pcrepattern man page

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• PCRE REGULAR EXPRESSION DETAILS
• NEWLINE CONVENTIONS
• CHARACTERS AND METACHARACTERS
• BACKSLASH
• CIRCUMFLEX AND DOLLAR
• FULL STOP (PERIOD, DOT)
• MATCHING A SINGLE BYTE
• SQUARE BRACKETS AND CHARACTER CLASSES
• POSIX CHARACTER CLASSES
• VERTICAL BAR
• INTERNAL OPTION SETTING
• SUBPATTERNS
• DUPLICATE SUBPATTERN NUMBERS
• NAMED SUBPATTERNS
• REPETITION
• ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
• BACK REFERENCES
• ASSERTIONS
• CONDITIONAL SUBPATTERNS
• COMMENTS
• RECURSIVE PATTERNS
• SUBPATTERNS AS SUBROUTINES
• ONIGURUMA SUBROUTINE SYNTAX
• CALLOUTS
• BACKTRACKING CONTROL
• SEE ALSO
• AUTHOR
• REVISION

The syntax and semantics of the regular expressions that are supported by PCRE are described in detail below. There is a quick-reference syntax summary in the pcresyntax page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE also supports some alternative regular expression syntax (which does not conflict with the Perl syntax) in order to provide some compatibility with regular expressions in Python, .NET, and Oniguruma.

Perl's regular expressions are described in its own documentation, and regular expressions in general are covered 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 description 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. There is also a special sequence that can be given at the start of a pattern:

  (*UTF8)

The remainder of this document discusses the patterns that are supported 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.

PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (linefeed) character, the two-character sequence CRLF, any of the three preceding, or any Unicode newline sequence. The pcreapi page has further discussion about newlines, and shows how to set the newline convention in the options arguments for the compiling and matching functions.

It is also possible to specify a newline convention by starting a pattern string with one of the following five sequences:

  (*CR)        carriage return
  (*LF)        linefeed
  (*CRLF)      carriage return, followed by linefeed
  (*ANYCRLF)   any of the three above
  (*ANY)       all Unicode newline sequences

  (*CR)a.b

The newline convention does not affect what the \R escape sequence matches. By default, this is any Unicode newline sequence, for Perl compatibility. However, this can be changed; see the description of \R in the section entitled "Newline sequences" below. A change of \R setting can be combined with a change of newline convention.

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

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 recognized 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

  \      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 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 backslash, 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 escaping 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 characters, you can do so by putting them between \Q and \E. This is different from Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable interpolation. 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

A second use of backslash provides a way of encoding non-printing characters 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        linefeed (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..

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 value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code point, which is 10FFFF.

If characters other than hexadecimal digits appear between \x{ and }, or if there is no terminating }, this form of escape is not recognized. Instead, the initial \x will be interpreted as a basic 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 handled. 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 complicated. Outside a character class, PCRE reads it and any following digits 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 generate 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"

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).

The sequence \g followed by an unsigned or a 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.

For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a "subroutine". Details are discussed later. Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call.

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

These character type sequences can appear both inside and outside character 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 character. 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 Unicode character property support is available. These sequences retain their original meanings from before UTF-8 support was available, mainly for efficiency reasons. Note that this also affects \b, because it is defined in terms of \w and \W.

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

  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 controlled 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.

Outside a character class, by default, 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)

In UTF-8 mode, two additional characters whose codepoints are greater than 255 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). Unicode character property support is not needed for these characters to be recognized.

It is possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. (BSR is an abbrevation for "backslash R".) This can be made the default when PCRE is built; if this is the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. It is also possible to specify these settings by starting a pattern string with one of the following sequences:

  (*BSR_ANYCRLF)   CR, LF, or CRLF only
  (*BSR_UNICODE)   any Unicode newline sequence

  (*ANY)(*BSR_ANYCRLF)

When PCRE is built with Unicode character property support, three additional 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

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}

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, Hiragana, 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 general 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

  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 Cs (Surrogate) property applies only to characters in the range U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity checking has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in the pcreapi page).

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) property. 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*)

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.

The escape sequence \K, which is a Perl 5.10 feature, causes any previously matched characters not to be included in the final matched sequence. For example, the pattern:

  foo\Kbar

  (foo)\Kbar

The final use of backslash is for certain simple assertions. An assertion 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

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 assertions 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, indicating 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 arguments, you can mimic Perl's /g option, and it is in this kind of implementation 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.

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 argument 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 subject, 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.

Outside a character class, a dot in the pattern matches any one character in the subject string except (by default) a character that signifies 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 Unicode 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 circumflex and dollar, the only relationship being that they both involve newlines. Dot has no special meaning in a character class.

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 characters into individual bytes, what remains in the string may be a malformed UTF-8 string. For this reason, the \C escape sequence is best avoided.

PCRE does not allow \C to appear in lookbehind assertions (described below), because in UTF-8 mode this would make it impossible to calculate the length of the lookbehind.

An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not special. 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 consumes 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 characters 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 character 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 interpreted 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 circumflex 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.

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:]%]

  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 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:]]

In UTF-8 mode, characters with values greater than 128 do not match any of the POSIX character classes.

Vertical bar characters are used to separate alternative patterns. For example, the pattern

  gilbert|sullivan

The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED options (which are Perl-compatible) 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

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.

When one of these option changes occurs at top level (that is, not inside subpattern 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

  (a(?i)b|c)

Note: There are other PCRE-specific options that can be set by the application when the compile or match functions are called. In some cases the pattern can contain special leading sequences such as (*CRLF) to override what the application has set or what has been defaulted. Details are given in the section entitled "Newline sequences" above. There is also the (*UTF8) leading sequence that can be used to set UTF-8 mode; this is equivalent to setting the PCRE_UTF8 option.

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|)

For example, if the string "the red king" is matched against the pattern

  the ((red|white) (king|queen))

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 capturing, 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))

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)

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

  # before  ---------------branch-reset----------- after
  / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
  # 1            2         2  3        2     3     4

An alternative approach to using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section.

Identifying capturing parentheses by number is simple, but it can be very hard to keep track of the numbers in complicated regular expressions. Furthermore, if an expression is modified, the numbers may change. To help with this difficulty, PCRE supports the naming of subpatterns. 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 syntax.

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 backreferences, 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)?

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 documentation.

Warning: You cannot use different names to distinguish between two subpatterns with the same number (see the previous section) because PCRE uses only the numbers when matching.

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)

  z{2,4}

  [aeiou]{3,}

  \d{8}

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 characters, 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. This may be useful for subpatterns that are referenced as subroutines from elsewhere in the pattern. Items other than subpatterns that have a {0} quantifier are omitted from the compiled pattern.

For convenience, the three most common quantifiers have single-character abbreviations:

  *    is equivalent to {0,}
  +    is equivalent to {1,}
  ?    is equivalent to {0,1}

  (a?)*

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

  /\*.*\*/

  /* first comment */  not comment  /* second comment */

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

  /\*.*?\*/

  \d??\d

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 (equivalent 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 newlines, it is worth setting PCRE_DOTALL in order to obtain this optimization, 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

When a capturing subpattern is repeated, the value captured is the substring that matched the final iteration. For example, after

  (tweedle[dume]{3}\s*)+

  /(a|(b))+/

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

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

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 prepared 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

  (abc|xyz){2,3}+

The possessive quantifier syntax is an extension to the Perl 5.8 syntax. Jeffrey Friedl originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he built Sun's Java package, and PCRE copied it from there. It ultimately found its way into Perl at release 5.10.

PCRE has an optimization that automatically "possessifies" certain simple 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+>)*[!?]

  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

  ((?>\D+)|<\d+>)*[!?]

Outside a character class, a backslash followed by a digit greater than 0 (and possibly further digits) is a back reference to a capturing subpattern 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 pattern. 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 iteration.

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 feature introduced in Perl 5.10. This escape must be followed by an unsigned number or a negative number, optionally enclosed in braces. These examples are all identical:

  (ring), \1
  (ring), \g1
  (ring), \g{1}

  (abc(def)ghi)\g{-1}

A back reference matches whatever actually matched the capturing subpattern 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

  ((?i)rah)\s+\1

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}

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

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 subpatterns. For example, the pattern

  (a|b\1)+

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 subpatterns 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 start with (?= for positive assertions and (?! for negative assertions. For example,

  \w+(?=;)

  foo(?!bar)

  (?!foo)bar

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 start with (?<= for positive assertions and (?<! for negative assertions. For example,

  (?<!foo)bar

  (?<=bullock|donkey)

  (?<!dogs?|cats?)

  (?<=ab(c|de))

  (?<=abc|abde)

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 current 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 impossible 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$

  ^.*abcd$

  ^.*+(?<=abcd)

Several assertions (of any sort) may occur in succession. For example,

  (?<=\d{3})(?<!999)foo

  (?<=\d{3}...)(?<!999)foo

Assertions can be nested in any combination. For example,

  (?<=(?<!foo)bar)baz

  (?<=\d{3}(?!999)...)foo

It is possible to cause the matching process to obey a subpattern conditionally or to choose between two alternative subpatterns, depending on the result of an assertion, or whether a previous capturing subpattern matched or not. The two possible forms of conditional subpattern are

  (?(condition)yes-pattern)
  (?(condition)yes-pattern|no-pattern)

There are four kinds of condition: references to subpatterns, references to recursion, a pseudo-condition called DEFINE, and assertions.

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 relative 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:

  ( \( )?    [^()]+    (?(1) \) )

If you were embedding this pattern in a larger one, you could use a relative reference:

  ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

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 syntax, 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 number, which must be greater than zero. Using subpattern names that consist entirely of digits is not recommended.

Rewriting the above example to use a named subpattern gives this:

  (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

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 ampersand follow the letter R, for example:

  (?(R3)...) or (?(R&name)...)

At "top level", all these recursion test conditions are false. Recursive patterns are described below.

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 referenced 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 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.

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 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.

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 expressions to recurse (amongst other things). It does this by interpolating Perl code in the expression at run time, and the code can refer to the expression itself. A Perl pattern using code interpolation to solve the parentheses problem can be created like this:

  $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

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 section.) 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):

  \( ( (?>[^()]+) | (?R) )* \)

If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you could use this:

  ( \( ( (?>[^()]+) | (?1) )* \) )

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 referenced. 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> \( ( (?>[^()]+) | (?&pn) )* \) )

This particular example pattern that we have been looking at 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

  (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

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)

  \( ( ( (?>[^()]+) | (?R) )* ) \)
     ^                        ^
     ^                        ^

Do not confuse the (?R) item with the condition (R), which tests for recursion. Consider this pattern, which matches text in angle brackets, allowing for arbitrary nesting. Only digits are allowed in nested brackets (that is, when recursing), whereas any characters are permitted at the outer level.

  < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

If the syntax for a recursive subpattern reference (either by number or by name) is used outside the parentheses to which it refers, it operates like a subroutine in a programming language. The "called" subpattern 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)...

  (sens|respons)e and \1ibility

  (sens|respons)e and (?1)ibility

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))

For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a subroutine, possibly recursively. Here are two of the examples used above, rewritten using this syntax:

  (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
  (sens|respons)e and \g'1'ibility

  (abc)(?i:\g<-1>)

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 substrings that match the same pair of parentheses when there is a repetition.

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

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 altogether. A complete description of the interface to the callout function is given in the pcrecallout documentation.

Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are described in the Perl documentation as "experimental and subject to change or removal in a future version of Perl". It goes on to say: "Their usage in production code should be noted to avoid problems during upgrades." The same remarks apply to the PCRE features described in this section.

Since these verbs are specifically related to backtracking, most of them can be used only when the pattern is to be matched using pcre_exec(), which uses a backtracking algorithm. With the exception of (*FAIL), which behaves like a failing negative assertion, they cause an error if encountered by pcre_dfa_exec().

The new verbs make use of what was previously invalid syntax: an opening parenthesis followed by an asterisk. In Perl, they are generally of the form (*VERB:ARG) but PCRE does not support the use of arguments, so its general form is just (*VERB). Any number of these verbs may occur in a pattern. There are two kinds:

The following verbs act as soon as they are encountered:

   (*ACCEPT)

  A(A|B(*ACCEPT)|C)D

  (*FAIL) or (*F)

  a+(?C)(*FAIL)

The following verbs do nothing when they are encountered. Matching continues with what follows, but if there is no subsequent match, a failure is forced. The verbs differ in exactly what kind of failure occurs.

  (*COMMIT)

  a+(*COMMIT)b

  (*PRUNE)

  (*SKIP)

  a+(*SKIP)b

  (*THEN)

  ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).

Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.

Last updated: 11 April 2009
Copyright © 1997-2009 University of Cambridge.

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