This file contains the PCRE man page that describes the regular expressions
-supported by PCRE version 7.0. 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.
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
+ 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 within square brackets. Outside square brackets,
+ 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
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
+ 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
+ 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 sequences \R and \X are interpreted as the characters "R"
- and "X", respectively. Outside a character class, these sequences have
+ 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. Back
- references are discussed later, following the discussion of parenthe-
- sized subpatterns.
+ 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
\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 an extension to Perl. In non-UTF-8 mode \R is
+ 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
+ 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,
+ 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
+ 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
+ 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, Balinese, Bengali, Bopomofo, Braille, Buginese,
- Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
+ 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,
+ 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,
+ 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
+ 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 final use of backslash is for certain simple assertions. An asser-
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.
"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
(?<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
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 Perl syntax \k<name> or
- \k'name' or the Python syntax (?P=name). We could rewrite the above
- example in either of the following ways:
+ 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
+ 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
+ There may be more than one back reference to the same subpattern. If a
+ subpattern has not actually been used in a particular match, any back
references to it always fail. For example, the pattern
(a|(bc))\2
- always fails if it starts to match "a" rather than "bc". Because there
- may be many capturing parentheses in a pattern, all digits following
- the backslash are taken as part of a potential back reference number.
+ always fails if it starts to match "a" rather than "bc". Because there
+ may be many capturing parentheses in a pattern, all digits following
+ the backslash are taken as part of a potential back reference number.
If the pattern continues with a digit character, some delimiter must be
- used to terminate the back reference. If the PCRE_EXTENDED option is
- set, this can be whitespace. Otherwise an empty comment (see "Com-
+ used to terminate the back reference. If the PCRE_EXTENDED option is
+ set, this can be whitespace. Otherwise an empty comment (see "Com-
ments" below) can be used.
- A back reference that occurs inside the parentheses to which it refers
- fails when the subpattern is first used, so, for example, (a\1) never
- matches. However, such references can be useful inside repeated sub-
+ A back reference that occurs inside the parentheses to which it refers
+ fails when the subpattern is first used, so, for example, (a\1) never
+ matches. However, such references can be useful inside repeated sub-
patterns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
- ation of the subpattern, the back reference matches the character
- string corresponding to the previous iteration. In order for this to
- work, the pattern must be such that the first iteration does not need
- to match the back reference. This can be done using alternation, as in
+ ation of the subpattern, the back reference matches the character
+ string corresponding to the previous iteration. In order for this to
+ work, the pattern must be such that the first iteration does not need
+ to match the back reference. This can be done using alternation, as in
the example above, or by a quantifier with a minimum of zero.
ASSERTIONS
- An assertion is a test on the characters following or preceding the
- current matching point that does not actually consume any characters.
- The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
+ An assertion is a test on the characters following or preceding the
+ current matching point that does not actually consume any characters.
+ The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
described above.
- More complicated assertions are coded as subpatterns. There are two
- kinds: those that look ahead of the current position in the subject
- string, and those that look behind it. An assertion subpattern is
- matched in the normal way, except that it does not cause the current
+ More complicated assertions are coded as subpatterns. There are two
+ kinds: those that look ahead of the current position in the subject
+ string, and those that look behind it. An assertion subpattern is
+ matched in the normal way, except that it does not cause the current
matching position to be changed.
- Assertion subpatterns are not capturing subpatterns, and may not be
- repeated, because it makes no sense to assert the same thing several
- times. If any kind of assertion contains capturing subpatterns within
- it, these are counted for the purposes of numbering the capturing sub-
+ Assertion subpatterns are not capturing subpatterns, and may not be
+ repeated, because it makes no sense to assert the same thing several
+ times. If any kind of assertion contains capturing subpatterns within
+ it, these are counted for the purposes of numbering the capturing sub-
patterns in the whole pattern. However, substring capturing is carried
- out only for positive assertions, because it does not make sense for
+ out only for positive assertions, because it does not make sense for
negative assertions.
Lookahead assertions
\w+(?=;)
- matches a word followed by a semicolon, but does not include the semi-
+ matches a word followed by a semicolon, but does not include the semi-
colon in the match, and
foo(?!bar)
- matches any occurrence of "foo" that is not followed by "bar". Note
+ matches any occurrence of "foo" that is not followed by "bar". Note
that the apparently similar pattern
(?!foo)bar
- does not find an occurrence of "bar" that is preceded by something
- other than "foo"; it finds any occurrence of "bar" whatsoever, because
+ does not find an occurrence of "bar" that is preceded by something
+ other than "foo"; it finds any occurrence of "bar" whatsoever, because
the assertion (?!foo) is always true when the next three characters are
"bar". A lookbehind assertion is needed to achieve the other effect.
If you want to force a matching failure at some point in a pattern, the
- most convenient way to do it is with (?!) because an empty string
- always matches, so an assertion that requires there not to be an empty
+ most convenient way to do it is with (?!) because an empty string
+ always matches, so an assertion that requires there not to be an empty
string must always fail.
Lookbehind assertions
- Lookbehind assertions start with (?<= for positive assertions and (?<!
+ Lookbehind assertions start with (?<= for positive assertions and (?<!
for negative assertions. For example,
(?<!foo)bar
- does find an occurrence of "bar" that is not preceded by "foo". The
- contents of a lookbehind assertion are restricted such that all the
+ does find an occurrence of "bar" that is not preceded by "foo". The
+ contents of a lookbehind assertion are restricted such that all the
strings it matches must have a fixed length. However, if there are sev-
- eral top-level alternatives, they do not all have to have the same
+ eral top-level alternatives, they do not all have to have the same
fixed length. Thus
(?<=bullock|donkey)
(?<!dogs?|cats?)
- causes an error at compile time. Branches that match different length
- strings are permitted only at the top level of a lookbehind assertion.
- This is an extension compared with Perl (at least for 5.8), which
- requires all branches to match the same length of string. An assertion
+ causes an error at compile time. Branches that match different length
+ strings are permitted only at the top level of a lookbehind assertion.
+ This is an extension compared with Perl (at least for 5.8), which
+ requires all branches to match the same length of string. An assertion
such as
(?<=ab(c|de))
- is not permitted, because its single top-level branch can match two
- different lengths, but it is acceptable if rewritten to use two top-
+ is not permitted, because its single top-level branch can match two
+ different lengths, but it is acceptable if rewritten to use two top-
level branches:
(?<=abc|abde)
- The implementation of lookbehind assertions is, for each alternative,
- to temporarily move the current position back by the fixed length and
+ In some cases, the Perl 5.10 escape sequence \K (see above) can be used
+ instead of a lookbehind assertion; this is not restricted to a fixed-
+ length.
+
+ The implementation of lookbehind assertions is, for each alternative,
+ to temporarily move the current position back by the fixed length and
then try to match. If there are insufficient characters before the cur-
rent position, the assertion fails.
PCRE does not allow the \C escape (which matches a single byte in UTF-8
- mode) to appear in lookbehind assertions, because it makes it impossi-
- ble to calculate the length of the lookbehind. The \X and \R escapes,
+ mode) to appear in lookbehind assertions, because it makes it impossi-
+ ble to calculate the length of the lookbehind. The \X and \R escapes,
which can match different numbers of bytes, are also not permitted.
- Possessive quantifiers can be used in conjunction with lookbehind
- assertions to specify efficient matching at the end of the subject
+ Possessive quantifiers can be used in conjunction with lookbehind
+ assertions to specify efficient matching at the end of the subject
string. Consider a simple pattern such as
abcd$
- when applied to a long string that does not match. Because matching
+ when applied to a long string that does not match. Because matching
proceeds from left to right, PCRE will look for each "a" in the subject
- and then see if what follows matches the rest of the pattern. If the
+ and then see if what follows matches the rest of the pattern. If the
pattern is specified as
^.*abcd$
- the initial .* matches the entire string at first, but when this fails
+ the initial .* matches the entire string at first, but when this fails
(because there is no following "a"), it backtracks to match all but the
- last character, then all but the last two characters, and so on. Once
- again the search for "a" covers the entire string, from right to left,
+ last character, then all but the last two characters, and so on. Once
+ again the search for "a" covers the entire string, from right to left,
so we are no better off. However, if the pattern is written as
^.*+(?<=abcd)
- there can be no backtracking for the .*+ item; it can match only the
- entire string. The subsequent lookbehind assertion does a single test
- on the last four characters. If it fails, the match fails immediately.
- For long strings, this approach makes a significant difference to the
+ there can be no backtracking for the .*+ item; it can match only the
+ entire string. The subsequent lookbehind assertion does a single test
+ on the last four characters. If it fails, the match fails immediately.
+ For long strings, this approach makes a significant difference to the
processing time.
Using multiple assertions
(?<=\d{3})(?<!999)foo
- matches "foo" preceded by three digits that are not "999". Notice that
- each of the assertions is applied independently at the same point in
- the subject string. First there is a check that the previous three
- characters are all digits, and then there is a check that the same
+ matches "foo" preceded by three digits that are not "999". Notice that
+ each of the assertions is applied independently at the same point in
+ the subject string. First there is a check that the previous three
+ characters are all digits, and then there is a check that the same
three characters are not "999". This pattern does not match "foo" pre-
- ceded by six characters, the first of which are digits and the last
- three of which are not "999". For example, it doesn't match "123abc-
+ ceded by six characters, the first of which are digits and the last
+ three of which are not "999". For example, it doesn't match "123abc-
foo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
- This time the first assertion looks at the preceding six characters,
+ This time the first assertion looks at the preceding six characters,
checking that the first three are digits, and then the second assertion
checks that the preceding three characters are not "999".
(?<=(?<!foo)bar)baz
- matches an occurrence of "baz" that is preceded by "bar" which in turn
+ matches an occurrence of "baz" that is preceded by "bar" which in turn
is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
- is another pattern that matches "foo" preceded by three digits and any
+ is another pattern that matches "foo" preceded by three digits and any
three characters that are not "999".
CONDITIONAL SUBPATTERNS
- It is possible to cause the matching process to obey a subpattern con-
- ditionally or to choose between two alternative subpatterns, depending
- on the result of an assertion, or whether a previous capturing subpat-
- tern matched or not. The two possible forms of conditional subpattern
+ It is possible to cause the matching process to obey a subpattern con-
+ ditionally or to choose between two alternative subpatterns, depending
+ on the result of an assertion, or whether a previous capturing subpat-
+ tern matched or not. The two possible forms of conditional subpattern
are
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
- If the condition is satisfied, the yes-pattern is used; otherwise the
- no-pattern (if present) is used. If there are more than two alterna-
+ If the condition is satisfied, the yes-pattern is used; otherwise the
+ no-pattern (if present) is used. If there are more than two alterna-
tives in the subpattern, a compile-time error occurs.
- There are four kinds of condition: references to subpatterns, refer-
+ 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.
+ 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
other words, this pattern matches a sequence of non-parentheses,
optionally enclosed in parentheses.
+ If you were embedding this pattern in a larger one, you could use a
+ relative reference:
+
+ ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
+
+ This makes the fragment independent of the parentheses in the larger
+ pattern.
+
Checking for a used subpattern by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
( \( ( (?>[^()]+) | (?1) )* \) )
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. The Perl syntax for this is
- (?&name); PCRE's earlier syntax (?P>name) is also supported. We could
- rewrite the above example as follows:
+ refer to them instead of the whole pattern.
+
+ In a larger pattern, keeping track of parenthesis numbers can be
+ tricky. This is made easier by the use of relative references. (A Perl
+ 5.10 feature.) Instead of (?1) in the pattern above you can write
+ (?-2) to refer to the second most recently opened parentheses preceding
+ the recursion. In other words, a negative number counts capturing
+ parentheses leftwards from the point at which it is encountered.
+
+ It is also possible to refer to subsequently opened parentheses, by
+ writing references such as (?+2). However, these cannot be recursive
+ because the reference is not inside the parentheses that are refer-
+ enced. They are always "subroutine" calls, as described in the next
+ section.
+
+ An alternative approach is to use named parentheses instead. The Perl
+ syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
+ supported. We could rewrite the above example as follows:
(?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
- If there is more than one subpattern with the same name, the earliest
- one is used. This particular example pattern 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
+ 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()
If the syntax for a recursive subpattern reference (either by number or
by name) is used outside the parentheses to which it refers, it oper-
ates like a subroutine in a programming language. The "called" subpat-
- tern may be defined before or after the reference. An earlier example
- pointed out that the pattern
+ 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
case-independence are fixed when the subpattern is defined. They cannot
be changed for different calls. For example, consider this pattern:
- (abc)(?i:(?1))
+ (abc)(?i:(?-1))
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
-Last updated: 06 December 2006
-Copyright (c) 1997-2006 University of Cambridge.
+
+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 / exim.git / blobdiff