+++ /dev/null
-This file contains the PCRE man page that describes the regular expressions
-supported by PCRE version 7.2. Note that not all of the features are relevant
-in the context of Exim. In particular, the version of PCRE that is compiled
-with Exim does not include UTF-8 support, there is no mechanism for changing
-the options with which the PCRE functions are called, and features such as
-callout are not accessible.
------------------------------------------------------------------------------
-
-PCREPATTERN(3) PCREPATTERN(3)
-
-
-NAME
- PCRE - Perl-compatible regular expressions
-
-
-PCRE REGULAR EXPRESSION DETAILS
-
- The syntax and semantics of the regular expressions supported by PCRE
- are described below. Regular expressions are also described in the Perl
- documentation and in a number of books, some of which have copious
- examples. Jeffrey Friedl's "Mastering Regular Expressions", published
- by O'Reilly, covers regular expressions in great detail. This descrip-
- tion of PCRE's regular expressions is intended as reference material.
-
- The original operation of PCRE was on strings of one-byte characters.
- However, there is now also support for UTF-8 character strings. To use
- this, you must build PCRE to include UTF-8 support, and then call
- pcre_compile() with the PCRE_UTF8 option. How this affects pattern
- matching is mentioned in several places below. There is also a summary
- of UTF-8 features in the section on UTF-8 support in the main pcre
- page.
-
- The remainder of this document discusses the patterns that are sup-
- ported by PCRE when its main matching function, pcre_exec(), is used.
- From release 6.0, PCRE offers a second matching function,
- pcre_dfa_exec(), which matches using a different algorithm that is not
- Perl-compatible. Some of the features discussed below are not available
- when pcre_dfa_exec() is used. The advantages and disadvantages of the
- alternative function, and how it differs from the normal function, are
- discussed in the pcrematching page.
-
-
-CHARACTERS AND METACHARACTERS
-
- A regular expression is a pattern that is matched against a subject
- string from left to right. Most characters stand for themselves in a
- pattern, and match the corresponding characters in the subject. As a
- trivial example, the pattern
-
- The quick brown fox
-
- matches a portion of a subject string that is identical to itself. When
- caseless matching is specified (the PCRE_CASELESS option), letters are
- matched independently of case. In UTF-8 mode, PCRE always understands
- the concept of case for characters whose values are less than 128, so
- caseless matching is always possible. For characters with higher val-
- ues, the concept of case is supported if PCRE is compiled with Unicode
- property support, but not otherwise. If you want to use caseless
- matching for characters 128 and above, you must ensure that PCRE is
- compiled with Unicode property support as well as with UTF-8 support.
-
- The power of regular expressions comes from the ability to include
- alternatives and repetitions in the pattern. These are encoded in the
- pattern by the use of metacharacters, which do not stand for themselves
- but instead are interpreted in some special way.
-
- There are two different sets of metacharacters: those that are recog-
- nized anywhere in the pattern except within square brackets, and those
- that are recognized within square brackets. Outside square brackets,
- the metacharacters are as follows:
-
- \ general escape character with several uses
- ^ assert start of string (or line, in multiline mode)
- $ assert end of string (or line, in multiline mode)
- . match any character except newline (by default)
- [ start character class definition
- | start of alternative branch
- ( start subpattern
- ) end subpattern
- ? extends the meaning of (
- also 0 or 1 quantifier
- also quantifier minimizer
- * 0 or more quantifier
- + 1 or more quantifier
- also "possessive quantifier"
- { start min/max quantifier
-
- Part of a pattern that is in square brackets is called a "character
- class". In a character class the only metacharacters are:
-
- \ general escape character
- ^ negate the class, but only if the first character
- - indicates character range
- [ POSIX character class (only if followed by POSIX
- syntax)
- ] terminates the character class
-
- The following sections describe the use of each of the metacharacters.
-
-
-BACKSLASH
-
- The backslash character has several uses. Firstly, if it is followed by
- a non-alphanumeric character, it takes away any special meaning that
- character may have. This use of backslash as an escape character
- applies both inside and outside character classes.
-
- For example, if you want to match a * character, you write \* in the
- pattern. This escaping action applies whether or not the following
- character would otherwise be interpreted as a metacharacter, so it is
- always safe to precede a non-alphanumeric with backslash to specify
- that it stands for itself. In particular, if you want to match a back-
- slash, you write \\.
-
- If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
- the pattern (other than in a character class) and characters between a
- # outside a character class and the next newline are ignored. An escap-
- ing backslash can be used to include a whitespace or # character as
- part of the pattern.
-
- If you want to remove the special meaning from a sequence of charac-
- ters, you can do so by putting them between \Q and \E. This is differ-
- ent from Perl in that $ and @ are handled as literals in \Q...\E
- sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
- tion. Note the following examples:
-
- Pattern PCRE matches Perl matches
-
- \Qabc$xyz\E abc$xyz abc followed by the
- contents of $xyz
- \Qabc\$xyz\E abc\$xyz abc\$xyz
- \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
-
- The \Q...\E sequence is recognized both inside and outside character
- classes.
-
- Non-printing characters
-
- A second use of backslash provides a way of encoding non-printing char-
- acters in patterns in a visible manner. There is no restriction on the
- appearance of non-printing characters, apart from the binary zero that
- terminates a pattern, but when a pattern is being prepared by text
- editing, it is usually easier to use one of the following escape
- sequences than the binary character it represents:
-
- \a alarm, that is, the BEL character (hex 07)
- \cx "control-x", where x is any character
- \e escape (hex 1B)
- \f formfeed (hex 0C)
- \n newline (hex 0A)
- \r carriage return (hex 0D)
- \t tab (hex 09)
- \ddd character with octal code ddd, or backreference
- \xhh character with hex code hh
- \x{hhh..} character with hex code hhh..
-
- The precise effect of \cx is as follows: if x is a lower case letter,
- it is converted to upper case. Then bit 6 of the character (hex 40) is
- inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
- becomes hex 7B.
-
- After \x, from zero to two hexadecimal digits are read (letters can be
- in upper or lower case). Any number of hexadecimal digits may appear
- between \x{ and }, but the value of the character code must be less
- than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is,
- the maximum hexadecimal value is 7FFFFFFF). If characters other than
- hexadecimal digits appear between \x{ and }, or if there is no termi-
- nating }, this form of escape is not recognized. Instead, the initial
- \x will be interpreted as a basic hexadecimal escape, with no following
- digits, giving a character whose value is zero.
-
- Characters whose value is less than 256 can be defined by either of the
- two syntaxes for \x. There is no difference in the way they are han-
- dled. For example, \xdc is exactly the same as \x{dc}.
-
- After \0 up to two further octal digits are read. If there are fewer
- than two digits, just those that are present are used. Thus the
- sequence \0\x\07 specifies two binary zeros followed by a BEL character
- (code value 7). Make sure you supply two digits after the initial zero
- if the pattern character that follows is itself an octal digit.
-
- The handling of a backslash followed by a digit other than 0 is compli-
- cated. Outside a character class, PCRE reads it and any following dig-
- its as a decimal number. If the number is less than 10, or if there
- have been at least that many previous capturing left parentheses in the
- expression, the entire sequence is taken as a back reference. A
- description of how this works is given later, following the discussion
- of parenthesized subpatterns.
-
- Inside a character class, or if the decimal number is greater than 9
- and there have not been that many capturing subpatterns, PCRE re-reads
- up to three octal digits following the backslash, and uses them to gen-
- erate a data character. Any subsequent digits stand for themselves. In
- non-UTF-8 mode, the value of a character specified in octal must be
- less than \400. In UTF-8 mode, values up to \777 are permitted. For
- example:
-
- \040 is another way of writing a space
- \40 is the same, provided there are fewer than 40
- previous capturing subpatterns
- \7 is always a back reference
- \11 might be a back reference, or another way of
- writing a tab
- \011 is always a tab
- \0113 is a tab followed by the character "3"
- \113 might be a back reference, otherwise the
- character with octal code 113
- \377 might be a back reference, otherwise
- the byte consisting entirely of 1 bits
- \81 is either a back reference, or a binary zero
- followed by the two characters "8" and "1"
-
- Note that octal values of 100 or greater must not be introduced by a
- leading zero, because no more than three octal digits are ever read.
-
- All the sequences that define a single character value can be used both
- inside and outside character classes. In addition, inside a character
- class, the sequence \b is interpreted as the backspace character (hex
- 08), and the sequences \R and \X are interpreted as the characters "R"
- and "X", respectively. Outside a character class, these sequences have
- different meanings (see below).
-
- Absolute and relative back references
-
- The sequence \g followed by a positive or negative number, optionally
- enclosed in braces, is an absolute or relative back reference. A named
- back reference can be coded as \g{name}. Back references are discussed
- later, following the discussion of parenthesized subpatterns.
-
- Generic character types
-
- Another use of backslash is for specifying generic character types. The
- following are always recognized:
-
- \d any decimal digit
- \D any character that is not a decimal digit
- \h any horizontal whitespace character
- \H any character that is not a horizontal whitespace character
- \s any whitespace character
- \S any character that is not a whitespace character
- \v any vertical whitespace character
- \V any character that is not a vertical whitespace character
- \w any "word" character
- \W any "non-word" character
-
- Each pair of escape sequences partitions the complete set of characters
- into two disjoint sets. Any given character matches one, and only one,
- of each pair.
-
- These character type sequences can appear both inside and outside char-
- acter classes. They each match one character of the appropriate type.
- If the current matching point is at the end of the subject string, all
- of them fail, since there is no character to match.
-
- For compatibility with Perl, \s does not match the VT character (code
- 11). This makes it different from the the POSIX "space" class. The \s
- characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
- "use locale;" is included in a Perl script, \s may match the VT charac-
- ter. In PCRE, it never does.
-
- In UTF-8 mode, characters with values greater than 128 never match \d,
- \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
- code character property support is available. These sequences retain
- their original meanings from before UTF-8 support was available, mainly
- for efficiency reasons.
-
- The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
- the other sequences, these do match certain high-valued codepoints in
- UTF-8 mode. The horizontal space characters are:
-
- U+0009 Horizontal tab
- U+0020 Space
- U+00A0 Non-break space
- U+1680 Ogham space mark
- U+180E Mongolian vowel separator
- U+2000 En quad
- U+2001 Em quad
- U+2002 En space
- U+2003 Em space
- U+2004 Three-per-em space
- U+2005 Four-per-em space
- U+2006 Six-per-em space
- U+2007 Figure space
- U+2008 Punctuation space
- U+2009 Thin space
- U+200A Hair space
- U+202F Narrow no-break space
- U+205F Medium mathematical space
- U+3000 Ideographic space
-
- The vertical space characters are:
-
- U+000A Linefeed
- U+000B Vertical tab
- U+000C Formfeed
- U+000D Carriage return
- U+0085 Next line
- U+2028 Line separator
- U+2029 Paragraph separator
-
- A "word" character is an underscore or any character less than 256 that
- is a letter or digit. The definition of letters and digits is con-
- trolled by PCRE's low-valued character tables, and may vary if locale-
- specific matching is taking place (see "Locale support" in the pcreapi
- page). For example, in a French locale such as "fr_FR" in Unix-like
- systems, or "french" in Windows, some character codes greater than 128
- are used for accented letters, and these are matched by \w. The use of
- locales with Unicode is discouraged.
-
- Newline sequences
-
- Outside a character class, the escape sequence \R matches any Unicode
- newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
- equivalent to the following:
-
- (?>\r\n|\n|\x0b|\f|\r|\x85)
-
- This is an example of an "atomic group", details of which are given
- below. This particular group matches either the two-character sequence
- CR followed by LF, or one of the single characters LF (linefeed,
- U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
- return, U+000D), or NEL (next line, U+0085). The two-character sequence
- is treated as a single unit that cannot be split.
-
- In UTF-8 mode, two additional characters whose codepoints are greater
- than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
- rator, U+2029). Unicode character property support is not needed for
- these characters to be recognized.
-
- Inside a character class, \R matches the letter "R".
-
- Unicode character properties
-
- When PCRE is built with Unicode character property support, three addi-
- tional escape sequences that match characters with specific properties
- are available. When not in UTF-8 mode, these sequences are of course
- limited to testing characters whose codepoints are less than 256, but
- they do work in this mode. The extra escape sequences are:
-
- \p{xx} a character with the xx property
- \P{xx} a character without the xx property
- \X an extended Unicode sequence
-
- The property names represented by xx above are limited to the Unicode
- script names, the general category properties, and "Any", which matches
- any character (including newline). Other properties such as "InMusical-
- Symbols" are not currently supported by PCRE. Note that \P{Any} does
- not match any characters, so always causes a match failure.
-
- Sets of Unicode characters are defined as belonging to certain scripts.
- A character from one of these sets can be matched using a script name.
- For example:
-
- \p{Greek}
- \P{Han}
-
- Those that are not part of an identified script are lumped together as
- "Common". The current list of scripts is:
-
- Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
- Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
- Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
- Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
- gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
- Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
- Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
- Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
- Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
-
- Each character has exactly one general category property, specified by
- a two-letter abbreviation. For compatibility with Perl, negation can be
- specified by including a circumflex between the opening brace and the
- property name. For example, \p{^Lu} is the same as \P{Lu}.
-
- If only one letter is specified with \p or \P, it includes all the gen-
- eral category properties that start with that letter. In this case, in
- the absence of negation, the curly brackets in the escape sequence are
- optional; these two examples have the same effect:
-
- \p{L}
- \pL
-
- The following general category property codes are supported:
-
- C Other
- Cc Control
- Cf Format
- Cn Unassigned
- Co Private use
- Cs Surrogate
-
- L Letter
- Ll Lower case letter
- Lm Modifier letter
- Lo Other letter
- Lt Title case letter
- Lu Upper case letter
-
- M Mark
- Mc Spacing mark
- Me Enclosing mark
- Mn Non-spacing mark
-
- N Number
- Nd Decimal number
- Nl Letter number
- No Other number
-
- P Punctuation
- Pc Connector punctuation
- Pd Dash punctuation
- Pe Close punctuation
- Pf Final punctuation
- Pi Initial punctuation
- Po Other punctuation
- Ps Open punctuation
-
- S Symbol
- Sc Currency symbol
- Sk Modifier symbol
- Sm Mathematical symbol
- So Other symbol
-
- Z Separator
- Zl Line separator
- Zp Paragraph separator
- Zs Space separator
-
- The special property L& is also supported: it matches a character that
- has the Lu, Ll, or Lt property, in other words, a letter that is not
- classified as a modifier or "other".
-
- The long synonyms for these properties that Perl supports (such as
- \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
- any of these properties with "Is".
-
- No character that is in the Unicode table has the Cn (unassigned) prop-
- erty. Instead, this property is assumed for any code point that is not
- in the Unicode table.
-
- Specifying caseless matching does not affect these escape sequences.
- For example, \p{Lu} always matches only upper case letters.
-
- The \X escape matches any number of Unicode characters that form an
- extended Unicode sequence. \X is equivalent to
-
- (?>\PM\pM*)
-
- That is, it matches a character without the "mark" property, followed
- by zero or more characters with the "mark" property, and treats the
- sequence as an atomic group (see below). Characters with the "mark"
- property are typically accents that affect the preceding character.
- None of them have codepoints less than 256, so in non-UTF-8 mode \X
- matches any one character.
-
- Matching characters by Unicode property is not fast, because PCRE has
- to search a structure that contains data for over fifteen thousand
- characters. That is why the traditional escape sequences such as \d and
- \w do not use Unicode properties in PCRE.
-
- Resetting the match start
-
- The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
- ously matched characters not to be included in the final matched
- sequence. For example, the pattern:
-
- foo\Kbar
-
- matches "foobar", but reports that it has matched "bar". This feature
- is similar to a lookbehind assertion (described below). However, in
- this case, the part of the subject before the real match does not have
- to be of fixed length, as lookbehind assertions do. The use of \K does
- not interfere with the setting of captured substrings. For example,
- when the pattern
-
- (foo)\Kbar
-
- matches "foobar", the first substring is still set to "foo".
-
- Simple assertions
-
- The final use of backslash is for certain simple assertions. An asser-
- tion specifies a condition that has to be met at a particular point in
- a match, without consuming any characters from the subject string. The
- use of subpatterns for more complicated assertions is described below.
- The backslashed assertions are:
-
- \b matches at a word boundary
- \B matches when not at a word boundary
- \A matches at the start of the subject
- \Z matches at the end of the subject
- also matches before a newline at the end of the subject
- \z matches only at the end of the subject
- \G matches at the first matching position in the subject
-
- These assertions may not appear in character classes (but note that \b
- has a different meaning, namely the backspace character, inside a char-
- acter class).
-
- A word boundary is a position in the subject string where the current
- character and the previous character do not both match \w or \W (i.e.
- one matches \w and the other matches \W), or the start or end of the
- string if the first or last character matches \w, respectively.
-
- The \A, \Z, and \z assertions differ from the traditional circumflex
- and dollar (described in the next section) in that they only ever match
- at the very start and end of the subject string, whatever options are
- set. Thus, they are independent of multiline mode. These three asser-
- tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
- affect only the behaviour of the circumflex and dollar metacharacters.
- However, if the startoffset argument of pcre_exec() is non-zero, indi-
- cating that matching is to start at a point other than the beginning of
- the subject, \A can never match. The difference between \Z and \z is
- that \Z matches before a newline at the end of the string as well as at
- the very end, whereas \z matches only at the end.
-
- The \G assertion is true only when the current matching position is at
- the start point of the match, as specified by the startoffset argument
- of pcre_exec(). It differs from \A when the value of startoffset is
- non-zero. By calling pcre_exec() multiple times with appropriate argu-
- ments, you can mimic Perl's /g option, and it is in this kind of imple-
- mentation where \G can be useful.
-
- Note, however, that PCRE's interpretation of \G, as the start of the
- current match, is subtly different from Perl's, which defines it as the
- end of the previous match. In Perl, these can be different when the
- previously matched string was empty. Because PCRE does just one match
- at a time, it cannot reproduce this behaviour.
-
- If all the alternatives of a pattern begin with \G, the expression is
- anchored to the starting match position, and the "anchored" flag is set
- in the compiled regular expression.
-
-
-CIRCUMFLEX AND DOLLAR
-
- Outside a character class, in the default matching mode, the circumflex
- character is an assertion that is true only if the current matching
- point is at the start of the subject string. If the startoffset argu-
- ment of pcre_exec() is non-zero, circumflex can never match if the
- PCRE_MULTILINE option is unset. Inside a character class, circumflex
- has an entirely different meaning (see below).
-
- Circumflex need not be the first character of the pattern if a number
- of alternatives are involved, but it should be the first thing in each
- alternative in which it appears if the pattern is ever to match that
- branch. If all possible alternatives start with a circumflex, that is,
- if the pattern is constrained to match only at the start of the sub-
- ject, it is said to be an "anchored" pattern. (There are also other
- constructs that can cause a pattern to be anchored.)
-
- A dollar character is an assertion that is true only if the current
- matching point is at the end of the subject string, or immediately
- before a newline at the end of the string (by default). Dollar need not
- be the last character of the pattern if a number of alternatives are
- involved, but it should be the last item in any branch in which it
- appears. Dollar has no special meaning in a character class.
-
- The meaning of dollar can be changed so that it matches only at the
- very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
- compile time. This does not affect the \Z assertion.
-
- The meanings of the circumflex and dollar characters are changed if the
- PCRE_MULTILINE option is set. When this is the case, a circumflex
- matches immediately after internal newlines as well as at the start of
- the subject string. It does not match after a newline that ends the
- string. A dollar matches before any newlines in the string, as well as
- at the very end, when PCRE_MULTILINE is set. When newline is specified
- as the two-character sequence CRLF, isolated CR and LF characters do
- not indicate newlines.
-
- For example, the pattern /^abc$/ matches the subject string "def\nabc"
- (where \n represents a newline) in multiline mode, but not otherwise.
- Consequently, patterns that are anchored in single line mode because
- all branches start with ^ are not anchored in multiline mode, and a
- match for circumflex is possible when the startoffset argument of
- pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
- PCRE_MULTILINE is set.
-
- Note that the sequences \A, \Z, and \z can be used to match the start
- and end of the subject in both modes, and if all branches of a pattern
- start with \A it is always anchored, whether or not PCRE_MULTILINE is
- set.
-
-
-FULL STOP (PERIOD, DOT)
-
- Outside a character class, a dot in the pattern matches any one charac-
- ter in the subject string except (by default) a character that signi-
- fies the end of a line. In UTF-8 mode, the matched character may be
- more than one byte long.
-
- When a line ending is defined as a single character, dot never matches
- that character; when the two-character sequence CRLF is used, dot does
- not match CR if it is immediately followed by LF, but otherwise it
- matches all characters (including isolated CRs and LFs). When any Uni-
- code line endings are being recognized, dot does not match CR or LF or
- any of the other line ending characters.
-
- The behaviour of dot with regard to newlines can be changed. If the
- PCRE_DOTALL option is set, a dot matches any one character, without
- exception. If the two-character sequence CRLF is present in the subject
- string, it takes two dots to match it.
-
- The handling of dot is entirely independent of the handling of circum-
- flex and dollar, the only relationship being that they both involve
- newlines. Dot has no special meaning in a character class.
-
-
-MATCHING A SINGLE BYTE
-
- Outside a character class, the escape sequence \C matches any one byte,
- both in and out of UTF-8 mode. Unlike a dot, it always matches any
- line-ending characters. The feature is provided in Perl in order to
- match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
- acters into individual bytes, what remains in the string may be a mal-
- formed UTF-8 string. For this reason, the \C escape sequence is best
- avoided.
-
- PCRE does not allow \C to appear in lookbehind assertions (described
- below), because in UTF-8 mode this would make it impossible to calcu-
- late the length of the lookbehind.
-
-
-SQUARE BRACKETS AND CHARACTER CLASSES
-
- An opening square bracket introduces a character class, terminated by a
- closing square bracket. A closing square bracket on its own is not spe-
- cial. If a closing square bracket is required as a member of the class,
- it should be the first data character in the class (after an initial
- circumflex, if present) or escaped with a backslash.
-
- A character class matches a single character in the subject. In UTF-8
- mode, the character may occupy more than one byte. A matched character
- must be in the set of characters defined by the class, unless the first
- character in the class definition is a circumflex, in which case the
- subject character must not be in the set defined by the class. If a
- circumflex is actually required as a member of the class, ensure it is
- not the first character, or escape it with a backslash.
-
- For example, the character class [aeiou] matches any lower case vowel,
- while [^aeiou] matches any character that is not a lower case vowel.
- Note that a circumflex is just a convenient notation for specifying the
- characters that are in the class by enumerating those that are not. A
- class that starts with a circumflex is not an assertion: it still con-
- sumes a character from the subject string, and therefore it fails if
- the current pointer is at the end of the string.
-
- In UTF-8 mode, characters with values greater than 255 can be included
- in a class as a literal string of bytes, or by using the \x{ escaping
- mechanism.
-
- When caseless matching is set, any letters in a class represent both
- their upper case and lower case versions, so for example, a caseless
- [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
- match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
- understands the concept of case for characters whose values are less
- than 128, so caseless matching is always possible. For characters with
- higher values, the concept of case is supported if PCRE is compiled
- with Unicode property support, but not otherwise. If you want to use
- caseless matching for characters 128 and above, you must ensure that
- PCRE is compiled with Unicode property support as well as with UTF-8
- support.
-
- Characters that might indicate line breaks are never treated in any
- special way when matching character classes, whatever line-ending
- sequence is in use, and whatever setting of the PCRE_DOTALL and
- PCRE_MULTILINE options is used. A class such as [^a] always matches one
- of these characters.
-
- The minus (hyphen) character can be used to specify a range of charac-
- ters in a character class. For example, [d-m] matches any letter
- between d and m, inclusive. If a minus character is required in a
- class, it must be escaped with a backslash or appear in a position
- where it cannot be interpreted as indicating a range, typically as the
- first or last character in the class.
-
- It is not possible to have the literal character "]" as the end charac-
- ter of a range. A pattern such as [W-]46] is interpreted as a class of
- two characters ("W" and "-") followed by a literal string "46]", so it
- would match "W46]" or "-46]". However, if the "]" is escaped with a
- backslash it is interpreted as the end of range, so [W-\]46] is inter-
- preted as a class containing a range followed by two other characters.
- The octal or hexadecimal representation of "]" can also be used to end
- a range.
-
- Ranges operate in the collating sequence of character values. They can
- also be used for characters specified numerically, for example
- [\000-\037]. In UTF-8 mode, ranges can include characters whose values
- are greater than 255, for example [\x{100}-\x{2ff}].
-
- If a range that includes letters is used when caseless matching is set,
- it matches the letters in either case. For example, [W-c] is equivalent
- to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
- character tables for a French locale are in use, [\xc8-\xcb] matches
- accented E characters in both cases. In UTF-8 mode, PCRE supports the
- concept of case for characters with values greater than 128 only when
- it is compiled with Unicode property support.
-
- The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
- in a character class, and add the characters that they match to the
- class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
- flex can conveniently be used with the upper case character types to
- specify a more restricted set of characters than the matching lower
- case type. For example, the class [^\W_] matches any letter or digit,
- but not underscore.
-
- The only metacharacters that are recognized in character classes are
- backslash, hyphen (only where it can be interpreted as specifying a
- range), circumflex (only at the start), opening square bracket (only
- when it can be interpreted as introducing a POSIX class name - see the
- next section), and the terminating closing square bracket. However,
- escaping other non-alphanumeric characters does no harm.
-
-
-POSIX CHARACTER CLASSES
-
- Perl supports the POSIX notation for character classes. This uses names
- enclosed by [: and :] within the enclosing square brackets. PCRE also
- supports this notation. For example,
-
- [01[:alpha:]%]
-
- matches "0", "1", any alphabetic character, or "%". The supported class
- names are
-
- alnum letters and digits
- alpha letters
- ascii character codes 0 - 127
- blank space or tab only
- cntrl control characters
- digit decimal digits (same as \d)
- graph printing characters, excluding space
- lower lower case letters
- print printing characters, including space
- punct printing characters, excluding letters and digits
- space white space (not quite the same as \s)
- upper upper case letters
- word "word" characters (same as \w)
- xdigit hexadecimal digits
-
- The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
- and space (32). Notice that this list includes the VT character (code
- 11). This makes "space" different to \s, which does not include VT (for
- Perl compatibility).
-
- The name "word" is a Perl extension, and "blank" is a GNU extension
- from Perl 5.8. Another Perl extension is negation, which is indicated
- by a ^ character after the colon. For example,
-
- [12[:^digit:]]
-
- matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
- POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
- these are not supported, and an error is given if they are encountered.
-
- In UTF-8 mode, characters with values greater than 128 do not match any
- of the POSIX character classes.
-
-
-VERTICAL BAR
-
- Vertical bar characters are used to separate alternative patterns. For
- example, the pattern
-
- gilbert|sullivan
-
- matches either "gilbert" or "sullivan". Any number of alternatives may
- appear, and an empty alternative is permitted (matching the empty
- string). The matching process tries each alternative in turn, from left
- to right, and the first one that succeeds is used. If the alternatives
- are within a subpattern (defined below), "succeeds" means matching the
- rest of the main pattern as well as the alternative in the subpattern.
-
-
-INTERNAL OPTION SETTING
-
- The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
- PCRE_EXTENDED options can be changed from within the pattern by a
- sequence of Perl option letters enclosed between "(?" and ")". The
- option letters are
-
- i for PCRE_CASELESS
- m for PCRE_MULTILINE
- s for PCRE_DOTALL
- x for PCRE_EXTENDED
-
- For example, (?im) sets caseless, multiline matching. It is also possi-
- ble to unset these options by preceding the letter with a hyphen, and a
- combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
- LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
- is also permitted. If a letter appears both before and after the
- hyphen, the option is unset.
-
- When an option change occurs at top level (that is, not inside subpat-
- tern parentheses), the change applies to the remainder of the pattern
- that follows. If the change is placed right at the start of a pattern,
- PCRE extracts it into the global options (and it will therefore show up
- in data extracted by the pcre_fullinfo() function).
-
- An option change within a subpattern (see below for a description of
- subpatterns) affects only that part of the current pattern that follows
- it, so
-
- (a(?i)b)c
-
- matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
- used). By this means, options can be made to have different settings
- in different parts of the pattern. Any changes made in one alternative
- do carry on into subsequent branches within the same subpattern. For
- example,
-
- (a(?i)b|c)
-
- matches "ab", "aB", "c", and "C", even though when matching "C" the
- first branch is abandoned before the option setting. This is because
- the effects of option settings happen at compile time. There would be
- some very weird behaviour otherwise.
-
- The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
- can be changed in the same way as the Perl-compatible options by using
- the characters J, U and X respectively.
-
-
-SUBPATTERNS
-
- Subpatterns are delimited by parentheses (round brackets), which can be
- nested. Turning part of a pattern into a subpattern does two things:
-
- 1. It localizes a set of alternatives. For example, the pattern
-
- cat(aract|erpillar|)
-
- matches one of the words "cat", "cataract", or "caterpillar". Without
- the parentheses, it would match "cataract", "erpillar" or an empty
- string.
-
- 2. It sets up the subpattern as a capturing subpattern. This means
- that, when the whole pattern matches, that portion of the subject
- string that matched the subpattern is passed back to the caller via the
- ovector argument of pcre_exec(). Opening parentheses are counted from
- left to right (starting from 1) to obtain numbers for the capturing
- subpatterns.
-
- For example, if the string "the red king" is matched against the pat-
- tern
-
- the ((red|white) (king|queen))
-
- the captured substrings are "red king", "red", and "king", and are num-
- bered 1, 2, and 3, respectively.
-
- The fact that plain parentheses fulfil two functions is not always
- helpful. There are often times when a grouping subpattern is required
- without a capturing requirement. If an opening parenthesis is followed
- by a question mark and a colon, the subpattern does not do any captur-
- ing, and is not counted when computing the number of any subsequent
- capturing subpatterns. For example, if the string "the white queen" is
- matched against the pattern
-
- the ((?:red|white) (king|queen))
-
- the captured substrings are "white queen" and "queen", and are numbered
- 1 and 2. The maximum number of capturing subpatterns is 65535.
-
- As a convenient shorthand, if any option settings are required at the
- start of a non-capturing subpattern, the option letters may appear
- between the "?" and the ":". Thus the two patterns
-
- (?i:saturday|sunday)
- (?:(?i)saturday|sunday)
-
- match exactly the same set of strings. Because alternative branches are
- tried from left to right, and options are not reset until the end of
- the subpattern is reached, an option setting in one branch does affect
- subsequent branches, so the above patterns match "SUNDAY" as well as
- "Saturday".
-
-
-DUPLICATE SUBPATTERN NUMBERS
-
- Perl 5.10 introduced a feature whereby each alternative in a subpattern
- uses the same numbers for its capturing parentheses. Such a subpattern
- starts with (?| and is itself a non-capturing subpattern. For example,
- consider this pattern:
-
- (?|(Sat)ur|(Sun))day
-
- Because the two alternatives are inside a (?| group, both sets of cap-
- turing parentheses are numbered one. Thus, when the pattern matches,
- you can look at captured substring number one, whichever alternative
- matched. This construct is useful when you want to capture part, but
- not all, of one of a number of alternatives. Inside a (?| group, paren-
- theses are numbered as usual, but the number is reset at the start of
- each branch. The numbers of any capturing buffers that follow the sub-
- pattern start after the highest number used in any branch. The follow-
- ing example is taken from the Perl documentation. The numbers under-
- neath show in which buffer the captured content will be stored.
-
- # before ---------------branch-reset----------- after
- / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
- # 1 2 2 3 2 3 4
-
- A backreference or a recursive call to a numbered subpattern always
- refers to the first one in the pattern with the given number.
-
- An alternative approach to using this "branch reset" feature is to use
- duplicate named subpatterns, as described in the next section.
-
-
-NAMED SUBPATTERNS
-
- Identifying capturing parentheses by number is simple, but it can be
- very hard to keep track of the numbers in complicated regular expres-
- sions. Furthermore, if an expression is modified, the numbers may
- change. To help with this difficulty, PCRE supports the naming of sub-
- patterns. This feature was not added to Perl until release 5.10. Python
- had the feature earlier, and PCRE introduced it at release 4.0, using
- the Python syntax. PCRE now supports both the Perl and the Python syn-
- tax.
-
- In PCRE, a subpattern can be named in one of three ways: (?<name>...)
- or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
- to capturing parentheses from other parts of the pattern, such as back-
- references, recursion, and conditions, can be made by name as well as
- by number.
-
- Names consist of up to 32 alphanumeric characters and underscores.
- Named capturing parentheses are still allocated numbers as well as
- names, exactly as if the names were not present. The PCRE API provides
- function calls for extracting the name-to-number translation table from
- a compiled pattern. There is also a convenience function for extracting
- a captured substring by name.
-
- By default, a name must be unique within a pattern, but it is possible
- to relax this constraint by setting the PCRE_DUPNAMES option at compile
- time. This can be useful for patterns where only one instance of the
- named parentheses can match. Suppose you want to match the name of a
- weekday, either as a 3-letter abbreviation or as the full name, and in
- both cases you want to extract the abbreviation. This pattern (ignoring
- the line breaks) does the job:
-
- (?<DN>Mon|Fri|Sun)(?:day)?|
- (?<DN>Tue)(?:sday)?|
- (?<DN>Wed)(?:nesday)?|
- (?<DN>Thu)(?:rsday)?|
- (?<DN>Sat)(?:urday)?
-
- There are five capturing substrings, but only one is ever set after a
- match. (An alternative way of solving this problem is to use a "branch
- reset" subpattern, as described in the previous section.)
-
- The convenience function for extracting the data by name returns the
- substring for the first (and in this example, the only) subpattern of
- that name that matched. This saves searching to find which numbered
- subpattern it was. If you make a reference to a non-unique named sub-
- pattern from elsewhere in the pattern, the one that corresponds to the
- lowest number is used. For further details of the interfaces for han-
- dling named subpatterns, see the pcreapi documentation.
-
-
-REPETITION
-
- Repetition is specified by quantifiers, which can follow any of the
- following items:
-
- a literal data character
- the dot metacharacter
- the \C escape sequence
- the \X escape sequence (in UTF-8 mode with Unicode properties)
- the \R escape sequence
- an escape such as \d that matches a single character
- a character class
- a back reference (see next section)
- a parenthesized subpattern (unless it is an assertion)
-
- The general repetition quantifier specifies a minimum and maximum num-
- ber of permitted matches, by giving the two numbers in curly brackets
- (braces), separated by a comma. The numbers must be less than 65536,
- and the first must be less than or equal to the second. For example:
-
- z{2,4}
-
- matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
- special character. If the second number is omitted, but the comma is
- present, there is no upper limit; if the second number and the comma
- are both omitted, the quantifier specifies an exact number of required
- matches. Thus
-
- [aeiou]{3,}
-
- matches at least 3 successive vowels, but may match many more, while
-
- \d{8}
-
- matches exactly 8 digits. An opening curly bracket that appears in a
- position where a quantifier is not allowed, or one that does not match
- the syntax of a quantifier, is taken as a literal character. For exam-
- ple, {,6} is not a quantifier, but a literal string of four characters.
-
- In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
- individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
- acters, each of which is represented by a two-byte sequence. Similarly,
- when Unicode property support is available, \X{3} matches three Unicode
- extended sequences, each of which may be several bytes long (and they
- may be of different lengths).
-
- The quantifier {0} is permitted, causing the expression to behave as if
- the previous item and the quantifier were not present.
-
- For convenience, the three most common quantifiers have single-charac-
- ter abbreviations:
-
- * is equivalent to {0,}
- + is equivalent to {1,}
- ? is equivalent to {0,1}
-
- It is possible to construct infinite loops by following a subpattern
- that can match no characters with a quantifier that has no upper limit,
- for example:
-
- (a?)*
-
- Earlier versions of Perl and PCRE used to give an error at compile time
- for such patterns. However, because there are cases where this can be
- useful, such patterns are now accepted, but if any repetition of the
- subpattern does in fact match no characters, the loop is forcibly bro-
- ken.
-
- By default, the quantifiers are "greedy", that is, they match as much
- as possible (up to the maximum number of permitted times), without
- causing the rest of the pattern to fail. The classic example of where
- this gives problems is in trying to match comments in C programs. These
- appear between /* and */ and within the comment, individual * and /
- characters may appear. An attempt to match C comments by applying the
- pattern
-
- /\*.*\*/
-
- to the string
-
- /* first comment */ not comment /* second comment */
-
- fails, because it matches the entire string owing to the greediness of
- the .* item.
-
- However, if a quantifier is followed by a question mark, it ceases to
- be greedy, and instead matches the minimum number of times possible, so
- the pattern
-
- /\*.*?\*/
-
- does the right thing with the C comments. The meaning of the various
- quantifiers is not otherwise changed, just the preferred number of
- matches. Do not confuse this use of question mark with its use as a
- quantifier in its own right. Because it has two uses, it can sometimes
- appear doubled, as in
-
- \d??\d
-
- which matches one digit by preference, but can match two if that is the
- only way the rest of the pattern matches.
-
- If the PCRE_UNGREEDY option is set (an option that is not available in
- Perl), the quantifiers are not greedy by default, but individual ones
- can be made greedy by following them with a question mark. In other
- words, it inverts the default behaviour.
-
- When a parenthesized subpattern is quantified with a minimum repeat
- count that is greater than 1 or with a limited maximum, more memory is
- required for the compiled pattern, in proportion to the size of the
- minimum or maximum.
-
- If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
- alent to Perl's /s) is set, thus allowing the dot to match newlines,
- the pattern is implicitly anchored, because whatever follows will be
- tried against every character position in the subject string, so there
- is no point in retrying the overall match at any position after the
- first. PCRE normally treats such a pattern as though it were preceded
- by \A.
-
- In cases where it is known that the subject string contains no new-
- lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
- mization, or alternatively using ^ to indicate anchoring explicitly.
-
- However, there is one situation where the optimization cannot be used.
- When .* is inside capturing parentheses that are the subject of a
- backreference elsewhere in the pattern, a match at the start may fail
- where a later one succeeds. Consider, for example:
-
- (.*)abc\1
-
- If the subject is "xyz123abc123" the match point is the fourth charac-
- ter. For this reason, such a pattern is not implicitly anchored.
-
- When a capturing subpattern is repeated, the value captured is the sub-
- string that matched the final iteration. For example, after
-
- (tweedle[dume]{3}\s*)+
-
- has matched "tweedledum tweedledee" the value of the captured substring
- is "tweedledee". However, if there are nested capturing subpatterns,
- the corresponding captured values may have been set in previous itera-
- tions. For example, after
-
- /(a|(b))+/
-
- matches "aba" the value of the second captured substring is "b".
-
-
-ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
-
- With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
- repetition, failure of what follows normally causes the repeated item
- to be re-evaluated to see if a different number of repeats allows the
- rest of the pattern to match. Sometimes it is useful to prevent this,
- either to change the nature of the match, or to cause it fail earlier
- than it otherwise might, when the author of the pattern knows there is
- no point in carrying on.
-
- Consider, for example, the pattern \d+foo when applied to the subject
- line
-
- 123456bar
-
- After matching all 6 digits and then failing to match "foo", the normal
- action of the matcher is to try again with only 5 digits matching the
- \d+ item, and then with 4, and so on, before ultimately failing.
- "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
- the means for specifying that once a subpattern has matched, it is not
- to be re-evaluated in this way.
-
- If we use atomic grouping for the previous example, the matcher gives
- up immediately on failing to match "foo" the first time. The notation
- is a kind of special parenthesis, starting with (?> as in this example:
-
- (?>\d+)foo
-
- This kind of parenthesis "locks up" the part of the pattern it con-
- tains once it has matched, and a failure further into the pattern is
- prevented from backtracking into it. Backtracking past it to previous
- items, however, works as normal.
-
- An alternative description is that a subpattern of this type matches
- the string of characters that an identical standalone pattern would
- match, if anchored at the current point in the subject string.
-
- Atomic grouping subpatterns are not capturing subpatterns. Simple cases
- such as the above example can be thought of as a maximizing repeat that
- must swallow everything it can. So, while both \d+ and \d+? are pre-
- pared to adjust the number of digits they match in order to make the
- rest of the pattern match, (?>\d+) can only match an entire sequence of
- digits.
-
- Atomic groups in general can of course contain arbitrarily complicated
- subpatterns, and can be nested. However, when the subpattern for an
- atomic group is just a single repeated item, as in the example above, a
- simpler notation, called a "possessive quantifier" can be used. This
- consists of an additional + character following a quantifier. Using
- this notation, the previous example can be rewritten as
-
- \d++foo
-
- Possessive quantifiers are always greedy; the setting of the
- PCRE_UNGREEDY option is ignored. They are a convenient notation for the
- simpler forms of atomic group. However, there is no difference in the
- meaning of a possessive quantifier and the equivalent atomic group,
- though there may be a performance difference; possessive quantifiers
- should be slightly faster.
-
- The possessive quantifier syntax is an extension to the Perl 5.8 syn-
- tax. Jeffrey Friedl originated the idea (and the name) in the first
- edition of his book. Mike McCloskey liked it, so implemented it when he
- built Sun's Java package, and PCRE copied it from there. It ultimately
- found its way into Perl at release 5.10.
-
- PCRE has an optimization that automatically "possessifies" certain sim-
- ple pattern constructs. For example, the sequence A+B is treated as
- A++B because there is no point in backtracking into a sequence of A's
- when B must follow.
-
- When a pattern contains an unlimited repeat inside a subpattern that
- can itself be repeated an unlimited number of times, the use of an
- atomic group is the only way to avoid some failing matches taking a
- very long time indeed. The pattern
-
- (\D+|<\d+>)*[!?]
-
- matches an unlimited number of substrings that either consist of non-
- digits, or digits enclosed in <>, followed by either ! or ?. When it
- matches, it runs quickly. However, if it is applied to
-
- aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
-
- it takes a long time before reporting failure. This is because the
- string can be divided between the internal \D+ repeat and the external
- * repeat in a large number of ways, and all have to be tried. (The
- example uses [!?] rather than a single character at the end, because
- both PCRE and Perl have an optimization that allows for fast failure
- when a single character is used. They remember the last single charac-
- ter that is required for a match, and fail early if it is not present
- in the string.) If the pattern is changed so that it uses an atomic
- group, like this:
-
- ((?>\D+)|<\d+>)*[!?]
-
- sequences of non-digits cannot be broken, and failure happens quickly.
-
-
-BACK REFERENCES
-
- Outside a character class, a backslash followed by a digit greater than
- 0 (and possibly further digits) is a back reference to a capturing sub-
- pattern earlier (that is, to its left) in the pattern, provided there
- have been that many previous capturing left parentheses.
-
- However, if the decimal number following the backslash is less than 10,
- it is always taken as a back reference, and causes an error only if
- there are not that many capturing left parentheses in the entire pat-
- tern. In other words, the parentheses that are referenced need not be
- to the left of the reference for numbers less than 10. A "forward back
- reference" of this type can make sense when a repetition is involved
- and the subpattern to the right has participated in an earlier itera-
- tion.
-
- It is not possible to have a numerical "forward back reference" to a
- subpattern whose number is 10 or more using this syntax because a
- sequence such as \50 is interpreted as a character defined in octal.
- See the subsection entitled "Non-printing characters" above for further
- details of the handling of digits following a backslash. There is no
- such problem when named parentheses are used. A back reference to any
- subpattern is possible using named parentheses (see below).
-
- Another way of avoiding the ambiguity inherent in the use of digits
- following a backslash is to use the \g escape sequence, which is a fea-
- ture introduced in Perl 5.10. This escape must be followed by a posi-
- tive or a negative number, optionally enclosed in braces. These exam-
- ples are all identical:
-
- (ring), \1
- (ring), \g1
- (ring), \g{1}
-
- A positive number specifies an absolute reference without the ambiguity
- that is present in the older syntax. It is also useful when literal
- digits follow the reference. A negative number is a relative reference.
- Consider this example:
-
- (abc(def)ghi)\g{-1}
-
- The sequence \g{-1} is a reference to the most recently started captur-
- ing subpattern before \g, that is, is it equivalent to \2. Similarly,
- \g{-2} would be equivalent to \1. The use of relative references can be
- helpful in long patterns, and also in patterns that are created by
- joining together fragments that contain references within themselves.
-
- A back reference matches whatever actually matched the capturing sub-
- pattern in the current subject string, rather than anything matching
- the subpattern itself (see "Subpatterns as subroutines" below for a way
- of doing that). So the pattern
-
- (sens|respons)e and \1ibility
-
- matches "sense and sensibility" and "response and responsibility", but
- not "sense and responsibility". If caseful matching is in force at the
- time of the back reference, the case of letters is relevant. For exam-
- ple,
-
- ((?i)rah)\s+\1
-
- matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
- original capturing subpattern is matched caselessly.
-
- There are several different ways of writing back references to named
- subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
- \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
- unified back reference syntax, in which \g can be used for both numeric
- and named references, is also supported. We could rewrite the above
- example in any of the following ways:
-
- (?<p1>(?i)rah)\s+\k<p1>
- (?'p1'(?i)rah)\s+\k{p1}
- (?P<p1>(?i)rah)\s+(?P=p1)
- (?<p1>(?i)rah)\s+\g{p1}
-
- A subpattern that is referenced by name may appear in the pattern
- before or after the reference.
-
- There may be more than one back reference to the same subpattern. If a
- subpattern has not actually been used in a particular match, any back
- references to it always fail. For example, the pattern
-
- (a|(bc))\2
-
- always fails if it starts to match "a" rather than "bc". Because there
- may be many capturing parentheses in a pattern, all digits following
- the backslash are taken as part of a potential back reference number.
- If the pattern continues with a digit character, some delimiter must be
- used to terminate the back reference. If the PCRE_EXTENDED option is
- set, this can be whitespace. Otherwise an empty comment (see "Com-
- ments" below) can be used.
-
- A back reference that occurs inside the parentheses to which it refers
- fails when the subpattern is first used, so, for example, (a\1) never
- matches. However, such references can be useful inside repeated sub-
- patterns. For example, the pattern
-
- (a|b\1)+
-
- matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
- ation of the subpattern, the back reference matches the character
- string corresponding to the previous iteration. In order for this to
- work, the pattern must be such that the first iteration does not need
- to match the back reference. This can be done using alternation, as in
- the example above, or by a quantifier with a minimum of zero.
-
-
-ASSERTIONS
-
- An assertion is a test on the characters following or preceding the
- current matching point that does not actually consume any characters.
- The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
- described above.
-
- More complicated assertions are coded as subpatterns. There are two
- kinds: those that look ahead of the current position in the subject
- string, and those that look behind it. An assertion subpattern is
- matched in the normal way, except that it does not cause the current
- matching position to be changed.
-
- Assertion subpatterns are not capturing subpatterns, and may not be
- repeated, because it makes no sense to assert the same thing several
- times. If any kind of assertion contains capturing subpatterns within
- it, these are counted for the purposes of numbering the capturing sub-
- patterns in the whole pattern. However, substring capturing is carried
- out only for positive assertions, because it does not make sense for
- negative assertions.
-
- Lookahead assertions
-
- Lookahead assertions start with (?= for positive assertions and (?! for
- negative assertions. For example,
-
- \w+(?=;)
-
- matches a word followed by a semicolon, but does not include the semi-
- colon in the match, and
-
- foo(?!bar)
-
- matches any occurrence of "foo" that is not followed by "bar". Note
- that the apparently similar pattern
-
- (?!foo)bar
-
- does not find an occurrence of "bar" that is preceded by something
- other than "foo"; it finds any occurrence of "bar" whatsoever, because
- the assertion (?!foo) is always true when the next three characters are
- "bar". A lookbehind assertion is needed to achieve the other effect.
-
- If you want to force a matching failure at some point in a pattern, the
- most convenient way to do it is with (?!) because an empty string
- always matches, so an assertion that requires there not to be an empty
- string must always fail.
-
- Lookbehind assertions
-
- Lookbehind assertions start with (?<= for positive assertions and (?<!
- for negative assertions. For example,
-
- (?<!foo)bar
-
- does find an occurrence of "bar" that is not preceded by "foo". The
- contents of a lookbehind assertion are restricted such that all the
- strings it matches must have a fixed length. However, if there are sev-
- eral top-level alternatives, they do not all have to have the same
- fixed length. Thus
-
- (?<=bullock|donkey)
-
- is permitted, but
-
- (?<!dogs?|cats?)
-
- causes an error at compile time. Branches that match different length
- strings are permitted only at the top level of a lookbehind assertion.
- This is an extension compared with Perl (at least for 5.8), which
- requires all branches to match the same length of string. An assertion
- such as
-
- (?<=ab(c|de))
-
- is not permitted, because its single top-level branch can match two
- different lengths, but it is acceptable if rewritten to use two top-
- level branches:
-
- (?<=abc|abde)
-
- In some cases, the Perl 5.10 escape sequence \K (see above) can be used
- instead of a lookbehind assertion; this is not restricted to a fixed-
- length.
-
- The implementation of lookbehind assertions is, for each alternative,
- to temporarily move the current position back by the fixed length and
- then try to match. If there are insufficient characters before the cur-
- rent position, the assertion fails.
-
- PCRE does not allow the \C escape (which matches a single byte in UTF-8
- mode) to appear in lookbehind assertions, because it makes it impossi-
- ble to calculate the length of the lookbehind. The \X and \R escapes,
- which can match different numbers of bytes, are also not permitted.
-
- Possessive quantifiers can be used in conjunction with lookbehind
- assertions to specify efficient matching at the end of the subject
- string. Consider a simple pattern such as
-
- abcd$
-
- when applied to a long string that does not match. Because matching
- proceeds from left to right, PCRE will look for each "a" in the subject
- and then see if what follows matches the rest of the pattern. If the
- pattern is specified as
-
- ^.*abcd$
-
- the initial .* matches the entire string at first, but when this fails
- (because there is no following "a"), it backtracks to match all but the
- last character, then all but the last two characters, and so on. Once
- again the search for "a" covers the entire string, from right to left,
- so we are no better off. However, if the pattern is written as
-
- ^.*+(?<=abcd)
-
- there can be no backtracking for the .*+ item; it can match only the
- entire string. The subsequent lookbehind assertion does a single test
- on the last four characters. If it fails, the match fails immediately.
- For long strings, this approach makes a significant difference to the
- processing time.
-
- Using multiple assertions
-
- Several assertions (of any sort) may occur in succession. For example,
-
- (?<=\d{3})(?<!999)foo
-
- matches "foo" preceded by three digits that are not "999". Notice that
- each of the assertions is applied independently at the same point in
- the subject string. First there is a check that the previous three
- characters are all digits, and then there is a check that the same
- three characters are not "999". This pattern does not match "foo" pre-
- ceded by six characters, the first of which are digits and the last
- three of which are not "999". For example, it doesn't match "123abc-
- foo". A pattern to do that is
-
- (?<=\d{3}...)(?<!999)foo
-
- This time the first assertion looks at the preceding six characters,
- checking that the first three are digits, and then the second assertion
- checks that the preceding three characters are not "999".
-
- Assertions can be nested in any combination. For example,
-
- (?<=(?<!foo)bar)baz
-
- matches an occurrence of "baz" that is preceded by "bar" which in turn
- is not preceded by "foo", while
-
- (?<=\d{3}(?!999)...)foo
-
- is another pattern that matches "foo" preceded by three digits and any
- three characters that are not "999".
-
-
-CONDITIONAL SUBPATTERNS
-
- It is possible to cause the matching process to obey a subpattern con-
- ditionally or to choose between two alternative subpatterns, depending
- on the result of an assertion, or whether a previous capturing subpat-
- tern matched or not. The two possible forms of conditional subpattern
- are
-
- (?(condition)yes-pattern)
- (?(condition)yes-pattern|no-pattern)
-
- If the condition is satisfied, the yes-pattern is used; otherwise the
- no-pattern (if present) is used. If there are more than two alterna-
- tives in the subpattern, a compile-time error occurs.
-
- There are four kinds of condition: references to subpatterns, refer-
- ences to recursion, a pseudo-condition called DEFINE, and assertions.
-
- Checking for a used subpattern by number
-
- If the text between the parentheses consists of a sequence of digits,
- the condition is true if the capturing subpattern of that number has
- previously matched. An alternative notation is to precede the digits
- with a plus or minus sign. In this case, the subpattern number is rela-
- tive rather than absolute. The most recently opened parentheses can be
- referenced by (?(-1), the next most recent by (?(-2), and so on. In
- looping constructs it can also make sense to refer to subsequent groups
- with constructs such as (?(+2).
-
- Consider the following pattern, which contains non-significant white
- space to make it more readable (assume the PCRE_EXTENDED option) and to
- divide it into three parts for ease of discussion:
-
- ( \( )? [^()]+ (?(1) \) )
-
- The first part matches an optional opening parenthesis, and if that
- character is present, sets it as the first captured substring. The sec-
- ond part matches one or more characters that are not parentheses. The
- third part is a conditional subpattern that tests whether the first set
- of parentheses matched or not. If they did, that is, if subject started
- with an opening parenthesis, the condition is true, and so the yes-pat-
- tern is executed and a closing parenthesis is required. Otherwise,
- since no-pattern is not present, the subpattern matches nothing. In
- other words, this pattern matches a sequence of non-parentheses,
- optionally enclosed in parentheses.
-
- If you were embedding this pattern in a larger one, you could use a
- relative reference:
-
- ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
-
- This makes the fragment independent of the parentheses in the larger
- pattern.
-
- Checking for a used subpattern by name
-
- Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
- used subpattern by name. For compatibility with earlier versions of
- PCRE, which had this facility before Perl, the syntax (?(name)...) is
- also recognized. However, there is a possible ambiguity with this syn-
- tax, because subpattern names may consist entirely of digits. PCRE
- looks first for a named subpattern; if it cannot find one and the name
- consists entirely of digits, PCRE looks for a subpattern of that num-
- ber, which must be greater than zero. Using subpattern names that con-
- sist entirely of digits is not recommended.
-
- Rewriting the above example to use a named subpattern gives this:
-
- (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
-
-
- Checking for pattern recursion
-
- If the condition is the string (R), and there is no subpattern with the
- name R, the condition is true if a recursive call to the whole pattern
- or any subpattern has been made. If digits or a name preceded by amper-
- sand follow the letter R, for example:
-
- (?(R3)...) or (?(R&name)...)
-
- the condition is true if the most recent recursion is into the subpat-
- tern whose number or name is given. This condition does not check the
- entire recursion stack.
-
- At "top level", all these recursion test conditions are false. Recur-
- sive patterns are described below.
-
- Defining subpatterns for use by reference only
-
- If the condition is the string (DEFINE), and there is no subpattern
- with the name DEFINE, the condition is always false. In this case,
- there may be only one alternative in the subpattern. It is always
- skipped if control reaches this point in the pattern; the idea of
- DEFINE is that it can be used to define "subroutines" that can be ref-
- erenced from elsewhere. (The use of "subroutines" is described below.)
- For example, a pattern to match an IPv4 address could be written like
- this (ignore whitespace and line breaks):
-
- (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
- \b (?&byte) (\.(?&byte)){3} \b
-
- The first part of the pattern is a DEFINE group inside which a another
- group named "byte" is defined. This matches an individual component of
- an IPv4 address (a number less than 256). When matching takes place,
- this part of the pattern is skipped because DEFINE acts like a false
- condition.
-
- The rest of the pattern uses references to the named group to match the
- four dot-separated components of an IPv4 address, insisting on a word
- boundary at each end.
-
- Assertion conditions
-
- If the condition is not in any of the above formats, it must be an
- assertion. This may be a positive or negative lookahead or lookbehind
- assertion. Consider this pattern, again containing non-significant
- white space, and with the two alternatives on the second line:
-
- (?(?=[^a-z]*[a-z])
- \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
-
- The condition is a positive lookahead assertion that matches an
- optional sequence of non-letters followed by a letter. In other words,
- it tests for the presence of at least one letter in the subject. If a
- letter is found, the subject is matched against the first alternative;
- otherwise it is matched against the second. This pattern matches
- strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
- letters and dd are digits.
-
-
-COMMENTS
-
- The sequence (?# marks the start of a comment that continues up to the
- next closing parenthesis. Nested parentheses are not permitted. The
- characters that make up a comment play no part in the pattern matching
- at all.
-
- If the PCRE_EXTENDED option is set, an unescaped # character outside a
- character class introduces a comment that continues to immediately
- after the next newline in the pattern.
-
-
-RECURSIVE PATTERNS
-
- Consider the problem of matching a string in parentheses, allowing for
- unlimited nested parentheses. Without the use of recursion, the best
- that can be done is to use a pattern that matches up to some fixed
- depth of nesting. It is not possible to handle an arbitrary nesting
- depth.
-
- For some time, Perl has provided a facility that allows regular expres-
- sions to recurse (amongst other things). It does this by interpolating
- Perl code in the expression at run time, and the code can refer to the
- expression itself. A Perl pattern using code interpolation to solve the
- parentheses problem can be created like this:
-
- $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
-
- The (?p{...}) item interpolates Perl code at run time, and in this case
- refers recursively to the pattern in which it appears.
-
- Obviously, PCRE cannot support the interpolation of Perl code. Instead,
- it supports special syntax for recursion of the entire pattern, and
- also for individual subpattern recursion. After its introduction in
- PCRE and Python, this kind of recursion was introduced into Perl at
- release 5.10.
-
- A special item that consists of (? followed by a number greater than
- zero and a closing parenthesis is a recursive call of the subpattern of
- the given number, provided that it occurs inside that subpattern. (If
- not, it is a "subroutine" call, which is described in the next sec-
- tion.) The special item (?R) or (?0) is a recursive call of the entire
- regular expression.
-
- In PCRE (like Python, but unlike Perl), a recursive subpattern call is
- always treated as an atomic group. That is, once it has matched some of
- the subject string, it is never re-entered, even if it contains untried
- alternatives and there is a subsequent matching failure.
-
- This PCRE pattern solves the nested parentheses problem (assume the
- PCRE_EXTENDED option is set so that white space is ignored):
-
- \( ( (?>[^()]+) | (?R) )* \)
-
- First it matches an opening parenthesis. Then it matches any number of
- substrings which can either be a sequence of non-parentheses, or a
- recursive match of the pattern itself (that is, a correctly parenthe-
- sized substring). Finally there is a closing parenthesis.
-
- If this were part of a larger pattern, you would not want to recurse
- the entire pattern, so instead you could use this:
-
- ( \( ( (?>[^()]+) | (?1) )* \) )
-
- We have put the pattern into parentheses, and caused the recursion to
- refer to them instead of the whole pattern.
-
- In a larger pattern, keeping track of parenthesis numbers can be
- tricky. This is made easier by the use of relative references. (A Perl
- 5.10 feature.) Instead of (?1) in the pattern above you can write
- (?-2) to refer to the second most recently opened parentheses preceding
- the recursion. In other words, a negative number counts capturing
- parentheses leftwards from the point at which it is encountered.
-
- It is also possible to refer to subsequently opened parentheses, by
- writing references such as (?+2). However, these cannot be recursive
- because the reference is not inside the parentheses that are refer-
- enced. They are always "subroutine" calls, as described in the next
- section.
-
- An alternative approach is to use named parentheses instead. The Perl
- syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
- supported. We could rewrite the above example as follows:
-
- (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
-
- If there is more than one subpattern with the same name, the earliest
- one is used.
-
- This particular example pattern that we have been looking at contains
- nested unlimited repeats, and so the use of atomic grouping for match-
- ing strings of non-parentheses is important when applying the pattern
- to strings that do not match. For example, when this pattern is applied
- to
-
- (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
-
- it yields "no match" quickly. However, if atomic grouping is not used,
- the match runs for a very long time indeed because there are so many
- different ways the + and * repeats can carve up the subject, and all
- have to be tested before failure can be reported.
-
- At the end of a match, the values set for any capturing subpatterns are
- those from the outermost level of the recursion at which the subpattern
- value is set. If you want to obtain intermediate values, a callout
- function can be used (see below and the pcrecallout documentation). If
- the pattern above is matched against
-
- (ab(cd)ef)
-
- the value for the capturing parentheses is "ef", which is the last
- value taken on at the top level. If additional parentheses are added,
- giving
-
- \( ( ( (?>[^()]+) | (?R) )* ) \)
- ^ ^
- ^ ^
-
- the string they capture is "ab(cd)ef", the contents of the top level
- parentheses. If there are more than 15 capturing parentheses in a pat-
- tern, PCRE has to obtain extra memory to store data during a recursion,
- which it does by using pcre_malloc, freeing it via pcre_free after-
- wards. If no memory can be obtained, the match fails with the
- PCRE_ERROR_NOMEMORY error.
-
- Do not confuse the (?R) item with the condition (R), which tests for
- recursion. Consider this pattern, which matches text in angle brack-
- ets, allowing for arbitrary nesting. Only digits are allowed in nested
- brackets (that is, when recursing), whereas any characters are permit-
- ted at the outer level.
-
- < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
-
- In this pattern, (?(R) is the start of a conditional subpattern, with
- two different alternatives for the recursive and non-recursive cases.
- The (?R) item is the actual recursive call.
-
-
-SUBPATTERNS AS SUBROUTINES
-
- If the syntax for a recursive subpattern reference (either by number or
- by name) is used outside the parentheses to which it refers, it oper-
- ates like a subroutine in a programming language. The "called" subpat-
- tern may be defined before or after the reference. A numbered reference
- can be absolute or relative, as in these examples:
-
- (...(absolute)...)...(?2)...
- (...(relative)...)...(?-1)...
- (...(?+1)...(relative)...
-
- An earlier example pointed out that the pattern
-
- (sens|respons)e and \1ibility
-
- matches "sense and sensibility" and "response and responsibility", but
- not "sense and responsibility". If instead the pattern
-
- (sens|respons)e and (?1)ibility
-
- is used, it does match "sense and responsibility" as well as the other
- two strings. Another example is given in the discussion of DEFINE
- above.
-
- Like recursive subpatterns, a "subroutine" call is always treated as an
- atomic group. That is, once it has matched some of the subject string,
- it is never re-entered, even if it contains untried alternatives and
- there is a subsequent matching failure.
-
- When a subpattern is used as a subroutine, processing options such as
- case-independence are fixed when the subpattern is defined. They cannot
- be changed for different calls. For example, consider this pattern:
-
- (abc)(?i:(?-1))
-
- It matches "abcabc". It does not match "abcABC" because the change of
- processing option does not affect the called subpattern.
-
-
-CALLOUTS
-
- Perl has a feature whereby using the sequence (?{...}) causes arbitrary
- Perl code to be obeyed in the middle of matching a regular expression.
- This makes it possible, amongst other things, to extract different sub-
- strings that match the same pair of parentheses when there is a repeti-
- tion.
-
- PCRE provides a similar feature, but of course it cannot obey arbitrary
- Perl code. The feature is called "callout". The caller of PCRE provides
- an external function by putting its entry point in the global variable
- pcre_callout. By default, this variable contains NULL, which disables
- all calling out.
-
- Within a regular expression, (?C) indicates the points at which the
- external function is to be called. If you want to identify different
- callout points, you can put a number less than 256 after the letter C.
- The default value is zero. For example, this pattern has two callout
- points:
-
- (?C1)abc(?C2)def
-
- If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
- automatically installed before each item in the pattern. They are all
- numbered 255.
-
- During matching, when PCRE reaches a callout point (and pcre_callout is
- set), the external function is called. It is provided with the number
- of the callout, the position in the pattern, and, optionally, one item
- of data originally supplied by the caller of pcre_exec(). The callout
- function may cause matching to proceed, to backtrack, or to fail alto-
- gether. A complete description of the interface to the callout function
- is given in the pcrecallout documentation.
-
-
-SEE 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.
projects / exim.git / blobdiff