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-revision 73 by nigel,
Sat Feb 24 21:40:30 2007 UTC
+revision 166 by ph10,
Wed May  9 14:48:28 2007 UTC
 Line 1
- .TH PCRE 3
+ .TH PCREPATTERN 3
  .SH NAME
  PCRE - Perl-compatible regular expressions
- .SH PCRE REGULAR EXPRESSION DETAILS
+ .SH "PCRE REGULAR EXPRESSION DETAILS"
  .rs
  .sp
  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 other books, some of which have copious
+ documentation and in a number of books, some of which have copious examples.
- examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
+ Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
- O'Reilly, covers them in great detail. The description here is intended as
+ regular expressions in great detail. This description of PCRE's regular
- reference documentation.
+ expressions is intended as reference material.
+ .P
- The basic operation of PCRE is on strings of bytes. However, there is also
+ The original operation of PCRE was on strings of one-byte characters. However,
- support for UTF-8 character strings. To use this support you must build PCRE to
+ there is now also support for UTF-8 character strings. To use this, you must
- include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8
+ build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
- option. How this affects the pattern matching is mentioned in several places
+ the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
- below. There is also a summary of UTF-8 features in the
+ places below. There is also a summary of UTF-8 features in the
  .\" HTML <a href="pcre.html#utf8support">
  .\" </a>
  section on UTF-8 support
  .\"
  in the main
  .\" HREF
- \fBpcre\fR
+ \fBpcre\fP
  .\"
  page.
+ .P
+ The remainder of this document discusses the patterns that are supported by
+ PCRE when its main matching function, \fBpcre_exec()\fP, is used.
+ From release 6.0, PCRE offers a second matching function,
+ \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
+ Perl-compatible. The advantages and disadvantages of the alternative function,
+ and how it differs from the normal function, are discussed in the
+ .\" HREF
+ \fBpcrematching\fP
+ .\"
+ page.
+ .
+ .
+ .SH "CHARACTERS AND METACHARACTERS"
+ .rs
+ .sp
  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
+ .sp
    The quick brown fox
+ .sp
- matches a portion of a subject string that is identical to itself. The power of
+ matches a portion of a subject string that is identical to itself. When
- regular expressions comes from the ability to include alternatives and
+ caseless matching is specified (the PCRE_CASELESS option), letters are matched
- repetitions in the pattern. These are encoded in the pattern by the use of
+ independently of case. In UTF-8 mode, PCRE always understands the concept of
- \fImeta-characters\fR, which do not stand for themselves but instead are
+ 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.
+ .P
+ 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
+ \fImetacharacters\fP, which do not stand for themselves but instead are
  interpreted in some special way.
+ .P
- There are two different sets of meta-characters: those that are recognized
+ There are two different sets of metacharacters: those that are recognized
  anywhere in the pattern except within square brackets, and those that are
- recognized in square brackets. Outside square brackets, the meta-characters are
+ recognized within square brackets. Outside square brackets, the metacharacters
- as follows:
+ are as follows:
+ .sp
-   \\      general escape character with several uses
+   \e      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)
-Line 58 
 as follows:
+Line 82 
 as follows:
    +      1 or more quantifier
           also "possessive quantifier"
    {      start min/max quantifier
+ .sp
  Part of a pattern that is in square brackets is called a "character class". In
- a character class the only meta-characters are:
+ a character class the only metacharacters are:
+ .sp
-   \\      general escape character
+   \e      general escape character
    ^      negate the class, but only if the first character
    -      indicates character range
+ .\" JOIN
    [      POSIX character class (only if followed by POSIX
             syntax)
    ]      terminates the character class
+ .sp
- The following sections describe the use of each of the meta-characters.
+ The following sections describe the use of each of the metacharacters.
+ .
+ .
  .SH BACKSLASH
  .rs
  .sp
  The backslash character has several uses. Firstly, if it is followed by a
- non-alphameric character, it takes away any special meaning that character may
+ non-alphanumeric character, it takes away any special meaning that character
- have. This use of backslash as an escape character applies both inside and
+ may have. This use of backslash as an escape character applies both inside and
  outside character classes.
+ .P
- For example, if you want to match a * character, you write \\* in the pattern.
+ For example, if you want to match a * character, you write \e* in the pattern.
  This escaping action applies whether or not the following character would
- otherwise be interpreted as a meta-character, so it is always safe to precede a
+ otherwise be interpreted as a metacharacter, so it is always safe to precede a
- non-alphameric with backslash to specify that it stands for itself. In
+ non-alphanumeric with backslash to specify that it stands for itself. In
- particular, if you want to match a backslash, you write \\\\.
+ particular, if you want to match a backslash, you write \e\e.
+ .P
  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
  pattern (other than in a character class) and characters between a # outside
- a character class and the next newline character are ignored. An escaping
+ a character class and the next newline are ignored. An escaping backslash can
- backslash can be used to include a whitespace or # character as part of the
+ be used to include a whitespace or # character as part of the pattern.
- pattern.
+ .P
  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
+ can do so by putting them between \eQ and \eE. This is different from Perl in
- that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in
+ that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
  Perl, $ and @ cause variable interpolation. Note the following examples:
+ .sp
    Pattern            PCRE matches   Perl matches
+ .sp
-   \\Qabc$xyz\\E        abc$xyz        abc followed by the
+ .\" JOIN
+   \eQabc$xyz\eE        abc$xyz        abc followed by the
                                        contents of $xyz
-   \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz
+   \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
-   \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz
+   \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
+ .sp
- The \\Q...\\E sequence is recognized both inside and outside character classes.
+ The \eQ...\eE sequence is recognized both inside and outside character classes.
+ .
+ .
+ .\" HTML <a name="digitsafterbackslash"></a>
+ .SS "Non-printing characters"
+ .rs
+ .sp
  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:
+ .sp
-   \\a        alarm, that is, the BEL character (hex 07)
+   \ea        alarm, that is, the BEL character (hex 07)
-   \\cx       "control-x", where x is any character
+   \ecx       "control-x", where x is any character
-   \\e        escape (hex 1B)
+   \ee        escape (hex 1B)
-   \\f        formfeed (hex 0C)
+   \ef        formfeed (hex 0C)
-   \\n        newline (hex 0A)
+   \en        newline (hex 0A)
-   \\r        carriage return (hex 0D)
+   \er        carriage return (hex 0D)
-   \\t        tab (hex 09)
+   \et        tab (hex 09)
-   \\ddd      character with octal code ddd, or backreference
+   \eddd      character with octal code ddd, or backreference
-   \\xhh      character with hex code hh
+   \exhh      character with hex code hh
-   \\x{hhh..} character with hex code hhh... (UTF-8 mode only)
+   \ex{hhh..} character with hex code hhh..
+ .sp
- The precise effect of \\cx is as follows: if x is a lower case letter, it
+ The precise effect of \ecx 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
+ Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
 B.
+ .P
- After \\x, from zero to two hexadecimal digits are read (letters can be in
+ After \ex, from zero to two hexadecimal digits are read (letters can be in
- upper or lower case). In UTF-8 mode, any number of hexadecimal digits may
+ upper or lower case). Any number of hexadecimal digits may appear between \ex{
- appear between \\x{ and }, but the value of the character code must be less
+ and }, but the value of the character code must be less than 256 in non-UTF-8
- than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters
+ mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
- other than hexadecimal digits appear between \\x{ and }, or if there is no
+ is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
- terminating }, this form of escape is not recognized. Instead, the initial
+ and }, or if there is no terminating }, this form of escape is not recognized.
- \\x will be interpreted as a basic hexadecimal escape, with no following
+ Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
- digits, giving a byte whose value is zero.
+ with no following digits, giving a character whose value is zero.
+ .P
  Characters whose value is less than 256 can be defined by either of the two
- syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the
+ syntaxes for \ex. There is no difference in the way they are handled. For
- way they are handled. For example, \\xdc is exactly the same as \\x{dc}.
+ example, \exdc is exactly the same as \ex{dc}.
+ .P
- After \\0 up to two further octal digits are read. In both cases, if there
+ After \e0 up to two further octal digits are read. If there are fewer than two
- are fewer than two digits, just those that are present are used. Thus the
+ digits, just those that are present are used. Thus the sequence \e0\ex\e07
- sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character
+ specifies two binary zeros followed by a BEL character (code value 7). Make
- (code value 7). Make sure you supply two digits after the initial zero if the
+ sure you supply two digits after the initial zero if the pattern character that
- character that follows is itself an octal digit.
+ follows is itself an octal digit.
+ .P
  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 \fIback reference\fR. A description of how this works is given
+ taken as a \fIback reference\fP. A description of how this works is given
- later, following the discussion of parenthesized subpatterns.
+ .\" HTML <a href="#backreferences">
+ .\" </a>
+ later,
+ .\"
+ following the discussion of
+ .\" HTML <a href="#subpattern">
+ .\" </a>
+ parenthesized subpatterns.
+ .\"
+ .P
  Inside a character class, or if the decimal number is greater than 9 and there
  have not been that many capturing subpatterns, PCRE re-reads up to three octal
- digits following the backslash, and generates a single byte from the least
+ digits following the backslash, and uses them to generate a data character. Any
- significant 8 bits of the value. Any subsequent digits stand for themselves.
+ subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
- For example:
+ character specified in octal must be less than \e400. In UTF-8 mode, values up
+ to \e777 are permitted. For example:
-   \\040   is another way of writing a space
+ .sp
-   \\40    is the same, provided there are fewer than 40
+   \e040   is another way of writing a space
+ .\" JOIN
+   \e40    is the same, provided there are fewer than 40
              previous capturing subpatterns
-   \\7     is always a back reference
+   \e7     is always a back reference
-   \\11    might be a back reference, or another way of
+ .\" JOIN
+   \e11    might be a back reference, or another way of
              writing a tab
-   \\011   is always a tab
+   \e011   is always a tab
-   \\0113  is a tab followed by the character "3"
+   \e0113  is a tab followed by the character "3"
-   \\113   might be a back reference, otherwise the
+ .\" JOIN
+   \e113   might be a back reference, otherwise the
              character with octal code 113
-   \\377   might be a back reference, otherwise
+ .\" JOIN
+   \e377   might be a back reference, otherwise
              the byte consisting entirely of 1 bits
-   \\81    is either a back reference, or a binary zero
+ .\" JOIN
+   \e81    is either a back reference, or a binary zero
              followed by the two characters "8" and "1"
+ .sp
  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.
+ .P
- All the sequences that define a single byte value or a single UTF-8 character
+ All the sequences that define a single character value can be used both inside
- (in UTF-8 mode) can be used both inside and outside character classes. In
+ and outside character classes. In addition, inside a character class, the
- addition, inside a character class, the sequence \\b is interpreted as the
+ sequence \eb is interpreted as the backspace character (hex 08), and the
- backspace character (hex 08). Outside a character class it has a different
+ sequences \eR and \eX are interpreted as the characters "R" and "X",
- meaning (see below).
+ respectively. Outside a character class, these sequences have different
+ meanings
- The third use of backslash is for specifying generic character types:
+ .\" HTML <a href="#uniextseq">
+ .\" </a>
-   \\d     any decimal digit
+ (see below).
-   \\D     any character that is not a decimal digit
+ .\"
-   \\s     any whitespace character
+ .
-   \\S     any character that is not a whitespace character
+ .
-   \\w     any "word" character
+ .SS "Absolute and relative back references"
-   \\W     any "non-word" character
+ .rs
+ .sp
+ The sequence \eg followed by a positive or negative number, optionally enclosed
+ in braces, is an absolute or relative back reference. Back references are
+ discussed
+ .\" HTML <a href="#backreferences">
+ .\" </a>
+ later,
+ .\"
+ following the discussion of
+ .\" HTML <a href="#subpattern">
+ .\" </a>
+ parenthesized subpatterns.
+ .\"
+ .
+ .
+ .SS "Generic character types"
+ .rs
+ .sp
+ Another use of backslash is for specifying generic character types. The
+ following are always recognized:
+ .sp
+   \ed     any decimal digit
+   \eD     any character that is not a decimal digit
+   \es     any whitespace character
+   \eS     any character that is not a whitespace character
+   \ew     any "word" character
+   \eW     any "non-word" character
+ .sp
  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.
+ .P
- In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or
+ These character type sequences can appear both inside and outside character
- \\w, and always match \\D, \\S, and \\W.
+ 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
- For compatibility with Perl, \\s does not match the VT character (code 11).
+ there is no character to match.
- This makes it different from the the POSIX "space" class. The \\s characters
+ .P
- are HT (9), LF (10), FF (12), CR (13), and space (32).
+ For compatibility with Perl, \es does not match the VT character (code 11).
+ This makes it different from the the POSIX "space" class. The \es characters
- A "word" character is any letter or digit or the underscore character, that is,
+ are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is
- any character which can be part of a Perl "word". The definition of letters and
+ included in a Perl script, \es may match the VT character. In PCRE, it never
- digits is controlled by PCRE's character tables, and may vary if locale-
+ does.)
- specific matching is taking place (see
+ .P
+ 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
  .\" HTML <a href="pcreapi.html#localesupport">
  .\" </a>
  "Locale support"
  .\"
  in the
  .\" HREF
- \fBpcreapi\fR
+ \fBpcreapi\fP
  .\"
- page). For example, in the "fr" (French) locale, some character codes greater
+ page). For example, in a French locale such as "fr_FR" in Unix-like systems,
- than 128 are used for accented letters, and these are matched by \\w.
+ or "french" in Windows, some character codes greater than 128 are used for
+ accented letters, and these are matched by \ew.
- These character type sequences can appear both inside and outside character
+ .P
- classes. They each match one character of the appropriate type. If the current
+ In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
- matching point is at the end of the subject string, all of them fail, since
+ \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
- there is no character to match.
+ character property support is available. The use of locales with Unicode is
+ discouraged.
- The fourth use of backslash is for certain simple assertions. An assertion
+ .
+ .
+ .SS "Newline sequences"
+ .rs
+ .sp
+ Outside a character class, the escape sequence \eR matches any Unicode newline
+ sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
+ the following:
+ .sp
+   (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
+ .sp
+ This is an example of an "atomic group", details of which are given
+ .\" HTML <a href="#atomicgroup">
+ .\" </a>
+ 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.
+ .P
+ 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.
+ .P
+ Inside a character class, \eR matches the letter "R".
+ .
+ .
+ .\" HTML <a name="uniextseq"></a>
+ .SS Unicode character properties
+ .rs
+ .sp
+ When PCRE is built with Unicode character property support, three additional
+ escape sequences to match character properties are available when UTF-8 mode
+ is selected. They are:
+ .sp
+   \ep{\fIxx\fP}   a character with the \fIxx\fP property
+   \eP{\fIxx\fP}   a character without the \fIxx\fP property
+   \eX       an extended Unicode sequence
+ .sp
+ The property names represented by \fIxx\fP above are limited to the Unicode
+ script names, the general category properties, and "Any", which matches any
+ character (including newline). Other properties such as "InMusicalSymbols" are
+ not currently supported by PCRE. Note that \eP{Any} does not match any
+ characters, so always causes a match failure.
+ .P
+ 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:
+ .sp
+   \ep{Greek}
+   \eP{Han}
+ .sp
+ Those that are not part of an identified script are lumped together as
+ "Common". The current list of scripts is:
+ .P
+ 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.
+ .P
+ 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, \ep{^Lu} is the same as \eP{Lu}.
+ .P
+ If only one letter is specified with \ep or \eP, 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:
+ .sp
+   \ep{L}
+   \epL
+ .sp
+ The following general category property codes are supported:
+ .sp
+   C     Other
+   Cc    Control
+   Cf    Format
+   Cn    Unassigned
+   Co    Private use
+   Cs    Surrogate
+ .sp
+   L     Letter
+   Ll    Lower case letter
+   Lm    Modifier letter
+   Lo    Other letter
+   Lt    Title case letter
+   Lu    Upper case letter
+ .sp
+   M     Mark
+   Mc    Spacing mark
+   Me    Enclosing mark
+   Mn    Non-spacing mark
+ .sp
+   N     Number
+   Nd    Decimal number
+   Nl    Letter number
+   No    Other number
+ .sp
+   P     Punctuation
+   Pc    Connector punctuation
+   Pd    Dash punctuation
+   Pe    Close punctuation
+   Pf    Final punctuation
+   Pi    Initial punctuation
+   Po    Other punctuation
+   Ps    Open punctuation
+ .sp
+   S     Symbol
+   Sc    Currency symbol
+   Sk    Modifier symbol
+   Sm    Mathematical symbol
+   So    Other symbol
+ .sp
+   Z     Separator
+   Zl    Line separator
+   Zp    Paragraph separator
+   Zs    Space separator
+ .sp
+ 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".
+ .P
+ The long synonyms for these properties that Perl supports (such as \ep{Letter})
+ are not supported by PCRE, nor is it permitted to prefix any of these
+ properties with "Is".
+ .P
+ 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.
+ .P
+ Specifying caseless matching does not affect these escape sequences. For
+ example, \ep{Lu} always matches only upper case letters.
+ .P
+ The \eX escape matches any number of Unicode characters that form an extended
+ Unicode sequence. \eX is equivalent to
+ .sp
+   (?>\ePM\epM*)
+ .sp
+ 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
+ .\" HTML <a href="#atomicgroup">
+ .\" </a>
+ (see below).
+ .\"
+ Characters with the "mark" property are typically accents that affect the
+ preceding character.
+ .P
+ 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 \ed and \ew do not use Unicode
+ properties in PCRE.
+ .
+ .
+ .\" HTML <a name="smallassertions"></a>
+ .SS "Simple assertions"
+ .rs
+ .sp
+ 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
+ subpatterns for more complicated assertions is described
- assertions are
+ .\" HTML <a href="#bigassertions">
+ .\" </a>
-   \\b     matches at a word boundary
+ below.
-   \\B     matches when not at a word boundary
+ .\"
-   \\A     matches at start of subject
+ The backslashed assertions are:
-   \\Z     matches at end of subject or before newline at end
+ .sp
-   \\z     matches at end of subject
+   \eb     matches at a word boundary
-   \\G     matches at first matching position in subject
+   \eB     matches when not at a word boundary
+   \eA     matches at the start of the subject
- These assertions may not appear in character classes (but note that \\b has a
+   \eZ     matches at the end of the subject
+           also matches before a newline at the end of the subject
+   \ez     matches only at the end of the subject
+   \eG     matches at the first matching position in the subject
+ .sp
+ These assertions may not appear in character classes (but note that \eb has a
  different meaning, namely the backspace character, inside a character class).
+ .P
  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
+ and the previous character do not both match \ew or \eW (i.e. one matches
- \\w and the other matches \\W), or the start or end of the string if the
+ \ew and the other matches \eW), or the start or end of the string if the
- first or last character matches \\w, respectively.
+ first or last character matches \ew, respectively.
+ .P
- The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
+ The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
- dollar (described below) in that they only ever match at the very start and end
+ dollar (described in the next section) in that they only ever match at the very
- of the subject string, whatever options are set. Thus, they are independent of
+ start and end of the subject string, whatever options are set. Thus, they are
- multiline mode.
+ 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
- They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the
+ circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
- \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that
+ argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
- matching is to start at a point other than the beginning of the subject, \\A
+ at a point other than the beginning of the subject, \eA can never match. The
- can never match. The difference between \\Z and \\z is that \\Z matches before
+ difference between \eZ and \ez is that \eZ matches before a newline at the end
- a newline that is the last character of the string as well as at the end of the
+ of the string as well as at the very end, whereas \ez matches only at the end.
- string, whereas \\z matches only at the end.
+ .P
+ The \eG assertion is true only when the current matching position is at the
- The \\G assertion is true only when the current matching position is at the
+ start point of the match, as specified by the \fIstartoffset\fP argument of
- start point of the match, as specified by the \fIstartoffset\fR argument of
+ \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
- \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is
+ non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
- non-zero. By calling \fBpcre_exec()\fR 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.
+ implementation where \eG can be useful.
+ .P
- Note, however, that PCRE's interpretation of \\G, as the start of the current
+ Note, however, that PCRE's interpretation of \eG, 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.
+ .P
- If all the alternatives of a pattern begin with \\G, the expression is anchored
+ If all the alternatives of a pattern begin with \eG, the expression is anchored
  to the starting match position, and the "anchored" flag is set in the compiled
  regular expression.
+ .
- .SH CIRCUMFLEX AND DOLLAR
+ .
+ .SH "CIRCUMFLEX AND DOLLAR"
  .rs
  .sp
  Outside a character class, in the default matching mode, the circumflex
- character is an assertion which is true only if the current matching point is
+ character is an assertion that is true only if the current matching point is
- at the start of the subject string. If the \fIstartoffset\fR argument of
+ at the start of the subject string. If the \fIstartoffset\fP argument of
- \fBpcre_exec()\fR is non-zero, circumflex can never match if the PCRE_MULTILINE
+ \fBpcre_exec()\fP 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).
+ meaning
+ .\" HTML <a href="#characterclass">
+ .\" </a>
+ (see below).
+ .\"
+ .P
  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
-Line 290 
 possible alternatives start with a circu
+Line 599 
 possible alternatives start with a circu
  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.)
+ .P
- A dollar character is an assertion which is true only if the current matching
+ A dollar character is an assertion that is true only if the current matching
  point is at the end of the subject string, or immediately before a newline
- character that is the last character in the string (by default). Dollar need
+ at the end of the string (by default). Dollar need not be the last character of
- not be the last character of the pattern if a number of alternatives are
+ the pattern if a number of alternatives are involved, but it should be the last
- involved, but it should be the last item in any branch in which it appears.
+ item in any branch in which it appears. Dollar has no special meaning in a
- Dollar has no special meaning in a character class.
+ character class.
+ .P
  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.
+ does not affect the \eZ assertion.
+ .P
  The meanings of the circumflex and dollar characters are changed if the
- PCRE_MULTILINE option is set. When this is the case, they match immediately
+ PCRE_MULTILINE option is set. When this is the case, a circumflex matches
- after and immediately before an internal newline character, respectively, in
+ immediately after internal newlines as well as at the start of the subject
- addition to matching at the start and end of the subject string. For example,
+ string. It does not match after a newline that ends the string. A dollar
- the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
+ matches before any newlines in the string, as well as at the very end, when
- but not otherwise. Consequently, patterns that are anchored in single line mode
+ PCRE_MULTILINE is set. When newline is specified as the two-character
- because all branches start with ^ are not anchored in multiline mode, and a
+ sequence CRLF, isolated CR and LF characters do not indicate newlines.
- match for circumflex is possible when the \fIstartoffset\fR argument of
+ .P
- \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
+ For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
- PCRE_MULTILINE is set.
+ \en represents a newline) in multiline mode, but not otherwise. Consequently,
+ patterns that are anchored in single line mode because all branches start with
- Note that the sequences \\A, \\Z, and \\z can be used to match the start and
+ ^ are not anchored in multiline mode, and a match for circumflex is possible
+ when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
+ PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
+ .P
+ Note that the sequences \eA, \eZ, and \ez can be used to match the start and
  end of the subject in both modes, and if all branches of a pattern start with
- \\A it is always anchored, whether PCRE_MULTILINE is set or not.
+ \eA it is always anchored, whether or not PCRE_MULTILINE is set.
+ .
- .SH FULL STOP (PERIOD, DOT)
+ .
+ .SH "FULL STOP (PERIOD, DOT)"
  .rs
  .sp
  Outside a character class, a dot in the pattern matches any one character in
- the subject, including a non-printing character, but not (by default) newline.
+ the subject string except (by default) a character that signifies the end of a
- In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
+ line. In UTF-8 mode, the matched character may be more than one byte long.
- byte long, except (by default) for newline. If the PCRE_DOTALL option is set,
+ .P
- dots match newlines as well. The handling of dot is entirely independent of the
+ When a line ending is defined as a single character, dot never matches that
- handling of circumflex and dollar, the only relationship being that they both
+ character; when the two-character sequence CRLF is used, dot does not match CR
- involve newline characters. Dot has no special meaning in a character class.
+ 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
- .SH MATCHING A SINGLE BYTE
+ recognized, dot does not match CR or LF or any of the other line ending
- .rs
+ characters.
- .sp
+ .P
- Outside a character class, the escape sequence \\C matches any one byte, both
+ The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
- in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The
+ option is set, a dot matches any one character, without exception. If the
- feature is provided in Perl in order to match individual bytes in UTF-8 mode.
+ two-character sequence CRLF is present in the subject string, it takes two dots
- Because it breaks up UTF-8 characters into individual bytes, what remains in
+ to match it.
- the string may be a malformed UTF-8 string. For this reason it is best avoided.
+ .P
+ The handling of dot is entirely independent of the handling of circumflex and
- PCRE does not allow \\C to appear in lookbehind assertions (see below), because
+ dollar, the only relationship being that they both involve newlines. Dot has no
- in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.
+ special meaning in a character class.
+ .
- .SH SQUARE BRACKETS
+ .
+ .SH "MATCHING A SINGLE BYTE"
+ .rs
+ .sp
+ Outside a character class, the escape sequence \eC 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 \eC escape sequence is best avoided.
+ .P
+ PCRE does not allow \eC to appear in lookbehind assertions
+ .\" HTML <a href="#lookbehind">
+ .\" </a>
+ (described below),
+ .\"
+ because in UTF-8 mode this would make it impossible to calculate the length of
+ the lookbehind.
+ .
+ .
+ .\" HTML <a name="characterclass"></a>
+ .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
  .rs
  .sp
  An opening square bracket introduces a character class, terminated by a closing
-Line 348 
 square bracket. A closing square bracket
+Line 683 
 square bracket. A closing square bracket
  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.
+ .P
  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
-Line 356 
 definition is a circumflex, in which cas
+Line 691 
 definition is a circumflex, in which cas
  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.
+ .P
  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 which
+ circumflex is just a convenient notation for specifying the characters that
- are in the class by enumerating those that are not. It is not an assertion: it
+ are in the class by enumerating those that are not. A class that starts with a
- still consumes a character from the subject string, and fails if the current
+ circumflex is not an assertion: it still consumes a character from the subject
- pointer is at the end of the string.
+ string, and therefore it fails if the current pointer is at the end of the
+ string.
+ .P
  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.
+ class as a literal string of bytes, or by using the \ex{ escaping mechanism.
+ .P
  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. PCRE does not support the concept of case for characters
+ caseful version would. In UTF-8 mode, PCRE always understands the concept of
- with values greater than 255.
+ 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
- The newline character is never treated in any special way in character classes,
+ supported if PCRE is compiled with Unicode property support, but not otherwise.
- whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
+ If you want to use caseless matching for characters 128 and above, you must
- such as [^a] will always match a newline.
+ ensure that PCRE is compiled with Unicode property support as well as with
+ UTF-8 support.
+ .P
+ 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.
+ .P
  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.
+ .P
  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 single class containing a
+ the end of range, so [W-\e]46] is interpreted as a class containing a range
- range followed by two separate characters. The octal or hexadecimal
+ followed by two other characters. The octal or hexadecimal representation of
- representation of "]" can also be used to end a range.
+ "]" can also be used to end a range.
+ .P
  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
+ used for characters specified numerically, for example [\e000-\e037]. In UTF-8
  mode, ranges can include characters whose values are greater than 255, for
- example [\\x{100}-\\x{2ff}].
+ example [\ex{100}-\ex{2ff}].
+ .P
  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 if character tables for the "fr"
+ [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
- locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.
+ tables for a French locale are in use, [\exc8-\excb] matches accented E
+ characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
- The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
+ characters with values greater than 128 only when it is compiled with Unicode
- character class, and add the characters that they match to the class. For
+ property support.
- example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
+ .P
+ The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
+ in a character class, and add the characters that they match to the class. For
+ example, [\edABCDEF] 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 class [^\eW_] matches any letter or digit, but not underscore.
+ .P
- All non-alphameric characters other than \\, -, ^ (at the start) and the
+ The only metacharacters that are recognized in character classes are backslash,
- terminating ] are non-special in character classes, but it does no harm if they
+ hyphen (only where it can be interpreted as specifying a range), circumflex
- are escaped.
+ (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
- .SH POSIX CHARACTER CLASSES
+ closing square bracket. However, escaping other non-alphanumeric characters
+ does no harm.
+ .
+ .
+ .SH "POSIX CHARACTER CLASSES"
  .rs
  .sp
- Perl supports the POSIX notation for character classes, which uses names
+ 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,
+ .sp
    [01[:alpha:]%]
+ .sp
  matches "0", "1", any alphabetic character, or "%". The supported class names
  are
+ .sp
    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)
+   digit    decimal digits (same as \ed)
    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)
+   space    white space (not quite the same as \es)
    upper    upper case letters
-   word     "word" characters (same as \\w)
+   word     "word" characters (same as \ew)
    xdigit   hexadecimal digits
+ .sp
  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
+ makes "space" different to \es, which does not include VT (for Perl
  compatibility).
+ .P
  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
 .8. Another Perl extension is negation, which is indicated by a ^ character
  after the colon. For example,
+ .sp
    [12[:^digit:]]
+ .sp
  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.
+ .P
- In UTF-8 mode, characters with values greater than 255 do not match any of
+ In UTF-8 mode, characters with values greater than 128 do not match any of
  the POSIX character classes.
+ .
- .SH VERTICAL BAR
+ .
+ .SH "VERTICAL BAR"
  .rs
  .sp
  Vertical bar characters are used to separate alternative patterns. For example,
  the pattern
+ .sp
    gilbert|sullivan
+ .sp
  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
- and an empty alternative is permitted (matching the empty string).
+ and an empty alternative is permitted (matching the empty string). The matching
- The matching process tries each alternative in turn, from left to right,
+ process tries each alternative in turn, from left to right, and the first one
- and the first one that succeeds is used. If the alternatives are within a
+ that succeeds is used. If the alternatives are within a subpattern
- subpattern (defined below), "succeeds" means matching the rest of the main
+ .\" HTML <a href="#subpattern">
- pattern as well as the alternative in the subpattern.
+ .\" </a>
+ (defined below),
- .SH INTERNAL OPTION SETTING
+ .\"
+ "succeeds" means matching the rest of the main pattern as well as the
+ alternative in the subpattern.
+ .
+ .
+ .SH "INTERNAL OPTION SETTING"
  .rs
  .sp
  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
+ .sp
    i  for PCRE_CASELESS
    m  for PCRE_MULTILINE
    s  for PCRE_DOTALL
    x  for PCRE_EXTENDED
+ .sp
  For example, (?im) sets caseless, multiline matching. It is also possible to
  unset these options by preceding the letter with a hyphen, and a combined
  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS 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.
+ .P
  When an option change 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
- \fBpcre_fullinfo()\fR function).
+ \fBpcre_fullinfo()\fP function).
+ .P
- An option change within a subpattern affects only that part of the current
+ An option change within a subpattern (see below for a description of
- pattern that follows it, so
+ subpatterns) affects only that part of the current pattern that follows it, so
+ .sp
    (a(?i)b)c
+ .sp
  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,
+ .sp
    (a(?i)b|c)
+ .sp
  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.
+ .P
- The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
+ The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
- same way as the Perl-compatible options by using the characters U and X
+ changed in the same way as the Perl-compatible options by using the characters
- respectively. The (?X) flag setting is special in that it must always occur
+ J, U and X respectively.
- earlier in the pattern than any of the additional features it turns on, even
+ .
- when it is at top level. It is best put at the start.
+ .
+ .\" HTML <a name="subpattern"></a>
  .SH SUBPATTERNS
  .rs
  .sp
  Subpatterns are delimited by parentheses (round brackets), which can be nested.
- Marking part of a pattern as a subpattern does two things:
+ Turning part of a pattern into a subpattern does two things:
+ .sp
 . It localizes a set of alternatives. For example, the pattern
+ .sp
    cat(aract|erpillar|)
+ .sp
  matches one of the words "cat", "cataract", or "caterpillar". Without the
- parentheses, it would match "cataract", "erpillar" or the empty string.
+ parentheses, it would match "cataract", "erpillar" or an empty string.
+ .sp
-. It sets up the subpattern as a capturing subpattern (as defined above).
+. It sets up the subpattern as a capturing subpattern. This means that, when
- When the whole pattern matches, that portion of the subject string that matched
+ the whole pattern matches, that portion of the subject string that matched the
- the subpattern is passed back to the caller via the \fIovector\fR argument of
+ subpattern is passed back to the caller via the \fIovector\fP argument of
- \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
+ \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
- from 1) to obtain the numbers of the capturing subpatterns.
+ from 1) to obtain numbers for the capturing subpatterns.
+ .P
  For example, if the string "the red king" is matched against the pattern
+ .sp
    the ((red|white) (king|queen))
+ .sp
  the captured substrings are "red king", "red", and "king", and are numbered 1,
 , and 3, respectively.
+ .P
  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
+ .sp
    the ((?:red|white) (king|queen))
+ .sp
  the captured substrings are "white queen" and "queen", and are numbered 1 and
-. The maximum number of capturing subpatterns is 65535, and the maximum depth
+. The maximum number of capturing subpatterns is 65535.
- of nesting of all subpatterns, both capturing and non-capturing, is 200.
+ .P
  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
+ .sp
    (?i:saturday|sunday)
    (?:(?i)saturday|sunday)
+ .sp
  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".
+ .
- .SH NAMED SUBPATTERNS
+ .
+ .SH "NAMED SUBPATTERNS"
  .rs
  .sp
  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 the
+ if an expression is modified, the numbers may change. To help with this
- difficulty, PCRE supports the naming of subpatterns, something that Perl does
+ difficulty, PCRE supports the naming of subpatterns. This feature was not
- not provide. The Python syntax (?P<name>...) is used. Names consist of
+ added to Perl until release 5.10. Python had the feature earlier, and PCRE
- alphanumeric characters and underscores, and must be unique within a pattern.
+ introduced it at release 4.0, using the Python syntax. PCRE now supports both
+ the Perl and the Python syntax.
- Named capturing parentheses are still allocated numbers as well as names. The
+ .P
- PCRE API provides function calls for extracting the name-to-number translation
+ In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
- table from a compiled pattern. For further details see the
+ (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
+ parentheses from other parts of the pattern, such as
+ .\" HTML <a href="#backreferences">
+ .\" </a>
+ backreferences,
+ .\"
+ .\" HTML <a href="#recursion">
+ .\" </a>
+ recursion,
+ .\"
+ and
+ .\" HTML <a href="#conditions">
+ .\" </a>
+ conditions,
+ .\"
+ can be made by name as well as by number.
+ .P
+ 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.
+ .P
+ 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:
+ .sp
+   (?<DN>Mon|Fri|Sun)(?:day)?|
+   (?<DN>Tue)(?:sday)?|
+   (?<DN>Wed)(?:nesday)?|
+   (?<DN>Thu)(?:rsday)?|
+   (?<DN>Sat)(?:urday)?
+ .sp
+ There are five capturing substrings, but only one is ever set after a match.
+ The convenience function for extracting the data by name returns the substring
+ for the first (and in this example, the only) subpattern of that name that
+ matched. This saves searching to find which numbered subpattern it was. If you
+ make a reference to a non-unique named subpattern from elsewhere in the
+ pattern, the one that corresponds to the lowest number is used. For further
+ details of the interfaces for handling named subpatterns, see the
  .\" HREF
- \fBpcreapi\fR
+ \fBpcreapi\fP
  .\"
  documentation.
+ .
+ .
  .SH REPETITION
  .rs
  .sp
  Repetition is specified by quantifiers, which can follow any of the following
  items:
+ .sp
    a literal data character
-   the . metacharacter
+   the dot metacharacter
-   the \\C escape sequence
+   the \eC escape sequence
-   escapes such as \\d that match single characters
+   the \eX escape sequence (in UTF-8 mode with Unicode properties)
+   the \eR escape sequence
+   an escape such as \ed that matches a single character
    a character class
    a back reference (see next section)
    a parenthesized subpattern (unless it is an assertion)
+ .sp
  The general repetition quantifier specifies a minimum and maximum number 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:
+ .sp
    z{2,4}
+ .sp
  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
+ .sp
    [aeiou]{3,}
+ .sp
  matches at least 3 successive vowels, but may match many more, while
+ .sp
-   \\d{8}
+   \ed{8}
+ .sp
  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 example, {,6} is not a
  quantifier, but a literal string of four characters.
+ .P
  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
+ bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
- which is represented by a two-byte sequence.
+ which is represented by a two-byte sequence. Similarly, when Unicode property
+ support is available, \eX{3} matches three Unicode extended sequences, each of
+ which may be several bytes long (and they may be of different lengths).
+ .P
  The quantifier {0} is permitted, causing the expression to behave as if the
  previous item and the quantifier were not present.
+ .P
- For convenience (and historical compatibility) the three most common
+ For convenience, the three most common quantifiers have single-character
- quantifiers have single-character abbreviations:
+ abbreviations:
+ .sp
    *    is equivalent to {0,}
    +    is equivalent to {1,}
    ?    is equivalent to {0,1}
+ .sp
  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:
+ .sp
    (a?)*
+ .sp
  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 broken.
+ .P
  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 the
+ is in trying to match comments in C programs. These appear between /* and */
- sequences /* and */ and within the sequence, individual * and / characters may
+ and within the comment, individual * and / characters may appear. An attempt to
- appear. An attempt to match C comments by applying the pattern
+ match C comments by applying the pattern
+ .sp
-   /\\*.*\\*/
+   /\e*.*\e*/
+ .sp
  to the string
+ .sp
-   /* first command */  not comment  /* second comment */
+   /* first comment */  not comment  /* second comment */
+ .sp
  fails, because it matches the entire string owing to the greediness of the .*
  item.
+ .P
  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
+ .sp
-   /\\*.*?\\*/
+   /\e*.*?\e*/
+ .sp
  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
+ .sp
-   \\d??\\d
+   \ed??\ed
+ .sp
  which matches one digit by preference, but can match two if that is the only
  way the rest of the pattern matches.
+ .P
- If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
+ 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.
+ .P
  When a parenthesized subpattern is quantified with a minimum repeat count that
- is greater than 1 or with a limited maximum, more store is required for the
+ 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.
+ .P
  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
- to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
+ 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.
+ pattern as though it were preceded by \eA.
+ .P
  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.
+ .P
  However, there is one situation where the optimization cannot be used. When .*
  is inside capturing parentheses that are the subject of a backreference
- elsewhere in the pattern, a match at the start may fail, and a later one
+ elsewhere in the pattern, a match at the start may fail where a later one
- succeed. Consider, for example:
+ succeeds. Consider, for example:
+ .sp
-   (.*)abc\\1
+   (.*)abc\e1
+ .sp
  If the subject is "xyz123abc123" the match point is the fourth character. For
  this reason, such a pattern is not implicitly anchored.
+ .P
  When a capturing subpattern is repeated, the value captured is the substring
  that matched the final iteration. For example, after
+ .sp
-   (tweedle[dume]{3}\\s*)+
+   (tweedle[dume]{3}\es*)+
+ .sp
  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 iterations. For
  example, after
+ .sp
    /(a|(b))+/
+ .sp
  matches "aba" the value of the second captured substring is "b".
+ .
- .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
+ .
+ .\" HTML <a name="atomicgroup"></a>
+ .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
  .rs
  .sp
- With both maximizing and minimizing repetition, failure of what follows
+ With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
- normally causes the repeated item to be re-evaluated to see if a different
+ repetition, failure of what follows normally causes the repeated item to be
- number of repeats allows the rest of the pattern to match. Sometimes it is
+ re-evaluated to see if a different number of repeats allows the rest of the
- useful to prevent this, either to change the nature of the match, or to cause
+ pattern to match. Sometimes it is useful to prevent this, either to change the
- it fail earlier than it otherwise might, when the author of the pattern knows
+ nature of the match, or to cause it fail earlier than it otherwise might, when
- there is no point in carrying on.
+ the author of the pattern knows there is no point in carrying on.
+ .P
- Consider, for example, the pattern \\d+foo when applied to the subject line
+ Consider, for example, the pattern \ed+foo when applied to the subject line
+ .sp
 bar
+ .sp
  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+
+ action of the matcher is to try again with only 5 digits matching the \ed+
  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.
+ .P
- If we use atomic grouping for the previous example, the matcher would give up
+ 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:
+ .sp
-   (?>\\d+)foo
+   (?>\ed+)foo
+ .sp
  This kind of parenthesis "locks up" the  part of the pattern it contains 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.
+ .P
  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.
+ .P
  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
+ everything it can. So, while both \ed+ and \ed+? 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.
+ (?>\ed+) can only match an entire sequence of digits.
+ .P
  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
+ .sp
-   \\d++bar
+   \ed++foo
+ .sp
  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
  option is ignored. They are a convenient notation for the simpler forms of
- atomic group. However, there is no difference in the meaning or processing of a
+ atomic group. However, there is no difference in the meaning of a possessive
- possessive quantifier and the equivalent atomic group.
+ 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 syntax. It
+ .P
- originates in Sun's Java package.
+ 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.
+ .P
+ 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.
+ .P
  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
+ .sp
-   (\\D+|<\\d+>)*[!?]
+   (\eD+|<\ed+>)*[!?]
+ .sp
  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
+ .sp
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+ .sp
  it takes a long time before reporting failure. This is because the string can
- be divided between the two repeats in a large number of ways, and all have to
+ be divided between the internal \eD+ repeat and the external * repeat in a
- be tried. (The example used [!?] rather than a single character at the end,
+ large number of ways, and all have to be tried. (The example uses [!?] rather
- because both PCRE and Perl have an optimization that allows for fast failure
+ than a single character at the end, because both PCRE and Perl have an
- when a single character is used. They remember the last single character that
+ optimization that allows for fast failure when a single character is used. They
- is required for a match, and fail early if it is not present in the string.)
+ remember the last single character that is required for a match, and fail early
- If the pattern is changed to
+ if it is not present in the string.) If the pattern is changed so that it uses
+ an atomic group, like this:
-   ((?>\\D+)|<\\d+>)*[!?]
+ .sp
+   ((?>\eD+)|<\ed+>)*[!?]
+ .sp
  sequences of non-digits cannot be broken, and failure happens quickly.
+ .
- .SH BACK REFERENCES
+ .
+ .\" HTML <a name="backreferences"></a>
+ .SH "BACK REFERENCES"
  .rs
  .sp
  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.
+ .P
  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. See the section entitled "Backslash" above for further
+ numbers less than 10. A "forward back reference" of this type can make sense
- details of the handling of digits following a backslash.
+ when a repetition is involved and the subpattern to the right has participated
+ in an earlier iteration.
+ .P
+ 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 \e50 is
+ interpreted as a character defined in octal. See the subsection entitled
+ "Non-printing characters"
+ .\" HTML <a href="#digitsafterbackslash">
+ .\" </a>
+ 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).
+ .P
+ Another way of avoiding the ambiguity inherent in the use of digits following a
+ backslash is to use the \eg escape sequence, which is a feature introduced in
+ Perl 5.10. This escape must be followed by a positive or a negative number,
+ optionally enclosed in braces. These examples are all identical:
+ .sp
+   (ring), \e1
+   (ring), \eg1
+   (ring), \eg{1}
+ .sp
+ 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:
+ .sp
+   (abc(def)ghi)\eg{-1}
+ .sp
+ The sequence \eg{-1} is a reference to the most recently started capturing
+ subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
+ would be equivalent to \e1. 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.
+ .P
  A back reference matches whatever actually matched the capturing subpattern in
  the current subject string, rather than anything matching the subpattern
  itself (see
-Line 832 
 itself (see
+Line 1281 
 itself (see
  "Subpatterns as subroutines"
  .\"
  below for a way of doing that). So the pattern
+ .sp
-   (sens|respons)e and \\1ibility
+   (sens|respons)e and \e1ibility
+ .sp
  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 example,
+ .sp
-   ((?i)rah)\\s+\\1
+   ((?i)rah)\es+\e1
+ .sp
  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
  capturing subpattern is matched caselessly.
+ .P
- Back references to named subpatterns use the Python syntax (?P=name). We could
+ Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'
- rewrite the above example as follows:
+ or the Python syntax (?P=name). We could rewrite the above example in either of
+ the following ways:
-   (?<p1>(?i)rah)\\s+(?P=p1)
+ .sp
+   (?<p1>(?i)rah)\es+\ek<p1>
+   (?P<p1>(?i)rah)\es+(?P=p1)
+ .sp
+ A subpattern that is referenced by name may appear in the pattern before or
+ after the reference.
+ .P
  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
+ .sp
-   (a|(bc))\\2
+   (a|(bc))\e2
+ .sp
  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 can be used.
+ Otherwise an empty comment (see
+ .\" HTML <a href="#comments">
+ .\" </a>
+ "Comments"
+ .\"
+ below) can be used.
+ .P
  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.
+ when the subpattern is first used, so, for example, (a\e1) never matches.
  However, such references can be useful inside repeated subpatterns. For
  example, the pattern
+ .sp
-   (a|b\\1)+
+   (a|b\e1)+
+ .sp
  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration 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.
+ .
+ .
+ .\" HTML <a name="bigassertions"></a>
  .SH ASSERTIONS
  .rs
  .sp
  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.
+ assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
+ .\" HTML <a href="#smallassertions">
+ .\" </a>
+ above.
+ .\"
+ .P
  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.
+ 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.
- An assertion subpattern is matched in the normal way, except that it does not
+ .P
- cause the current matching position to be changed. Lookahead assertions start
+ Assertion subpatterns are not capturing subpatterns, and may not be repeated,
- with (?= for positive assertions and (?! for negative assertions. For example,
+ 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
-   \\w+(?=;)
+ 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.
+ .
+ .
+ .SS "Lookahead assertions"
+ .rs
+ .sp
+ Lookahead assertions start with (?= for positive assertions and (?! for
+ negative assertions. For example,
+ .sp
+   \ew+(?=;)
+ .sp
  matches a word followed by a semicolon, but does not include the semicolon in
  the match, and
+ .sp
    foo(?!bar)
+ .sp
  matches any occurrence of "foo" that is not followed by "bar". Note that the
  apparently similar pattern
+ .sp
    (?!foo)bar
+ .sp
  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 this effect.
+ lookbehind assertion is needed to achieve the other effect.
+ .P
  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.
+ .
+ .
+ .\" HTML <a name="lookbehind"></a>
+ .SS "Lookbehind assertions"
+ .rs
+ .sp
  Lookbehind assertions start with (?<= for positive assertions and (?<! for
  negative assertions. For example,
+ .sp
    (?<!foo)bar
+ .sp
  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 several alternatives, they do not
+ have a fixed length. However, if there are several top-level alternatives, they
- all have to have the same fixed length. Thus
+ do not all have to have the same fixed length. Thus
+ .sp
    (?<=bullock|donkey)
+ .sp
  is permitted, but
+ .sp
    (?<!dogs?|cats?)
+ .sp
  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
+ .sp
    (?<=ab(c|de))
+ .sp
  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:
+ .sp
    (?<=abc|abde)
+ .sp
  The implementation of lookbehind assertions is, for each alternative, to
- temporarily move the current position back by the fixed width and then try 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
- match is deemed to fail.
+ assertion fails.
+ .P
- PCRE does not allow the \\C escape (which matches a single byte in UTF-8 mode)
+ PCRE does not allow the \eC 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 length of the lookbehind. The \eX and \eR escapes, which can match
+ different numbers of bytes, are also not permitted.
- Atomic groups can be used in conjunction with lookbehind assertions to specify
+ .P
- efficient matching at the end of the subject string. Consider a simple pattern
+ Possessive quantifiers can be used in conjunction with lookbehind assertions to
- such as
+ specify efficient matching at the end of the subject string. Consider a simple
+ pattern such as
+ .sp
    abcd$
+ .sp
  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
+ .sp
    ^.*abcd$
+ .sp
  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
+ .sp
-   ^(?>.*)(?<=abcd)
- or, equivalently,
    ^.*+(?<=abcd)
+ .sp
- there can be no backtracking for the .* item; it can match only the entire
+ 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.
+ .
+ .
+ .SS "Using multiple assertions"
+ .rs
+ .sp
  Several assertions (of any sort) may occur in succession. For example,
+ .sp
-   (?<=\\d{3})(?<!999)foo
+   (?<=\ed{3})(?<!999)foo
+ .sp
  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 \fInot\fR match "foo" preceded by six characters, the first
+ This pattern does \fInot\fP match "foo" preceded by six characters, the first
  of which are digits and the last three of which are not "999". For example, it
  doesn't match "123abcfoo". A pattern to do that is
+ .sp
-   (?<=\\d{3}...)(?<!999)foo
+   (?<=\ed{3}...)(?<!999)foo
+ .sp
  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".
+ .P
  Assertions can be nested in any combination. For example,
+ .sp
    (?<=(?<!foo)bar)baz
+ .sp
  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
  preceded by "foo", while
+ .sp
-   (?<=\\d{3}(?!999)...)foo
+   (?<=\ed{3}(?!999)...)foo
+ .sp
- is another pattern which matches "foo" preceded by three digits and any three
+ is another pattern that matches "foo" preceded by three digits and any three
  characters that are not "999".
+ .
- 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
+ .\" HTML <a name="conditions"></a>
- of assertion contains capturing subpatterns within it, these are counted for
+ .SH "CONDITIONAL SUBPATTERNS"
- 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.
- .SH CONDITIONAL SUBPATTERNS
  .rs
  .sp
  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
+ .sp
    (?(condition)yes-pattern)
    (?(condition)yes-pattern|no-pattern)
+ .sp
  If the condition is satisfied, the yes-pattern is used; otherwise the
  no-pattern (if present) is used. If there are more than two alternatives in the
  subpattern, a compile-time error occurs.
+ .P
- There are three kinds of condition. If the text between the parentheses
+ There are four kinds of condition: references to subpatterns, references to
- consists of a sequence of digits, the condition is satisfied if the capturing
+ recursion, a pseudo-condition called DEFINE, and assertions.
- subpattern of that number has previously matched. The number must be greater
+ .
- than zero. Consider the following pattern, which contains non-significant white
+ .SS "Checking for a used subpattern by number"
- space to make it more readable (assume the PCRE_EXTENDED option) and to divide
+ .rs
- it into three parts for ease of discussion:
+ .sp
+ If the text between the parentheses consists of a sequence of digits, the
-   ( \\( )?    [^()]+    (?(1) \\) )
+ condition is true if the capturing subpattern of that number has previously
+ matched.
+ .P
+ 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:
+ .sp
+   ( \e( )?    [^()]+    (?(1) \e) )
+ .sp
  The first part matches an optional opening parenthesis, and if that
  character is present, sets it as the first captured substring. The second part
  matches one or more characters that are not parentheses. The third part is a
-Line 1047 
 the condition is true, and so the yes-pa
+Line 1532 
 the condition is true, and so the yes-pa
  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 the condition is the string (R), it is satisfied if a recursive call to the
+ .SS "Checking for a used subpattern by name"
- pattern or subpattern has been made. At "top level", the condition is false.
+ .rs
- This is a PCRE extension. Recursive patterns are described in the next section.
+ .sp
+ Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
- If the condition is not a sequence of digits or (R), it must be an assertion.
+ 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.
+ .P
+ Rewriting the above example to use a named subpattern gives this:
+ .sp
+   (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
+ .sp
+ .
+ .SS "Checking for pattern recursion"
+ .rs
+ .sp
+ 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:
+ .sp
+   (?(R3)...) or (?(R&name)...)
+ .sp
+ the condition is true if the most recent recursion is into the subpattern whose
+ number or name is given. This condition does not check the entire recursion
+ stack.
+ .P
+ At "top level", all these recursion test conditions are false. Recursive
+ patterns are described below.
+ .
+ .SS "Defining subpatterns for use by reference only"
+ .rs
+ .sp
+ 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):
+ .sp
+   (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
+   \eb (?&byte) (\e.(?&byte)){3} \eb
+ .sp
+ 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.
+ .P
+ 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.
+ .
+ .SS "Assertion conditions"
+ .rs
+ .sp
+ 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:
+ .sp
    (?(?=[^a-z]*[a-z])
-   \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )
+   \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )
+ .sp
  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.
+ .
+ .
+ .\" HTML <a name="comments"></a>
  .SH COMMENTS
  .rs
  .sp
- The sequence (?# marks the start of a comment which continues up to the next
+ 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.
+ .P
  If the PCRE_EXTENDED option is set, an unescaped # character outside a
- character class introduces a comment that continues up to the next newline
+ character class introduces a comment that continues to immediately after the
- character in the pattern.
+ next newline in the pattern.
+ .
- .SH RECURSIVE PATTERNS
+ .
+ .\" HTML <a name="recursion"></a>
+ .SH "RECURSIVE PATTERNS"
  .rs
  .sp
  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. Perl has provided an
+ is not possible to handle an arbitrary nesting depth.
- experimental facility that allows regular expressions to recurse (amongst other
+ .P
- things). It does this by interpolating Perl code in the expression at run time,
+ For some time, Perl has provided a facility that allows regular expressions to
- and the code can refer to the expression itself. A Perl pattern to solve the
+ recurse (amongst other things). It does this by interpolating Perl code in the
- parentheses problem can be created like this:
+ 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
-   $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;
+ created like this:
+ .sp
+   $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
+ .sp
  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
+ recursively to the pattern in which it appears.
- the interpolation of Perl code. Instead, it supports some special syntax for
+ .P
- recursion of the entire pattern, and also for individual subpattern recursion.
+ Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
+ supports special syntax for recursion of the entire pattern, and also for
- The special item that consists of (? followed by a number greater than zero and
+ individual subpattern recursion. After its introduction in PCRE and Python,
- a closing parenthesis is a recursive call of the subpattern of the given
+ this kind of recursion was introduced into Perl at release 5.10.
- number, provided that it occurs inside that subpattern. (If not, it is a
+ .P
- "subroutine" call, which is described in the next section.) The special item
+ A special item that consists of (? followed by a number greater than zero and a
- (?R) is a recursive call of the entire regular expression.
+ 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"
- For example, this PCRE pattern solves the nested parentheses problem (assume
+ call, which is described in the next section.) The special item (?R) or (?0) is
- the PCRE_EXTENDED option is set so that white space is ignored):
+ a recursive call of the entire regular expression.
+ .P
-   \\( ( (?>[^()]+) | (?R) )* \\)
+ 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.
+ .P
+ This PCRE pattern solves the nested parentheses problem (assume the
+ PCRE_EXTENDED option is set so that white space is ignored):
+ .sp
+   \e( ( (?>[^()]+) | (?R) )* \e)
+ .sp
  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 parenthesized substring).
+ match of the pattern itself (that is, a correctly parenthesized substring).
  Finally there is a closing parenthesis.
+ .P
  If this were part of a larger pattern, you would not want to recurse the entire
  pattern, so instead you could use this:
+ .sp
-   ( \\( ( (?>[^()]+) | (?1) )* \\) )
+   ( \e( ( (?>[^()]+) | (?1) )* \e) )
+ .sp
  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
+ them instead of the whole pattern.
- parenthesis numbers can be tricky. It may be more convenient to use named
+ .P
- parentheses instead. For this, PCRE uses (?P>name), which is an extension to
+ In a larger pattern, keeping track of parenthesis numbers can be tricky. This
- the Python syntax that PCRE uses for named parentheses (Perl does not provide
+ is made easier by the use of relative references. (A Perl 5.10 feature.)
- named parentheses). We could rewrite the above example as follows:
+ 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
-   (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )
+ negative number counts capturing parentheses leftwards from the point at which
+ it is encountered.
- This particular example pattern contains nested unlimited repeats, and so the
+ .P
- use of atomic grouping for matching strings of non-parentheses is important
+ It is also possible to refer to subsequently opened parentheses, by writing
- when applying the pattern to strings that do not match. For example, when this
+ references such as (?+2). However, these cannot be recursive because the
- pattern is applied to
+ reference is not inside the parentheses that are referenced. They are always
+ "subroutine" calls, as described in the next section.
+ .P
+ 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:
+ .sp
+   (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
+ .sp
+ If there is more than one subpattern with the same name, the earliest one is
+ used.
+ .P
+ 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
+ .sp
    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
+ .sp
  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.
+ .P
  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
  .\" HREF
- \fBpcrecallout\fR
+ \fBpcrecallout\fP
  .\"
  documentation). If the pattern above is matched against
+ .sp
    (ab(cd)ef)
+ .sp
  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
+ .sp
-   \\( ( ( (?>[^()]+) | (?R) )* ) \\)
+   \e( ( ( (?>[^()]+) | (?R) )* ) \e)
       ^                        ^
       ^                        ^
+ .sp
  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 pattern, PCRE
  has to obtain extra memory to store data during a recursion, which it does by
- using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no
+ using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
+ .P
  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.
+ .sp
-   < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >
+   < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
+ .sp
  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.
+ .
+ .
  .\" HTML <a name="subpatternsassubroutines"></a>
- .SH SUBPATTERNS AS SUBROUTINES
+ .SH "SUBPATTERNS AS SUBROUTINES"
  .rs
  .sp
  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. An earlier example pointed out that the
+ subroutine in a programming language. The "called" subpattern may be defined
- pattern
+ before or after the reference. A numbered reference can be absolute or
+ relative, as in these examples:
-   (sens|respons)e and \\1ibility
+ .sp
+   (...(absolute)...)...(?2)...
+   (...(relative)...)...(?-1)...
+   (...(?+1)...(relative)...
+ .sp
+ An earlier example pointed out that the pattern
+ .sp
+   (sens|respons)e and \e1ibility
+ .sp
  matches "sense and sensibility" and "response and responsibility", but not
  "sense and responsibility". If instead the pattern
+ .sp
    (sens|respons)e and (?1)ibility
+ .sp
  is used, it does match "sense and responsibility" as well as the other two
- strings. Such references must, however, follow the subpattern to which they
+ strings. Another example is given in the discussion of DEFINE above.
- refer.
+ .P
+ 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.
+ .P
+ 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:
+ .sp
+   (abc)(?i:(?-1))
+ .sp
+ It matches "abcabc". It does not match "abcABC" because the change of
+ processing option does not affect the called subpattern.
+ .
+ .
  .SH CALLOUTS
  .rs
  .sp
-Line 1201 
 Perl has a feature whereby using the seq
+Line 1793 
 Perl has a feature whereby using the seq
  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.
+ .P
  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 \fIpcre_callout\fR.
+ function by putting its entry point in the global variable \fIpcre_callout\fP.
  By default, this variable contains NULL, which disables all calling out.
+ .P
  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:
+ .sp
-   (?C1)\dabc(?C2)def
+   (?C1)abc(?C2)def
+ .sp
- During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is
+ If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
+ automatically installed before each item in the pattern. They are all numbered
+.
+ .P
+ During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
  set), the external function is called. It is provided with the number of the
- callout, and, optionally, one item of data originally supplied by the caller of
+ callout, the position in the pattern, and, optionally, one item of data
- \fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to
+ originally supplied by the caller of \fBpcre_exec()\fP. The callout function
- fail altogether. A complete description of the interface to the callout
+ may cause matching to proceed, to backtrack, or to fail altogether. A complete
- function is given in the
+ description of the interface to the callout function is given in the
  .\" HREF
- \fBpcrecallout\fR
+ \fBpcrecallout\fP
  .\"
  documentation.
+ .
- .in 0
+ .
- Last updated: 03 February 2003
+ .SH "SEE ALSO"
- .br
+ .rs
- Copyright (c) 1997-2003 University of Cambridge.
+ .sp
+ \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
+ .
+ .
+ .SH AUTHOR
+ .rs
+ .sp
+ .nf
+ Philip Hazel
+ University Computing Service
+ Cambridge CB2 3QH, England.
+ .fi
+ .
+ .
+ .SH REVISION
+ .rs
+ .sp
+ .nf
+ Last updated: 06 March 2007
+ Copyright (c) 1997-2007 University of Cambridge.
+ .fi

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