, when recursing), whereas any characters are permitted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpattern, with two different alternatives for the
recursive and non-recursive cases. The (?R) item is the actual recursive call.
Differences in recursion processing between PCRE and Perl
Recursion processing in PCRE differs from Perl in two important ways. In PCRE (like Python, but unlike
Perl), a recursive subpattern call is always treated as an atomic group. That is, once it has matched
some of the subject string, it is never re-entered, even if it contains untried alternatives and there is
a subsequent matching failure. This can be illustrated by the following pattern, which purports to match
a palindromic string that contains an odd number of characters (for example, "a", "aba", "abcba",
"abcdcba"):
^(.|(.)(?1)\2)$
The idea is that it either matches a single character, or two identical characters surrounding a sub-
palindrome. In Perl, this pattern works; in PCRE it does not if the pattern is longer than three
characters. Consider the subject string "abcba":
At the top level, the first character is matched, but as it is not at the end of the string, the first
alternative fails; the second alternative is taken and the recursion kicks in. The recursive call to
subpattern 1 successfully matches the next character ("b"). (Note that the beginning and end of line
tests are not part of the recursion).
Back at the top level, the next character ("c") is compared with what subpattern 2 matched, which was
"a". This fails. Because the recursion is treated as an atomic group, there are now no backtracking
points, and so the entire match fails. (Perl is able, at this point, to re-enter the recursion and try
the second alternative.) However, if the pattern is written with the alternatives in the other order,
things are different:
^((.)(?1)\2|.)$
This time, the recursing alternative is tried first, and continues to recurse until it runs out of
characters, at which point the recursion fails. But this time we do have another alternative to try at
the higher level. That is the big difference: in the previous case the remaining alternative is at a
deeper recursion level, which PCRE cannot use.
To change the pattern so that it matches all palindromic strings, not just those with an odd number of
characters, it is tempting to change the pattern to this:
^((.)(?1)\2|.?)$
Again, this works in Perl, but not in PCRE, and for the same reason. When a deeper recursion has matched
a single character, it cannot be entered again in order to match an empty string. The solution is to
separate the two cases, and write out the odd and even cases as alternatives at the higher level:
^(?:((.)(?1)\2|)|((.)(?3)\4|.))
If you want to match typical palindromic phrases, the pattern has to ignore all non-word characters,
which can be done like this:
^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
If run with the PCRE_CASELESS option, this pattern matches phrases such as "A man, a plan, a canal:
Panama!" and it works well in both PCRE and Perl. Note the use of the possessive quantifier *+ to avoid
backtracking into sequences of non-word characters. Without this, PCRE takes a great deal longer (ten
times or more) to match typical phrases, and Perl takes so long that you think it has gone into a loop.
WARNING: The palindrome-matching patterns above work only if the subject string does not start with a
palindrome that is shorter than the entire string. For example, although "abcba" is correctly matched,
if the subject is "ababa", PCRE finds the palindrome "aba" at the start, then fails at top level because
the end of the string does not follow. Once again, it cannot jump back into the recursion to try other
alternatives, so the entire match fails.
The second way in which PCRE and Perl differ in their recursion processing is in the handling of captured
values. In Perl, when a subpattern is called recursively or as a subpattern (see the next section), it
has no access to any values that were captured outside the recursion, whereas in PCRE these values can be
referenced. Consider this pattern:
^(.)(\1|a(?2))
In PCRE, this pattern matches "bab". The first capturing parentheses match "b", then in the second group,
when the back reference \1 fails to match "b", the second alternative matches "a" and then recurses. In
the recursion, \1 does now match "b" and so the whole match succeeds. In Perl, the pattern fails to match
because inside the recursive call \1 cannot access the externally set value.
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 subject string, it is never re-entered, even if it contains untried
alternatives and there is a subsequent matching failure. Any capturing parentheses that are set during
the subroutine call revert to their previous values afterwards.
Processing options such as case-independence are fixed when a subpattern is defined, so if it is used as
a subroutine, such options cannot be changed for different calls. For example, consider this pattern:
(abc)(?i:(?-1))
It matches "abcabc". It does not match "abcABC" because the change of processing option does not affect
the called subpattern.
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, possibly recursively. Here are two of the examples used above, rewritten using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
PCRE supports an extension to Oniguruma: if a number is preceded by a plus or a minus sign it is taken as
a relative reference. For example:
(abc)(?i:\g<-1>)
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back
reference; the latter is a subroutine call.
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl code to be obeyed in the
middle of matching a regular expression. This makes it possible, amongst other things, to extract
different substrings that match the same pair of parentheses when there is a repetition.
PCRE provides a similar feature, but of course it cannot obey arbitrary Perl code. The feature is called
"callout". The caller of PCRE provides an external function by putting its entry point in the global
variable pcre_callout (8-bit library) or pcre16_callout (16-bit library). By default, this variable
contains NULL, which disables all calling out.
Within a regular expression, (?C) indicates the points at which the external function is to be called. If
you want to identify different callout points, you can put a number less than 256 after the letter C. The
default value is zero. For example, this pattern has two callout points:
(?C1)abc(?C2)def
If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are automatically installed
before each item in the pattern. They are all numbered 255.
During matching, when PCRE reaches a callout point, the external function is called. It is provided with
the number of the callout, the position in the pattern, and, optionally, one item of data originally
supplied by the caller of the matching function. The callout function may cause matching to proceed, to
backtrack, or to fail altogether. A complete description of the interface to the callout function is
given in the pcrecallout documentation.
BACKTRACKING CONTROL
Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are 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.
Since these verbs are specifically related to backtracking, most of them can be used only when the
pattern is to be matched using one of the traditional matching functions, which use a backtracking
algorithm. With the exception of (*FAIL), which behaves like a failing negative assertion, they cause an
error if encountered by a DFA matching function.
If any of these verbs are used in an assertion or in a subpattern that is called as a subroutine (whether
or not recursively), their effect is confined to that subpattern; it does not extend to the surrounding
pattern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in a
successful positive assertion is passed back when a match succeeds (compare capturing parentheses in
assertions). Note that such subpatterns are processed as anchored at the point where they are tested.
Note also that Perl's treatment of subroutines and assertions is different in some cases.
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, with
differing behaviour, depending on whether or not an argument 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 library. If the name is empty, that is, if the closing parenthesis
immediately follows the colon, the effect is as if the colon were not there. Any number of these verbs
may occur in a pattern.
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 suppresses the running of a match,
any included backtracking verbs will not, of course, be processed. You can suppress the start-of-match
optimizations by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_compile() or pcre_exec(), or
by starting the pattern with (*NO_START_OPT). There is more discussion of this option in the section
entitled "Option bits for pcre_exec()" in the pcreapi documentation.
Experiments with Perl suggest that it too has similar optimizations, sometimes leading to anomalous
results.
Verbs that act immediately
The following verbs act as soon as they are encountered. They may not be followed by a name.
(*ACCEPT)
This verb causes the match to end successfully, skipping the remainder of the pattern. However, when it
is inside a subpattern that is called as a subroutine, only that subpattern is ended successfully.
Matching then continues at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so far
is captured. For example:
A((?:A|B(*ACCEPT)|C)D)
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by the outer parentheses.
(*FAIL) or (*F)
This verb causes a matching failure, forcing backtracking to occur. It is equivalent to (?!) but easier
to read. The Perl documentation notes that it is probably useful only when combined with (?{}) or (??{}).
Those are, of course, Perl features that are not present in PCRE. The nearest equivalent is the callout
feature, as for example in this pattern:
a+(?C)(*FAIL)
A match with the string "aaaa" always fails, but the callout is taken before each backtrack happens (in
this example, 10 times).
Recording which path was taken
There is one verb whose main purpose is to track how a match was arrived at, though it also has a
secondary use in conjunction with advancing the match starting point (see (*SKIP) below).
(*MARK:NAME) or (*:NAME)
A name is always required with this verb. There may be as many instances of (*MARK) as you like in a
pattern, and their names do not have to be unique.
When a match succeeds, the name of the last-encountered (*MARK) on the matching path is passed back to
the caller as described in the section entitled "Extra data for pcre_exec()" in the pcreapi
documentation. Here is an example of pcretest output, where the /K modifier requests the retrieval and
outputting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XY
0: XY
MK: A
XZ
0: XZ
MK: B
The (*MARK) name is tagged with "MK:" in this output, and in this example it indicates which of the two
alternatives matched. This is a more efficient way of obtaining this information than putting each
alternative in its own capturing parentheses.
If (*MARK) is encountered in a positive assertion, its name is recorded and passed back if it is the
last-encountered. This does not happen for negative assertions.
After a partial match or a failed match, the name of the last encountered (*MARK) in the entire match
process is returned. For example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XP
No match, mark = B
Note that in this unanchored example the mark is retained from the match attempt that started at the
letter "X" in the subject. Subsequent match attempts starting at "P" and then with an empty string do not
get as far as the (*MARK) item, but nevertheless do not reset it.
If you are interested in (*MARK) values after failed matches, you should probably set the
PCRE_NO_START_OPTIMIZE option (see above) to ensure that the match is always attempted.
Verbs that act after backtracking
The following verbs do nothing when they are encountered. Matching continues with what follows, but if
there is no subsequent match, causing a backtrack to the verb, a failure is forced. That is, backtracking
cannot pass to the left of the verb. However, when one of these verbs appears inside an atomic group, its
effect is confined to that group, because once the group has been matched, there is never any
backtracking into it. In this situation, backtracking can "jump back" to the left of the entire atomic
group. (Remember also, as stated above, that this localization also applies in subroutine calls and
assertions.)
These verbs differ in exactly what kind of failure occurs when backtracking reaches them.
(*COMMIT)
This verb, which may not be followed by a name, causes the whole match to fail outright if the rest of
the pattern does not match. Even if the pattern is unanchored, no further attempts to find a match by
advancing the starting point take place. Once (*COMMIT) 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.
Note that (*COMMIT) at the start of a pattern is not the same as an anchor, unless PCRE's start-of-match
optimizations are turned off, as shown in this pcretest example:
re> /(*COMMIT)abc/
data> xyzabc
0: abc
xyzabc\Y
No match
PCRE knows that any match must start with "a", so the optimization skips along the subject to "a" before
running the first match attempt, which succeeds. When the optimization is disabled by the \Y escape in
the second subject, the match starts at "x" and so the (*COMMIT) causes it 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 the rest of the
pattern does not match. If the pattern is unanchored, the normal "bumpalong" advance to the next starting
character then happens. Backtracking can occur as usual to the left of (*PRUNE), before it is reached, or
when matching to the right of (*PRUNE), but if there is no match to the right, backtracking cannot cross
(*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative to an atomic group or possessive
quantifier, but there are some uses of (*PRUNE) that cannot be expressed in any other way. The behaviour
of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an anchored pattern (*PRUNE) has the same effect
as (*COMMIT).
(*SKIP)
This verb, when given without a name, is like (*PRUNE), except that if the pattern is unanchored, the
"bumpalong" advance is not to the next character, but to the position in the subject where (*SKIP) was
encountered. (*SKIP) signifies that whatever text was matched leading up to it cannot be part of a
successful match. Consider:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt fails (starting at the first character in the
string), the starting point skips on to start the next attempt at "c". Note that a possessive quantifer
does not have the same effect as this example; although it would suppress backtracking during the first
match attempt, the second attempt would start at the second character instead of skipping on to "c".
(*SKIP:NAME)
When (*SKIP) has an associated name, its behaviour is modified. If the following pattern fails to match,
the previous path through the pattern is searched for the most recent (*MARK) that has the same name. If
one is found, the "bumpalong" advance is to the subject position that corresponds to that (*MARK) instead
of to where (*SKIP) was encountered. If no (*MARK) with a matching name is found, the (*SKIP) is ignored.
(*THEN) or (*THEN:NAME)
This verb causes a skip to the next innermost alternative if the rest of the pattern does not match. That
is, it cancels pending backtracking, but only 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. The behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN). If (*THEN) is not
inside an alternation, it acts like (*PRUNE).
Note that a subpattern that does not contain a | character is just a part of the enclosing alternative;
it is not a nested alternation with only one alternative. The effect of (*THEN) extends beyond such a
subpattern to the enclosing alternative. Consider this pattern, where A, B, etc. are complex pattern
fragments that do not contain any | characters at this level:
A (B(*THEN)C) | D
If A and B are matched, but there is a failure in C, matching does not backtrack into A; instead it moves
to the next alternative, that is, D. However, if the subpattern containing (*THEN) is given an
alternative, it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner subpattern. After a failure in C, matching moves to
(*FAIL), which causes the whole subpattern to fail because there are no more alternatives to try. In this
case, matching does now backtrack into A.
Note also that a conditional subpattern is not considered as having two alternatives, because only one is
ever used. In other words, the | character in a conditional subpattern has a different meaning. Ignoring
white space, consider:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. Because .*? is ungreedy, it initially matches zero
characters. The condition (?=a) then fails, the character "b" is matched, but "c" is not. At this point,
matching does not backtrack to .*? as might perhaps be expected from the presence of the | character. The
conditional subpattern is part of the single alternative that comprises the whole pattern, and so the
match fails. (If there was a backtrack into .*?, allowing it to match "b", the match would succeed.)
The verbs just described provide four different "strengths" of control when subsequent matching fails.
(*THEN) is the weakest, carrying on the match at the next alternative. (*PRUNE) comes next, failing the
match at the current starting position, but allowing an advance to the next character (for an unanchored
pattern). (*SKIP) is similar, except that the advance may be more than one character. (*COMMIT) is the
strongest, causing the entire match to fail.
If more than one such verb is present in a pattern, the "strongest" one wins. For example, consider this
pattern, where A, B, etc. are complex pattern fragments:
(A(*COMMIT)B(*THEN)C|D)
Once A has matched, PCRE is committed to this match, at the current starting position. If subsequently B
matches, but C does not, the normal (*THEN) action of trying the next alternative (that is, D) does not
happen because (*COMMIT) overrides.
SEE ALSO
pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3), pcre16(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 17 June 2012
Copyright (c) 1997-2012 University of Cambridge.