This file contains the PCRE man page that describes the regular expressions
-supported by PCRE version 6.7. Note that not all of the features are relevant
+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
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. The advantages and disadvantages of the alternative
- function, and how it differs from the normal function, are discussed in
- the pcrematching page.
+ 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.
- 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
+
+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
+ 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
+ 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:
+ 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)
also "possessive quantifier"
{ start min/max quantifier
- Part of a pattern that is in square brackets is called a "character
+ 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
syntax)
] terminates the character class
- The following sections describe the use of each of the metacharacters.
+ 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
+ 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-
+ 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
+ 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
+ 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-
+ 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\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
- The \Q...\E sequence is recognized both inside and outside character
+ 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
+ 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)
\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;
+ 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
+ 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
+ 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-
+ 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
+ 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
+ (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
+ 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
+ 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, ane 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
+ 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
\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
+ 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 sequence \X is interpreted as the character "X". Outside a
- character class, these sequences have different meanings (see below).
+ 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). (If
+ 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.)
+ 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. The use of locales with
- Unicode is discouraged.
+ 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 character properties 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
- The property names represented by xx above are limited to the Unicode
+ 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
+ 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.
+ 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
+ Those that are not part of an identified script are lumped together as
"Common". The current list of scripts is:
- Arabic, Armenian, Bengali, Bopomofo, Braille, Buginese, Buhid, Cana-
- dian_Aboriginal, Cherokee, Common, Coptic, Cypriot, Cyrillic, Deseret,
- Devanagari, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati,
- Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Inherited, Kannada,
- Katakana, Kharoshthi, Khmer, Lao, Latin, Limbu, Linear_B, Malayalam,
- Mongolian, Myanmar, New_Tai_Lue, Ogham, Old_Italic, Old_Persian, Oriya,
- Osmanya, Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tag-
- banwa, Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
- Ugaritic, Yi.
-
- Each character has exactly one general category property, specified by
+ 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
+ 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
+ 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}
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
+ 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
+ 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.
+ 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
+ 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.
+ 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
+ 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-
+ 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.
\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
has a different meaning, namely the backspace character, inside a char-
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 (CR or LF), dot never matches that character; when the two-
- character sequence CRLF is used, dot does not match CR if it is immedi-
- ately followed by LF, but otherwise it matches all characters (includ-
- ing isolated CRs and LFs).
-
- 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 newline is defined as the two-character sequence CRLF, 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
+ 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 CR and
- LF. 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.
+ 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-
PCRE is compiled with Unicode property support as well as with UTF-8
support.
- Characters that might indicate line breaks (CR and LF) 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.
+ 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.
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_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
- can be changed in the same way as the Perl-compatible options by using
+ 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.
cat(aract|erpillar|)
- matches one of the words "cat", "cataract", or "caterpillar". Without
- the parentheses, it would match "cataract", "erpillar" or the empty
+ 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
+ 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
+ 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-
+ 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
+ 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, 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
+ 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
+ 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, something that Perl does not provide. The Python syntax
- (?P<name>...) is used. References to capturing parentheses from other
- parts of the pattern, such as backreferences, recursion, and condi-
- tions, 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.
+ 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
both cases you want to extract the abbreviation. This pattern (ignoring
the line breaks) does the job:
- (?P<DN>Mon|Fri|Sun)(?:day)?|
- (?P<DN>Tue)(?:sday)?|
- (?P<DN>Wed)(?:nesday)?|
- (?P<DN>Thu)(?:rsday)?|
- (?P<DN>Sat)(?:urday)?
+ (?<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. 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 subpattern from elsewhere in the pattern, the one that
- corresponds to the lowest number is used. For further details of the
- interfaces for handling named subpatterns, see the pcreapi documenta-
- tion.
+ 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.
- 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.
-
- 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. 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-
+ 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 sub-
- pattern whose number is 10 or more. However, a back reference to any
- subpattern is possible using named parentheses (see below). See also
- the subsection entitled "Non-printing characters" above for further
- details of the handling of digits following a backslash.
+ 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
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+\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.
(?<=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, or a sequence of alphanumeric charac-
- ters and underscores, the condition is satisfied if the capturing sub-
- pattern of that number or name has previously matched. There is a pos-
- sible ambiguity here, because subpattern names may consist entirely of
- digits. PCRE looks first for a named subpattern; if it cannot find one
- and the text consists entirely of digits, it looks for a subpattern of
- that number, which must be greater than zero. Using subpattern names
- that consist entirely of digits is not recommended.
+ 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
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. Rewriting it to use a named subpat-
- tern gives this:
+ optionally enclosed in parentheses.
+
+ If you were embedding this pattern in a larger one, you could use a
+ relative reference:
- (?P<OPEN> \( )? [^()]+ (?(OPEN) \) )
+ ...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 satisfied if a recursive call to the pattern
- or subpattern has been made. At "top level", the condition is false.
- This is a PCRE extension. Recursive patterns are described in the next
- section.
+ 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.
- If the condition is not a sequence of digits or (R), it must be an
+ 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:
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.
+
+ 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.
- The special item that consists of (? followed by a number greater than
+ 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.
- 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 subse-
- quent matching failure.
+ 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
+ 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.
- (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )
+ 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.
- 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
+ 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
+ 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, if given numerically, must follow the
- subpattern to which they refer. However, named references can refer to
- later subpatterns.
+ 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
+ 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: 06 June 2006
-Copyright (c) 1997-2006 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.
git://git.exim.org / users / jgh / exim.git / blobdiff