PCREPATTERN(3)PCREPATTERN(3)NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions supported by PCRE
are described below. Regular expressions are also described in the Perl
documentation and 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 descrip-
tion 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 character strings. To use
this, you must build PCRE to include UTF-8 support, and then call
pcre_compile() with the PCRE_UTF8 option. How this affects pattern
matching is mentioned in several places below. There is also a summary
of UTF-8 features in the section on UTF-8 support in the main pcre
page.
The remainder of this document discusses the patterns that are sup-
ported by PCRE when its main matching function, pcre_exec(), is used.
From release 6.0, PCRE offers a second matching function,
pcre_dfa_exec(), which matches using a different algorithm that is not
Perl-compatible. Some of the features discussed below are not available
when pcre_dfa_exec() is used. The advantages and disadvantages of the
alternative function, and how it differs from the normal function, are
discussed in the pcrematching page.
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 UTF-8 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 val-
ues, 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-8 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 recog-
nized anywhere in the pattern except within square brackets, and those
that are recognized within square brackets. Outside square brackets,
the metacharacters are as follows:
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
Part of a pattern that is in square brackets is called a "character
class". In a character class the only metacharacters are:
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (only if followed by POSIX
syntax)
] terminates the character class
The following sections describe the use of each of the metacharacters.
BACKSLASH
The backslash character has several uses. Firstly, if it is followed by
a non-alphanumeric character, 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 back-
slash, you write \\.
If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
the pattern (other than in a character class) and characters between a
# outside a character class and the next newline are ignored. An escap-
ing backslash can be used to include a whitespace or # character as
part of the pattern.
If you want to remove the special meaning from a sequence of charac-
ters, you can do so by putting them between \Q and \E. This is differ-
ent from Perl in that $ and @ are handled as literals in \Q...\E
sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
tion. 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.
Non-printing characters
A second use of backslash provides a way of encoding non-printing char-
acters in patterns in a visible manner. There is no restriction on the
appearance of non-printing characters, apart from the binary zero that
terminates a pattern, but when a pattern is being prepared by text
editing, it is usually 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 character
\e escape (hex 1B)
\f formfeed (hex 0C)
\n newline (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\ddd character with octal code ddd, or backreference
\xhh character with hex code hh
\x{hhh..} character with hex code hhh..
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, but \c{ becomes hex 3B, while \c;
becomes hex 7B.
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 value of the character code must be less
than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is,
the maximum hexadecimal value is 7FFFFFFF). If characters other than
hexadecimal digits appear between \x{ and }, or if there is no termi-
nating }, 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.
Characters whose value is less than 256 can be defined by either of the
two syntaxes for \x. There is no difference in the way they are han-
dled. For example, \xdc is exactly the same as \x{dc}.
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 compli-
cated. Outside a character class, PCRE reads it and any following dig-
its 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 gen-
erate a data character. Any subsequent digits stand for themselves. In
non-UTF-8 mode, the value of a character specified in octal must be
less than \400. In UTF-8 mode, values up to \777 are permitted. 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 byte consisting entirely of 1 bits
\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, the sequence \b is interpreted as the backspace character (hex
08), and the sequences \R and \X are interpreted as the characters "R"
and "X", respectively. Outside a character class, these sequences have
different meanings (see below).
Absolute and relative back references
The sequence \g followed by a positive or 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.
Generic character types
Another use of backslash is for specifying generic character types. The
following are always recognized:
\d any decimal digit
\D any character that is not a decimal digit
\h any horizontal whitespace character
\H any character that is not a horizontal whitespace character
\s any whitespace character
\S any character that is not a whitespace character
\v any vertical whitespace character
\V any character that is not a vertical whitespace character
\w any "word" character
\W any "non-word" character
Each pair of escape sequences partitions the complete set of characters
into two disjoint sets. Any given character matches one, and only one,
of each pair.
These character type sequences can appear both inside and outside char-
acter 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, since 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 charac-
ter. In PCRE, it never does.
In UTF-8 mode, characters with values greater than 128 never match \d,
\s, or \w, and always match \D, \S, and \W. This is true even when Uni-
code character property support is available. These sequences retain
their original meanings from before UTF-8 support was available, mainly
for efficiency reasons.
The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
the other sequences, these do match certain high-valued codepoints in
UTF-8 mode. 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 Formfeed
U+000D Carriage return
U+0085 Next line
U+2028 Line separator
U+2029 Paragraph separator
A "word" character is an underscore or any character less than 256 that
is a letter or digit. The definition of letters and digits is con-
trolled 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 matched by \w. The use of
locales with Unicode is discouraged.
Newline sequences
Outside a character class, the escape sequence \R matches any Unicode
newline sequence. This is a Perl 5.10 feature. In 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 (formfeed, 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 UTF-8 mode, two additional characters whose codepoints are greater
than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
rator, U+2029). Unicode character property support is not needed for
these characters to be recognized.
Inside a character class, \R matches the letter "R".
Unicode character properties
When PCRE is built with Unicode character property support, three addi-
tional escape sequences that match characters with specific properties
are available. When not in 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, and "Any", which matches
any character (including newline). Other properties such as "InMusical-
Symbols" are not currently supported by PCRE. Note that \P{Any} does
not match any characters, so always causes a match failure.
Sets of Unicode characters are defined as belonging to certain scripts.
A character from one of these sets can be matched using a script name.
For example:
\p{Greek}
\P{Han}
Those that are not part of an identified script are lumped together as
"Common". The current list of scripts is:
Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
Each character has exactly one 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 gen-
eral category properties that start with that letter. In this case, in
the absence of negation, the curly brackets in the escape sequence are
optional; these two examples have the same effect:
\p{L}
\pL
The following general category property codes are supported:
C Other
Cc Control
Cf Format
Cn Unassigned
Co Private use
Cs Surrogate
L Letter
Ll Lower case letter
Lm Modifier letter
Lo Other letter
Lt Title case letter
Lu Upper case letter
M Mark
Mc Spacing mark
Me Enclosing mark
Mn Non-spacing mark
N Number
Nd Decimal number
Nl Letter number
No Other number
P Punctuation
Pc Connector punctuation
Pd Dash punctuation
Pe Close punctuation
Pf Final punctuation
Pi Initial punctuation
Po Other punctuation
Ps Open punctuation
S Symbol
Sc Currency symbol
Sk Modifier symbol
Sm Mathematical symbol
So Other symbol
Z Separator
Zl Line separator
Zp Paragraph separator
Zs Space separator
The special property L& is also supported: it matches a character that
has the Lu, Ll, or Lt property, in other words, a letter that is not
classified as a modifier or "other".
The long synonyms for these properties 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) prop-
erty. Instead, this property is assumed for any code point that is not
in the Unicode table.
Specifying caseless matching does not affect these escape sequences.
For example, \p{Lu} always matches only upper case letters.
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 non-UTF-8 mode \X
matches any one character.
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.
Resetting the match start
The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
ously 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".
Simple assertions
The final use of backslash is for certain simple assertions. An asser-
tion specifies a condition that has to be met at a particular point in
a match, without consuming any characters from the subject string. The
use of subpatterns for more complicated assertions is described below.
The backslashed assertions are:
\b matches at a word boundary
\B matches when not at a word boundary
\A matches at the start of the subject
\Z matches at the end of the subject
also matches before a newline at the end of the subject
\z matches only at the end of the subject
\G matches at the first matching position in the subject
These assertions may not appear in character classes (but note that \b
has a different meaning, namely the backspace character, inside a char-
acter class).
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.
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 asser-
tions 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, indi-
cating 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 argu-
ments, you can mimic Perl's /g option, and it is in this kind of imple-
mentation 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 argu-
ment 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 sub-
ject, 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)
Outside a character class, a dot in the pattern matches any one charac-
ter in the subject string except (by default) a character that signi-
fies the end of a line. In UTF-8 mode, the matched character may be
more than one byte long.
When a line ending is defined as a single character, dot never matches
that character; when the two-character sequence CRLF is used, dot does
not match CR if it is immediately followed by LF, but otherwise it
matches all characters (including isolated CRs and LFs). When any Uni-
code line endings are being recognized, dot does not match CR or LF or
any of the other line ending characters.
The behaviour of dot with regard to newlines can be changed. If the
PCRE_DOTALL option is set, a dot matches any one character, without
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 circum-
flex and dollar, the only relationship being that they both involve
newlines. Dot has no special meaning in a character class.
MATCHING A SINGLE BYTE
Outside a character class, the escape sequence \C matches any one byte,
both in and out of UTF-8 mode. Unlike a dot, it always matches any
line-ending characters. The feature is provided in Perl in order to
match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
acters into individual bytes, what remains in the string may be a mal-
formed UTF-8 string. For this reason, the \C escape sequence is best
avoided.
PCRE does not allow \C to appear in lookbehind assertions (described
below), because in UTF-8 mode this would make it impossible to calcu-
late the length of the lookbehind.
SQUARE BRACKETS AND CHARACTER CLASSES
An opening square bracket introduces a character class, terminated by a
closing square bracket. A closing square bracket on its own is not spe-
cial. 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 UTF-8
mode, the character may occupy more than one byte. 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 con-
sumes a character from the subject string, and therefore it fails if
the current pointer is at the end of the string.
In UTF-8 mode, characters with values greater than 255 can be included
in a class as a literal string of bytes, 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 UTF-8 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-8
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 charac-
ters in a character class. For example, [d-m] matches any letter
between d and m, inclusive. If a minus character is required in a
class, it must be escaped with a backslash or appear in a position
where it cannot be interpreted as indicating a range, typically as the
first or last character in the class.
It is not possible to have the literal character "]" as the end charac-
ter of a range. A pattern such as [W-]46] is interpreted as a class of
two characters ("W" and "-") followed by a literal string "46]", so it
would match "W46]" or "-46]". However, if the "]" is escaped with a
backslash it is interpreted as the end of range, so [W-\]46] is inter-
preted as a class containing a range followed by two other characters.
The octal or hexadecimal representation of "]" can also be used to end
a range.
Ranges operate in the collating sequence of character values. They can
also be used for characters specified numerically, for example
[\000-\037]. In UTF-8 mode, ranges can include characters whose values
are greater than 255, for example [\x{100}-\x{2ff}].
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 non-UTF-8 mode, if
character tables for a French locale are in use, [\xc8-\xcb] matches
accented E characters in both cases. In UTF-8 mode, PCRE supports the
concept of case for characters with values greater than 128 only when
it is compiled with Unicode property support.
The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
in a character class, and add the characters that they match to the
class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
flex 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.
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
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.
In UTF-8 mode, characters with values greater than 128 do not match any
of the POSIX character classes.
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 can be changed from within the pattern by a
sequence of Perl option letters enclosed between "(?" and ")". The
option letters are
i for PCRE_CASELESS
m for PCRE_MULTILINE
s for PCRE_DOTALL
x for PCRE_EXTENDED
For example, (?im) sets caseless, multiline matching. It is also possi-
ble to unset these options by preceding the letter with a hyphen, and a
combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
is also permitted. If a letter appears both before and after the
hyphen, the option is unset.
When an option change occurs at top level (that is, not inside subpat-
tern 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 current pattern 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.
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.
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 one of the words "cat", "cataract", or "caterpillar". 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 pcre_exec(). 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 pat-
tern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king", and are num-
bered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not always
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 captur-
ing, 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 cap-
turing parentheses are numbered one. Thus, when the pattern matches,
you can look at captured substring number one, whichever alternative
matched. This construct is useful when you want to capture part, but
not all, of one of a number of alternatives. Inside a (?| group, paren-
theses are numbered as usual, but the number is reset at the start of
each branch. The numbers of any capturing buffers that follow the sub-
pattern start after the highest number used in any branch. The follow-
ing example is taken from the Perl documentation. The numbers under-
neath 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 backreference or a recursive call to a numbered subpattern always
refers to the first one in the pattern with the given number.
An alternative approach to using this "branch reset" feature is to use
duplicate named subpatterns, as described in the next section.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated regular expres-
sions. Furthermore, if an expression is modified, the numbers may
change. To help with this difficulty, PCRE supports the naming of sub-
patterns. This feature was not added to Perl until release 5.10. Python
had the feature earlier, and PCRE introduced it at release 4.0, using
the Python syntax. PCRE now supports both the Perl and the Python syn-
tax.
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. This 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 reference to a non-unique named sub-
pattern from elsewhere in the pattern, the one that corresponds to the
lowest number is used. For further details of the interfaces for han-
dling named subpatterns, see the pcreapi documentation.
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 (in UTF-8 mode with Unicode properties)
the \R escape sequence
an escape such as \d that matches a single character
a character class
a back reference (see next section)
a parenthesized subpattern (unless it is an assertion)
The general repetition quantifier specifies a minimum and maximum num-
ber 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 exam-
ple, {,6} is not a quantifier, but a literal string of four characters.
In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
acters, each of which is represented by a two-byte sequence. Similarly,
when Unicode property support is available, \X{3} matches three Unicode
extended sequences, each of which may be several bytes 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.
For convenience, the three most common quantifiers have single-charac-
ter abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
It is possible to construct infinite loops by following a subpattern
that can match no characters with a quantifier that has no upper limit,
for example:
(a?)*
Earlier versions of Perl and PCRE used to give an error at compile time
for such patterns. However, because there are cases where this can be
useful, such patterns are now accepted, but if any repetition of the
subpattern does in fact match no characters, the loop is forcibly bro-
ken.
By default, the quantifiers are "greedy", that is, they match as much
as possible (up to the maximum number of permitted times), without
causing the rest of the pattern to fail. The classic example of where
this gives problems is in trying to match comments in C programs. These
appear 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 (equiv-
alent 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 new-
lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
mization, 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
backreference elsewhere in the pattern, a match at the start may fail
where a later one succeeds. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the fourth charac-
ter. For this reason, such a pattern is not implicitly anchored.
When a capturing subpattern is repeated, the value captured is the sub-
string that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the captured substring
is "tweedledee". However, if there are nested capturing subpatterns,
the corresponding captured values may have been set in previous itera-
tions. 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 con-
tains 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 pre-
pared 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
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 syn-
tax. Jeffrey Friedl originated the idea (and the name) in the first
edition of his book. Mike McCloskey liked it, so implemented it when he
built Sun's Java package, and PCRE copied it from there. It ultimately
found its way into Perl at release 5.10.
PCRE has an optimization that automatically "possessifies" certain sim-
ple pattern constructs. For example, the sequence A+B is treated as
A++B because there is no point in backtracking into a sequence of A's
when B must follow.
When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of an
atomic group is the only way to avoid some failing matches taking a
very long time indeed. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of non-
digits, or digits enclosed in <>, followed by either ! or ?. When it
matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because the
string can be divided between the internal \D+ repeat and the external
* repeat in a large number of ways, and all have to be tried. (The
example uses [!?] rather than a single character at the end, because
both PCRE and Perl have an optimization that allows for fast failure
when a single character is used. They remember the last single charac-
ter 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 sub-
pattern earlier (that is, to its left) in the pattern, provided there
have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is less than 10,
it is always taken as a back reference, and causes an error only if
there are not that many capturing left parentheses in the entire pat-
tern. In other words, the parentheses that are referenced need not be
to the left of the reference for numbers less than 10. A "forward back
reference" of this type can make sense when a repetition is involved
and the subpattern to the right has participated in an earlier itera-
tion.
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, which is a fea-
ture introduced in Perl 5.10. This escape must be followed by a posi-
tive or a negative number, optionally enclosed in braces. These exam-
ples are all identical:
(ring), \1
(ring), \g1
(ring), \g{1}
A positive 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 captur-
ing subpattern before \g, that is, is it equivalent to \2. 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 sub-
pattern in the current subject string, rather than anything matching
the subpattern itself (see "Subpatterns as subroutines" below for a way
of doing that). So the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If caseful matching is in force at the
time of the back reference, the case of letters is relevant. For exam-
ple,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capturing subpattern is matched caselessly.
There are several different ways of writing back references to named
subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
\k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
unified back reference syntax, in which \g can be used for both numeric
and named references, is also supported. We could rewrite the above
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. For example, the pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc". Because there
may be many capturing parentheses in a pattern, all digits following
the 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 whitespace. Otherwise an empty comment (see "Com-
ments" below) can be used.
A back reference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, (a\1) never
matches. However, such references can be useful inside repeated sub-
patterns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
ation 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.
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, and may not be
repeated, because it makes no sense to assert the same thing several
times. If any kind of assertion contains capturing subpatterns within
it, these are counted for the purposes of numbering the capturing sub-
patterns in the whole pattern. However, substring capturing is carried
out only for positive assertions, because it does not make sense for
negative assertions.
Lookahead assertions
Lookahead assertions start with (?= for positive assertions and (?! for
negative assertions. For example,
\w+(?=;)
matches a word followed by a semicolon, but does not include the semi-
colon in the match, and
foo(?!bar)
matches any occurrence of "foo" that is not followed by "bar". Note
that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something
other than "foo"; it finds any occurrence of "bar" whatsoever, because
the 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.
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 sev-
eral 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 (at least for 5.8), 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 if rewritten to use two top-
level branches:
(?<=abc|abde)
In some cases, the Perl 5.10 escape sequence \K (see above) can be used
instead of a lookbehind assertion; this is not restricted to a fixed-
length.
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 cur-
rent position, the assertion fails.
PCRE does not allow the \C escape (which matches a single byte in UTF-8
mode) to appear in lookbehind assertions, because it makes it impossi-
ble to calculate the length of the lookbehind. The \X and \R escapes,
which can match different numbers of bytes, are also not permitted.
Possessive quantifiers can be used in conjunction with lookbehind
assertions to specify efficient matching at the end of the subject
string. 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" pre-
ceded by six characters, the first of which are digits and the last
three of which are not "999". For example, it doesn't match "123abc-
foo". 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 con-
ditionally or to choose between two alternative subpatterns, depending
on the result of an assertion, or whether a previous capturing subpat-
tern matched or not. 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 alterna-
tives in the subpattern, a compile-time error occurs.
There are four kinds of condition: references to subpatterns, refer-
ences 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 the capturing subpattern of that number has
previously matched. An alternative notation is to precede the digits
with a plus or minus sign. In this case, the subpattern number is rela-
tive rather than absolute. The most recently opened parentheses can be
referenced by (?(-1), the next most recent by (?(-2), and so on. In
looping constructs it can also make sense to refer to subsequent groups
with constructs such as (?(+2).
Consider the following pattern, which contains non-significant white
space to make it more readable (assume the PCRE_EXTENDED option) and to
divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The sec-
ond part matches one or more characters that are not parentheses. The
third part is a conditional subpattern that tests whether the first set
of parentheses matched or not. If they did, that is, if subject started
with an opening parenthesis, the condition is true, and so the yes-pat-
tern 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 syn-
tax, 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 num-
ber, which must be greater than zero. Using subpattern names that con-
sist entirely of digits is not recommended.
Rewriting the above example to use a named subpattern gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
Checking for pattern recursion
If the condition is the string (R), and there is no subpattern with the
name R, the condition is true if a recursive call to the whole pattern
or any subpattern has been made. If digits or a name preceded by amper-
sand follow the letter R, for example:
(?(R3)...) or (?(R&name)...)
the condition is true if the most recent recursion is into the subpat-
tern whose number or name is given. This condition does not check the
entire recursion stack.
At "top level", all these recursion test conditions are false. Recur-
sive patterns are 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 ref-
erenced from elsewhere. (The use of "subroutines" is described below.)
For example, a pattern to match an IPv4 address could be written like
this (ignore whitespace 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
The sequence (?# marks the start of a comment that continues up to the
next closing parenthesis. Nested parentheses are not permitted. The
characters that make up a comment play no part in the pattern matching
at all.
If the PCRE_EXTENDED option is set, an unescaped # character outside a
character class introduces a comment that continues to immediately
after the next newline in the pattern.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best
that can be done is to use a pattern that matches up to some fixed
depth of nesting. It is not possible to handle an arbitrary nesting
depth.
For some time, Perl has provided a facility that allows regular expres-
sions to recurse (amongst other things). It does this by interpolating
Perl code in the expression at run time, and the code can refer to the
expression itself. A Perl pattern using code interpolation to solve the
parentheses problem can be created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and in this case
refers recursively to the pattern in which it appears.
Obviously, PCRE cannot support the interpolation of Perl code. Instead,
it supports special syntax for recursion of the entire pattern, and
also for individual subpattern recursion. After its introduction in
PCRE and Python, this kind of recursion was 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 call of the subpattern of
the given number, provided that it occurs inside that subpattern. (If
not, it is a "subroutine" call, which is described in the next sec-
tion.) The special item (?R) or (?0) is a recursive call of the entire
regular expression.
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 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 parenthe-
sized substring). Finally there is a closing parenthesis.
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. (A Perl
5.10 feature.) 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 refer-
enced. They are always "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 atomic grouping for match-
ing 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 atomic grouping 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 set for any capturing subpatterns are
those from the outermost level of the recursion at which the subpattern
value is set. 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 capturing parentheses is "ef", which is the last
value taken on at the top level. If additional parentheses are added,
giving
\( ( ( (?>[^()]+) | (?R) )* ) \)
^ ^
^ ^
the string they capture is "ab(cd)ef", the contents of the top level
parentheses. If there are more than 15 capturing parentheses in a pat-
tern, PCRE has to obtain extra memory to store data during a recursion,
which it does by using pcre_malloc, freeing it via pcre_free after-
wards. 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 brack-
ets, allowing for arbitrary nesting. Only digits are allowed in nested
brackets (that is, when recursing), whereas any characters are permit-
ted 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.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern reference (either by number or
by name) is used outside the parentheses to which it refers, it oper-
ates like a subroutine in a programming language. The "called" subpat-
tern 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.
Like recursive subpatterns, a "subroutine" 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.
When a subpattern is used as a subroutine, processing options such as
case-independence are fixed when the subpattern is defined. They 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.
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary
Perl code to be obeyed in the middle of matching a regular expression.
This makes it possible, amongst other things, to extract different sub-
strings that match the same pair of parentheses when there is a repeti-
tion.
PCRE provides a similar feature, but of course it cannot obey arbitrary
Perl code. The feature is called "callout". The caller of PCRE provides
an external function by putting its entry point in the global variable
pcre_callout. 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 pcre_compile(), callouts are
automatically installed before each item in the pattern. They are all
numbered 255.
During matching, when PCRE reaches a callout point (and pcre_callout is
set), 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 pcre_exec(). The callout
function may cause matching to proceed, to backtrack, or to fail alto-
gether. A complete description of the interface to the callout function
is given in the pcrecallout documentation.
SEE ALSOpcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 19 June 2007
Copyright (c) 1997-2007 University of Cambridge.
PCREPATTERN(3)
