-This file contains the PCRE man page that describes the regular expressions
-supported by PCRE version 5.0. Note that not all of the features are relevant
+This file contains the PCRE man page that describes the regular expressions
+supported by PCRE version 7.2. Note that not all of the features are relevant
in the context of Exim. In particular, the version of PCRE that is compiled
with Exim does not include UTF-8 support, there is no mechanism for changing
the options with which the PCRE functions are called, and features such as
callout are not accessible.
-----------------------------------------------------------------------------
-PCRE(3) PCRE(3)
-
+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
of UTF-8 features in the section on UTF-8 support in the main pcre
page.
- 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
+ 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. The
- power of regular expressions comes from the ability to include alterna-
- tives 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.
+ 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 in square brackets. Outside square brackets, the
- metacharacters are as follows:
+ 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)
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 character are ignored.
- An escaping backslash can be used to include a whitespace or # charac-
- ter as part of the pattern.
+ # 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-
\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... (UTF-8 mode only)
+ \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
becomes hex 7B.
After \x, from zero to two hexadecimal digits are read (letters can be
- in upper or lower case). In UTF-8 mode, any number of hexadecimal dig-
- its may appear between \x{ and }, but the value of the character code
- must be less than 2**31 (that is, the maximum hexadecimal value is
- 7FFFFFFF). 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 hex-
- adecimal escape, with no following digits, giving a character whose
- value is zero.
+ 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 when PCRE is in UTF-8 mode. There is no difference
- in the way they are handled. For example, \xdc is exactly the same as
- \x{dc}.
+ 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. In both cases, 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.
+ 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-
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 generates a sin-
- gle byte from the least significant 8 bits of the value. Any subsequent
- digits stand for themselves. For example:
+ 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
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 byte value or a single UTF-8
- character (in UTF-8 mode) 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 sequence \X is
- interpreted as the character "X". Outside a character class, these
- sequences have different meanings (see below).
+ 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
- The third use of backslash is for specifying generic character types.
- The following are always recognized:
+ 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,
+ 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
+ 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).
+ 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.
- 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 the "fr_FR" (French) locale, some character
- codes greater than 128 are used for accented letters, and these are
- matched by \w.
-
- In UTF-8 mode, characters with values greater than 128 never match \d,
+ 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.
+ 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 to match generic character types are available
- when UTF-8 mode is selected. They are:
+ 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
+ \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
- general category properties. Each character has exactly one such prop-
- erty, 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
- 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:
+ 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 property codes are supported:
+ The following general category property codes are supported:
C Other
Cc Control
Zp Paragraph separator
Zs Space separator
- Extended properties such as "Greek" or "InMusicalSymbols" are not sup-
- ported by PCRE.
+ 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.
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.
+ 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 fourth 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 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 start of subject
- \Z matches at end of subject or before newline at end
- \z matches at end of subject
- \G matches at first matching position in subject
+ \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
+ 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
+ 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
+ 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-
+ 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-
+ 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 that is the last character of the
- string as well as at the end of the string, whereas \z matches only at
- the end.
+ 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
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 character that is the last character in 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
+ 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, they match immedi-
- ately after and immediately before an internal newline character,
- respectively, in addition to matching at the start and end of the sub-
- ject string. For example, the pattern /^abc$/ matches the subject
- string "def\nabc" (where \n represents a newline character) in multi-
- line mode, but not otherwise. Consequently, patterns that are anchored
- in single line mode because all branches start with ^ are not anchored
- in multiline mode, and a match for circumflex is possible when the
- startoffset argument of pcre_exec() is non-zero. The PCRE_DOL-
- LAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
+ PCRE_MULTILINE option is set. When this is the case, a circumflex
+ matches immediately after internal newlines as well as at the start of
+ the subject string. It does not match after a newline that ends the
+ string. A dollar matches before any newlines in the string, as well as
+ at the very end, when PCRE_MULTILINE is set. When newline is specified
+ as the two-character sequence CRLF, isolated CR and LF characters do
+ not indicate newlines.
+
+ For example, the pattern /^abc$/ 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 PCRE_MULTILINE is set or
- not.
+ 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, including a non-printing character, but not (by
- default) newline. In UTF-8 mode, a dot matches any UTF-8 character,
- which might be more than one byte long, except (by default) newline. If
- the PCRE_DOTALL option is set, dots match newlines as well. The han-
- dling of dot is entirely independent of the handling of circumflex and
- dollar, the only relationship being that they both involve newline
- characters. Dot has no special meaning in a character class.
+ 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 can match a newline.
- The feature is provided in Perl in order to match individual bytes in
- UTF-8 mode. Because it breaks up UTF-8 characters into individual
- bytes, what remains in the string may be a malformed 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-
+ 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.
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
+ 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
+ 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
+ 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.
+ 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
+ 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
+ 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. When running 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 newline character is never treated in any special way in character
- classes, whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE
- options is. A class such as [^a] will always match a newline.
+ 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
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 the "fr_FR" locale are in use, [\xc8-\xcb] matches
+ 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.
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 alterna-
- tives are within a subpattern (defined below), "succeeds" means match-
- ing the rest of the main pattern as well as the alternative in the sub-
- pattern.
+ 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
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 affects only that part of the cur-
- rent pattern that follows it, so
+ 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
+ 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
+ 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_UNGREEDY and PCRE_EXTRA can be changed
- in the same way as the Perl-compatible options by using the characters
- U and X respectively. The (?X) flag setting is special in that it must
- always occur earlier in the pattern than any of the additional features
- it turns on, even when it is at top level. It is best to put it at the
- start.
+ 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
cat(aract|erpillar|)
matches one of the words "cat", "cataract", or "caterpillar". Without
- the parentheses, it would match "cataract", "erpillar" or the empty
+ the parentheses, it would match "cataract", "erpillar" or an empty
string.
2. It sets up the subpattern as a capturing subpattern. This means
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, and the
- maximum depth of nesting of all subpatterns, both capturing and non-
- capturing, is 200.
+ 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
"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, something that Perl does not provide. The Python syntax
- (?P<name>...) is used. Names consist of alphanumeric characters and
- underscores, and must be unique within a pattern.
-
+ 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. The PCRE API provides function calls for extracting the name-to-
- number translation table from a compiled pattern. There is also a con-
- venience function for extracting a captured substring by name. For fur-
- ther details see the pcreapi documentation.
+ 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
following items:
a literal data character
- the . metacharacter
+ 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)
The quantifier {0} is permitted, causing the expression to behave as if
the previous item and the quantifier were not present.
- For convenience (and historical compatibility) the three most common
- quantifiers have single-character abbreviations:
+ For convenience, the three most common quantifiers have single-charac-
+ ter abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
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 which is not available in
+ 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.
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 . 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-
+ 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,
- and a later one succeed. Consider, for example:
+ 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-
+ 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-
(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-
+ 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))+/
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
- With both maximizing and minimizing repetition, failure of what follows
- normally causes the repeated item to be re-evaluated to see if a dif-
- ferent number of repeats allows the rest of the pattern to match. Some-
- times 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.
+ 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
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 would
- give up immediately on failing to match "foo" the first time. The nota-
- tion is a kind of special parenthesis, starting with (?> as in this
- example:
+ 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
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 or processing of a possessive quantifier and the equivalent
- atomic group.
-
- The possessive quantifier syntax is an extension to the Perl syntax. It
- originates in Sun's Java package.
-
- 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
+ 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 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
+ 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.
+ 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
+ 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. See the subsec-
- tion entitled "Non-printing characters" above for further details of
- the handling of digits following a backslash.
-
- A back reference matches whatever actually matched the capturing sub-
- pattern in the current subject string, rather than anything matching
+ 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-
+ 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
+ matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capturing subpattern is matched caselessly.
- Back references to named subpatterns use the Python syntax (?P=name).
- We could rewrite the above example as follows:
+ 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+(?P=p1)
+ (?<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
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 alternatives, they do not all have to have the same fixed length.
- Thus
+ eral top-level alternatives, they do not all have to have the same
+ fixed length. Thus
(?<=bullock|donkey)
(?<=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 width and
+ 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 match is deemed to fail.
+ 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 escape, which can
- match different numbers of bytes, is also not permitted.
+ ble to calculate the length of the lookbehind. The \X and \R escapes,
+ which can match different numbers of bytes, are also not permitted.
- Atomic groups 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
+ 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$
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)
-
- or, equivalently, using the possessive quantifier syntax,
-
^.*+(?<=abcd)
- there can be no backtracking for the .* item; it can match only the
+ 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
no-pattern (if present) is used. If there are more than two alterna-
tives in the subpattern, a compile-time error occurs.
- There are three kinds of condition. If the text between the parentheses
- consists of a sequence of digits, the condition is satisfied if the
- capturing subpattern of that number has previously matched. The number
- must be greater than zero. Consider the following pattern, which con-
- tains 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:
+ 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:
( \( )? [^()]+ (?(1) \) )
other words, this pattern matches a sequence of non-parentheses,
optionally enclosed in parentheses.
- If the condition is the string (R), it is satisfied if a recursive call
- to the pattern or subpattern has been made. At "top level", the condi-
- tion is false. This is a PCRE extension. Recursive patterns are
- described in the next section.
+ 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.
- If the condition is not a sequence of digits or (R), it must be an
+ 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:
at all.
If the PCRE_EXTENDED option is set, an unescaped # character outside a
- character class introduces a comment that continues up to the next new-
- line character in the pattern.
+ character class introduces a comment that continues to immediately
+ after the next newline in the pattern.
RECURSIVE PATTERNS
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. Perl provides 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 to solve the parentheses problem can be created
- like this:
+ 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
- some special syntax for recursion of the entire pattern, and also for
- individual subpattern recursion.
+ refers recursively to the pattern in which it appears.
- The special item that consists of (? followed by a number greater than
+ 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) is a recursive call of the entire regular
- expression.
+ 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.
- For example, this PCRE pattern solves the nested parentheses problem
- (assume the PCRE_EXTENDED option is set so that white space is
- ignored):
+ 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-
+ 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
+ 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, keep-
- ing track of parenthesis numbers can be tricky. It may be more conve-
- nient to use named parentheses instead. For this, PCRE uses (?P>name),
- which is an extension to the Python syntax that PCRE uses for named
- parentheses (Perl does not provide named parentheses). We could rewrite
- the above example as follows:
+ 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.
- (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )
+ 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.
- This particular example pattern contains nested unlimited repeats, and
- so the use of atomic grouping 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
+ 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
+ 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 the next section and the pcrecallout documen-
- tation). If the pattern above is matched against
+ 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,
+ 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-
+ 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
+ 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-
+ 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.
+ 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. An earlier example
- pointed out that the pattern
+ 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
(sens|respons)e and (?1)ibility
is used, it does match "sense and responsibility" as well as the other
- two strings. Such references must, however, follow the subpattern to
- which they refer.
+ 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
gether. A complete description of the interface to the callout function
is given in the pcrecallout documentation.
-Last updated: 09 September 2004
-Copyright (c) 1997-2004 University of Cambridge.
+
+SEE ALSO
+
+ pcreapi(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.
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