/vendor/pcre/doc/html/pcrepattern.html
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- <html>
- <head>
- <title>pcrepattern specification</title>
- </head>
- <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
- <h1>pcrepattern man page</h1>
- <p>
- Return to the <a href="index.html">PCRE index page</a>.
- </p>
- <p>
- This page is part of the PCRE HTML documentation. It was generated automatically
- from the original man page. If there is any nonsense in it, please consult the
- man page, in case the conversion went wrong.
- <br>
- <ul>
- <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
- <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
- <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
- <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
- <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
- <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
- <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
- <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
- <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
- <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
- <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
- <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
- <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
- <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
- <li><a name="TOC15" href="#SEC15">REPETITION</a>
- <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
- <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
- <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
- <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
- <li><a name="TOC20" href="#SEC20">COMMENTS</a>
- <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
- <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
- <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
- <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
- <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
- <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
- <li><a name="TOC27" href="#SEC27">AUTHOR</a>
- <li><a name="TOC28" href="#SEC28">REVISION</a>
- </ul>
- <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
- <P>
- The syntax and semantics of the regular expressions that are supported by PCRE
- are described in detail below. There is a quick-reference syntax summary in the
- <a href="pcresyntax.html"><b>pcresyntax</b></a>
- page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
- also supports some alternative regular expression syntax (which does not
- conflict with the Perl syntax) in order to provide some compatibility with
- regular expressions in Python, .NET, and Oniguruma.
- </P>
- <P>
- Perl's regular expressions are described in its own documentation, and
- regular expressions in general are covered in a number of books, some of which
- have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
- published by O'Reilly, covers regular expressions in great detail. This
- description of PCRE's regular expressions is intended as reference material.
- </P>
- <P>
- The original operation of PCRE was on strings of one-byte characters. However,
- there is now also support for UTF-8 character strings. To use this, you must
- build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
- the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
- places below. There is also a summary of UTF-8 features in the
- <a href="pcre.html#utf8support">section on UTF-8 support</a>
- in the main
- <a href="pcre.html"><b>pcre</b></a>
- page.
- </P>
- <P>
- The remainder of this document discusses the patterns that are supported by
- PCRE when its main matching function, <b>pcre_exec()</b>, is used.
- From release 6.0, PCRE offers a second matching function,
- <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
- Perl-compatible. Some of the features discussed below are not available when
- <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
- alternative function, and how it differs from the normal function, are
- discussed in the
- <a href="pcrematching.html"><b>pcrematching</b></a>
- page.
- </P>
- <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
- <P>
- PCRE supports five different conventions for indicating line breaks in
- strings: a single CR (carriage return) character, a single LF (linefeed)
- character, the two-character sequence CRLF, any of the three preceding, or any
- Unicode newline sequence. The
- <a href="pcreapi.html"><b>pcreapi</b></a>
- page has
- <a href="pcreapi.html#newlines">further discussion</a>
- about newlines, and shows how to set the newline convention in the
- <i>options</i> arguments for the compiling and matching functions.
- </P>
- <P>
- It is also possible to specify a newline convention by starting a pattern
- string with one of the following five sequences:
- <pre>
- (*CR) carriage return
- (*LF) linefeed
- (*CRLF) carriage return, followed by linefeed
- (*ANYCRLF) any of the three above
- (*ANY) all Unicode newline sequences
- </pre>
- These override the default and the options given to <b>pcre_compile()</b>. For
- example, on a Unix system where LF is the default newline sequence, the pattern
- <pre>
- (*CR)a.b
- </pre>
- changes the convention to CR. That pattern matches "a\nb" because LF is no
- longer a newline. Note that these special settings, which are not
- Perl-compatible, are recognized only at the very start of a pattern, and that
- they must be in upper case. If more than one of them is present, the last one
- is used.
- </P>
- <P>
- The newline convention does not affect what the \R escape sequence matches. By
- default, this is any Unicode newline sequence, for Perl compatibility. However,
- this can be changed; see the description of \R in the section entitled
- <a href="#newlineseq">"Newline sequences"</a>
- below. A change of \R setting can be combined with a change of newline
- convention.
- </P>
- <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
- <P>
- 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
- <pre>
- The quick brown fox
- </pre>
- matches a portion of a subject string that is identical to itself. When
- caseless matching is specified (the PCRE_CASELESS option), letters are matched
- independently of case. In UTF-8 mode, PCRE always understands the concept of
- case for characters whose values are less than 128, so caseless matching is
- always possible. For characters with higher 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>
- <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
- <i>metacharacters</i>, which do not stand for themselves but instead are
- interpreted in some special way.
- </P>
- <P>
- There are two different sets of metacharacters: those that are recognized
- anywhere in the pattern except within square brackets, and those that are
- recognized within square brackets. Outside square brackets, the metacharacters
- are as follows:
- <pre>
- \ general escape character with several uses
- ^ assert start of string (or line, in multiline mode)
- $ assert end of string (or line, in multiline mode)
- . match any character except newline (by default)
- [ start character class definition
- | start of alternative branch
- ( start subpattern
- ) end subpattern
- ? extends the meaning of (
- also 0 or 1 quantifier
- also quantifier minimizer
- * 0 or more quantifier
- + 1 or more quantifier
- also "possessive quantifier"
- { start min/max quantifier
- </pre>
- Part of a pattern that is in square brackets is called a "character class". In
- a character class the only metacharacters are:
- <pre>
- \ general escape character
- ^ negate the class, but only if the first character
- - indicates character range
- [ POSIX character class (only if followed by POSIX syntax)
- ] terminates the character class
- </pre>
- The following sections describe the use of each of the metacharacters.
- </P>
- <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
- <P>
- The backslash character has several uses. Firstly, if it is followed by a
- non-alphanumeric character, it takes away any special meaning that character
- may have. This use of backslash as an escape character applies both inside and
- outside character classes.
- </P>
- <P>
- For example, if you want to match a * character, you write \* in the pattern.
- This escaping action applies whether or not the following character would
- otherwise be interpreted as a metacharacter, so it is always safe to precede a
- non-alphanumeric with backslash to specify that it stands for itself. In
- particular, if you want to match a backslash, you write \\.
- </P>
- <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 are ignored. An escaping backslash can
- be used to include a whitespace or # character as part of the pattern.
- </P>
- <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
- that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
- Perl, $ and @ cause variable interpolation. Note the following examples:
- <pre>
- Pattern PCRE matches Perl matches
- \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
- \Qabc\$xyz\E abc\$xyz abc\$xyz
- \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
- </pre>
- The \Q...\E sequence is recognized both inside and outside character classes.
- <a name="digitsafterbackslash"></a></P>
- <br><b>
- Non-printing characters
- </b><br>
- <P>
- 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:
- <pre>
- \a alarm, that is, the BEL character (hex 07)
- \cx "control-x", where x is any character
- \e escape (hex 1B)
- \f formfeed (hex 0C)
- \n linefeed (hex 0A)
- \r carriage return (hex 0D)
- \t tab (hex 09)
- \ddd character with octal code ddd, or backreference
- \xhh character with hex code hh
- \x{hhh..} character with hex code hhh..
- </pre>
- The precise effect of \cx is as follows: if x is a lower case letter, it
- is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
- Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
- 7B.
- </P>
- <P>
- After \x, from zero to two hexadecimal digits are read (letters can be in
- upper or lower case). Any number of hexadecimal digits may appear between \x{
- and }, but the value of the character code must be less than 256 in non-UTF-8
- mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
- hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
- point, which is 10FFFF.
- </P>
- <P>
- If characters other than hexadecimal digits appear between \x{ and }, or if
- there is no terminating }, this form of escape is not recognized. Instead, the
- initial \x will be interpreted as a basic hexadecimal escape, with no
- following digits, giving a character whose value is zero.
- </P>
- <P>
- Characters whose value is less than 256 can be defined by either of the two
- syntaxes for \x. There is no difference in the way they are handled. For
- example, \xdc is exactly the same as \x{dc}.
- </P>
- <P>
- After \0 up to two further octal digits are read. If there are fewer than two
- digits, just those that are present are used. Thus the sequence \0\x\07
- specifies two binary zeros followed by a BEL character (code value 7). Make
- sure you supply two digits after the initial zero if the pattern character that
- follows is itself an octal digit.
- </P>
- <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 <i>back reference</i>. A description of how this works is given
- <a href="#backreferences">later,</a>
- following the discussion of
- <a href="#subpattern">parenthesized subpatterns.</a>
- </P>
- <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 uses them to generate a data character. Any
- subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
- character specified in octal must be less than \400. In UTF-8 mode, values up
- to \777 are permitted. For example:
- <pre>
- \040 is another way of writing a space
- \40 is the same, provided there are fewer than 40 previous capturing subpatterns
- \7 is always a back reference
- \11 might be a back reference, or another way of writing a tab
- \011 is always a tab
- \0113 is a tab followed by the character "3"
- \113 might be a back reference, otherwise the character with octal code 113
- \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
- \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
- </pre>
- 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>
- <P>
- All the sequences that define a single character value can be used both inside
- and outside character classes. In addition, inside a character class, the
- sequence \b is interpreted as the backspace character (hex 08), and the
- sequences \R and \X are interpreted as the characters "R" and "X",
- respectively. Outside a character class, these sequences have different
- meanings
- <a href="#uniextseq">(see below).</a>
- </P>
- <br><b>
- Absolute and relative back references
- </b><br>
- <P>
- The sequence \g followed by an unsigned or a negative number, optionally
- enclosed in braces, is an absolute or relative back reference. A named back
- reference can be coded as \g{name}. Back references are discussed
- <a href="#backreferences">later,</a>
- following the discussion of
- <a href="#subpattern">parenthesized subpatterns.</a>
- </P>
- <br><b>
- Absolute and relative subroutine calls
- </b><br>
- <P>
- For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
- a number enclosed either in angle brackets or single quotes, is an alternative
- syntax for referencing a subpattern as a "subroutine". Details are discussed
- <a href="#onigurumasubroutines">later.</a>
- Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
- synonymous. The former is a back reference; the latter is a subroutine call.
- </P>
- <br><b>
- Generic character types
- </b><br>
- <P>
- Another use of backslash is for specifying generic character types. The
- following are always recognized:
- <pre>
- \d any decimal digit
- \D any character that is not a decimal digit
- \h any horizontal whitespace character
- \H any character that is not a horizontal whitespace character
- \s any whitespace character
- \S any character that is not a whitespace character
- \v any vertical whitespace character
- \V any character that is not a vertical whitespace character
- \w any "word" character
- \W any "non-word" character
- </pre>
- 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>
- <P>
- These character type sequences can appear both inside and outside character
- classes. They each match one character of the appropriate type. If the current
- matching point is at the end of the subject string, all of them fail, since
- there is no character to match.
- </P>
- <P>
- For compatibility with Perl, \s does not match the VT character (code 11).
- This makes it different from the the POSIX "space" class. The \s characters
- are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
- included in a Perl script, \s may match the VT character. In PCRE, it never
- does.
- </P>
- <P>
- In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
- \w, and always match \D, \S, and \W. This is true even when Unicode
- character property support is available. These sequences retain their original
- meanings from before UTF-8 support was available, mainly for efficiency
- reasons.
- </P>
- <P>
- The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
- other sequences, these do match certain high-valued codepoints in UTF-8 mode.
- The horizontal space characters are:
- <pre>
- U+0009 Horizontal tab
- U+0020 Space
- U+00A0 Non-break space
- U+1680 Ogham space mark
- U+180E Mongolian vowel separator
- U+2000 En quad
- U+2001 Em quad
- U+2002 En space
- U+2003 Em space
- U+2004 Three-per-em space
- U+2005 Four-per-em space
- U+2006 Six-per-em space
- U+2007 Figure space
- U+2008 Punctuation space
- U+2009 Thin space
- U+200A Hair space
- U+202F Narrow no-break space
- U+205F Medium mathematical space
- U+3000 Ideographic space
- </pre>
- The vertical space characters are:
- <pre>
- U+000A Linefeed
- U+000B Vertical tab
- U+000C Formfeed
- U+000D Carriage return
- U+0085 Next line
- U+2028 Line separator
- U+2029 Paragraph separator
- </PRE>
- </P>
- <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
- <a href="pcreapi.html#localesupport">"Locale support"</a>
- in the
- <a href="pcreapi.html"><b>pcreapi</b></a>
- page). For example, in a French locale such as "fr_FR" in Unix-like systems,
- or "french" in Windows, some character codes greater than 128 are used for
- accented letters, and these are matched by \w. The use of locales with Unicode
- is discouraged.
- <a name="newlineseq"></a></P>
- <br><b>
- Newline sequences
- </b><br>
- <P>
- Outside a character class, by default, the escape sequence \R matches any
- Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
- equivalent to the following:
- <pre>
- (?>\r\n|\n|\x0b|\f|\r|\x85)
- </pre>
- This is an example of an "atomic group", details of which are given
- <a href="#atomicgroup">below.</a>
- 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>
- <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>
- <P>
- It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
- complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
- either at compile time or when the pattern is matched. (BSR is an abbrevation
- for "backslash R".) This can be made the default when PCRE is built; if this is
- the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
- It is also possible to specify these settings by starting a pattern string with
- one of the following sequences:
- <pre>
- (*BSR_ANYCRLF) CR, LF, or CRLF only
- (*BSR_UNICODE) any Unicode newline sequence
- </pre>
- These override the default and the options given to <b>pcre_compile()</b>, but
- they can be overridden by options given to <b>pcre_exec()</b>. Note that these
- special settings, which are not Perl-compatible, are recognized only at the
- very start of a pattern, and that they must be in upper case. If more than one
- of them is present, the last one is used. They can be combined with a change of
- newline convention, for example, a pattern can start with:
- <pre>
- (*ANY)(*BSR_ANYCRLF)
- </pre>
- Inside a character class, \R matches the letter "R".
- <a name="uniextseq"></a></P>
- <br><b>
- Unicode character properties
- </b><br>
- <P>
- When PCRE is built with Unicode character property support, three additional
- escape sequences that match characters with specific properties are available.
- When not in UTF-8 mode, these sequences are of course limited to testing
- characters whose codepoints are less than 256, but they do work in this mode.
- The extra escape sequences are:
- <pre>
- \p{<i>xx</i>} a character with the <i>xx</i> property
- \P{<i>xx</i>} a character without the <i>xx</i> property
- \X an extended Unicode sequence
- </pre>
- The property names represented by <i>xx</i> 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 \P{Any} does not match any
- characters, so always causes a match failure.
- </P>
- <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:
- <pre>
- \p{Greek}
- \P{Han}
- </pre>
- Those that are not part of an identified script are lumped together as
- "Common". The current list of scripts is:
- </P>
- <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>
- <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, \p{^Lu} is the same as \P{Lu}.
- </P>
- <P>
- If only one letter is specified with \p or \P, it includes all the general
- category properties that start with that letter. In this case, in the absence
- of negation, the curly brackets in the escape sequence are optional; these two
- examples have the same effect:
- <pre>
- \p{L}
- \pL
- </pre>
- The following general category property codes are supported:
- <pre>
- C Other
- Cc Control
- Cf Format
- Cn Unassigned
- Co Private use
- Cs Surrogate
- L Letter
- Ll Lower case letter
- Lm Modifier letter
- Lo Other letter
- Lt Title case letter
- Lu Upper case letter
- M Mark
- Mc Spacing mark
- Me Enclosing mark
- Mn Non-spacing mark
- N Number
- Nd Decimal number
- Nl Letter number
- No Other number
- P Punctuation
- Pc Connector punctuation
- Pd Dash punctuation
- Pe Close punctuation
- Pf Final punctuation
- Pi Initial punctuation
- Po Other punctuation
- Ps Open punctuation
- S Symbol
- Sc Currency symbol
- Sk Modifier symbol
- Sm Mathematical symbol
- So Other symbol
- Z Separator
- Zl Line separator
- Zp Paragraph separator
- Zs Space separator
- </pre>
- 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>
- <P>
- The Cs (Surrogate) property applies only to characters in the range U+D800 to
- U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
- cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
- (see the discussion of PCRE_NO_UTF8_CHECK in the
- <a href="pcreapi.html"><b>pcreapi</b></a>
- page).
- </P>
- <P>
- The long synonyms for these properties that Perl supports (such as \p{Letter})
- are not supported by PCRE, nor is it permitted to prefix any of these
- properties with "Is".
- </P>
- <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>
- <P>
- Specifying caseless matching does not affect these escape sequences. For
- example, \p{Lu} always matches only upper case letters.
- </P>
- <P>
- The \X escape matches any number of Unicode characters that form an extended
- Unicode sequence. \X is equivalent to
- <pre>
- (?>\PM\pM*)
- </pre>
- 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
- <a href="#atomicgroup">(see below).</a>
- Characters with the "mark" property are typically accents that affect the
- preceding character. None of them have codepoints less than 256, so in
- non-UTF-8 mode \X matches any one character.
- </P>
- <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 \d and \w do not use Unicode
- properties in PCRE.
- <a name="resetmatchstart"></a></P>
- <br><b>
- Resetting the match start
- </b><br>
- <P>
- The escape sequence \K, which is a Perl 5.10 feature, causes any previously
- matched characters not to be included in the final matched sequence. For
- example, the pattern:
- <pre>
- foo\Kbar
- </pre>
- matches "foobar", but reports that it has matched "bar". This feature is
- similar to a lookbehind assertion
- <a href="#lookbehind">(described below).</a>
- However, in this case, the part of the subject before the real match does not
- have to be of fixed length, as lookbehind assertions do. The use of \K does
- not interfere with the setting of
- <a href="#subpattern">captured substrings.</a>
- For example, when the pattern
- <pre>
- (foo)\Kbar
- </pre>
- matches "foobar", the first substring is still set to "foo".
- <a name="smallassertions"></a></P>
- <br><b>
- Simple assertions
- </b><br>
- <P>
- 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
- <a href="#bigassertions">below.</a>
- The backslashed assertions are:
- <pre>
- \b matches at a word boundary
- \B matches when not at a word boundary
- \A matches at the start of the subject
- \Z matches at the end of the subject
- also matches before a newline at the end of the subject
- \z matches only at the end of the subject
- \G matches at the first matching position in the subject
- </pre>
- These assertions may not appear in character classes (but note that \b has a
- different meaning, namely the backspace character, inside a character class).
- </P>
- <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
- \w and the other matches \W), or the start or end of the string if the
- first or last character matches \w, respectively.
- </P>
- <P>
- The \A, \Z, and \z assertions differ from the traditional circumflex and
- dollar (described in the next section) in that they only ever match at the very
- start and end of the subject string, whatever options are set. Thus, they are
- independent of multiline mode. These three assertions are not affected by the
- PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
- circumflex and dollar metacharacters. However, if the <i>startoffset</i>
- argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
- at a point other than the beginning of the subject, \A can never match. The
- difference between \Z and \z is that \Z matches before a newline at the end
- of the string as well as at the very end, whereas \z matches only at the end.
- </P>
- <P>
- The \G assertion is true only when the current matching position is at the
- start point of the match, as specified by the <i>startoffset</i> argument of
- <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
- non-zero. By calling <b>pcre_exec()</b> 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.
- </P>
- <P>
- Note, however, that PCRE's interpretation of \G, as the start of the current
- match, is subtly different from Perl's, which defines it as the end of the
- previous match. In Perl, these can be different when the previously matched
- string was empty. Because PCRE does just one match at a time, it cannot
- reproduce this behaviour.
- </P>
- <P>
- If all the alternatives of a pattern begin with \G, the expression is anchored
- to the starting match position, and the "anchored" flag is set in the compiled
- regular expression.
- </P>
- <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
- <P>
- Outside a character class, in the default matching mode, the circumflex
- character is an assertion that is true only if the current matching point is
- at the start of the subject string. If the <i>startoffset</i> argument of
- <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
- option is unset. Inside a character class, circumflex has an entirely different
- meaning
- <a href="#characterclass">(see below).</a>
- </P>
- <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
- possible alternatives start with a circumflex, that is, if the pattern is
- constrained to match only at the start of the subject, it is said to be an
- "anchored" pattern. (There are also other constructs that can cause a pattern
- to be anchored.)
- </P>
- <P>
- A dollar character is an assertion that is true only if the current matching
- point is at the end of the subject string, or immediately before a newline
- at the end of the string (by default). Dollar need not be the last character of
- the pattern if a number of alternatives are involved, but it should be the last
- item in any branch in which it appears. Dollar has no special meaning in a
- character class.
- </P>
- <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.
- </P>
- <P>
- The meanings of the circumflex and dollar characters are changed if the
- PCRE_MULTILINE option is set. When this is the case, a circumflex matches
- immediately after internal newlines as well as at the start of the subject
- string. It does not match after a newline that ends the string. A dollar
- matches before any newlines in the string, as well as at the very end, when
- PCRE_MULTILINE is set. When newline is specified as the two-character
- sequence CRLF, isolated CR and LF characters do not indicate newlines.
- </P>
- <P>
- For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
- \n represents a newline) in multiline mode, but not otherwise. Consequently,
- patterns that are anchored in single line mode because all branches start with
- ^ are not anchored in multiline mode, and a match for circumflex is possible
- when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
- PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
- </P>
- <P>
- Note that the sequences \A, \Z, and \z can be used to match the start and
- end of the subject in both modes, and if all branches of a pattern start with
- \A it is always anchored, whether or not PCRE_MULTILINE is set.
- </P>
- <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
- <P>
- Outside a character class, a dot in the pattern matches any one character in
- the subject string except (by default) a character that signifies the end of a
- line. In UTF-8 mode, the matched character may be more than one byte long.
- </P>
- <P>
- When a line ending is defined as a single character, dot never matches that
- character; when the two-character sequence CRLF is used, dot does not match CR
- if it is immediately followed by LF, but otherwise it matches all characters
- (including isolated CRs and LFs). When any Unicode line endings are being
- recognized, dot does not match CR or LF or any of the other line ending
- characters.
- </P>
- <P>
- The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
- option is set, a dot matches any one character, without exception. If the
- two-character sequence CRLF is present in the subject string, it takes two dots
- to match it.
- </P>
- <P>
- The handling of dot is entirely independent of the handling of circumflex and
- dollar, the only relationship being that they both involve newlines. Dot has no
- special meaning in a character class.
- </P>
- <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
- <P>
- Outside a character class, the escape sequence \C matches any one byte, both
- in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
- characters. The feature is provided in Perl in order to match individual bytes
- in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
- what remains in the string may be a malformed UTF-8 string. For this reason,
- the \C escape sequence is best avoided.
- </P>
- <P>
- PCRE does not allow \C to appear in lookbehind assertions
- <a href="#lookbehind">(described below),</a>
- because in UTF-8 mode this would make it impossible to calculate the length of
- the lookbehind.
- <a name="characterclass"></a></P>
- <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
- <P>
- An opening square bracket introduces a character class, terminated by a closing
- square bracket. A closing square bracket on its own is not special. If a
- closing square bracket is required as a member of the class, it should be the
- first data character in the class (after an initial circumflex, if present) or
- escaped with a backslash.
- </P>
- <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
- definition is a circumflex, in which case the subject character must not be in
- the set defined by the class. If a circumflex is actually required as a member
- of the class, ensure it is not the first character, or escape it with a
- backslash.
- </P>
- <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 that
- are in the class by enumerating those that are not. A class that starts with a
- circumflex is not an assertion: it still consumes a character from the subject
- string, and therefore it fails if the current pointer is at the end of the
- string.
- </P>
- <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.
- </P>
- <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. In UTF-8 mode, PCRE always understands the concept of
- case for characters whose values are less than 128, so caseless matching is
- always possible. For characters with higher values, the concept of case is
- supported if PCRE is compiled with Unicode property support, but not otherwise.
- If you want to use caseless matching for characters 128 and above, you must
- ensure that PCRE is compiled with Unicode property support as well as with
- UTF-8 support.
- </P>
- <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>
- <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>
- <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 class containing a range
- followed by two other characters. The octal or hexadecimal representation of
- "]" can also be used to end a range.
- </P>
- <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
- mode, ranges can include characters whose values are greater than 255, for
- example [\x{100}-\x{2ff}].
- </P>
- <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 in non-UTF-8 mode, if character
- tables for a French locale are in use, [\xc8-\xcb] matches accented E
- characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
- characters with values greater than 128 only when it is compiled with Unicode
- property support.
- </P>
- <P>
- The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
- in a character class, and add the characters that they match to the class. For
- example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
- conveniently be used with the upper case character types to specify a more
- restricted set of characters than the matching lower case type. For example,
- the class [^\W_] matches any letter or digit, but not underscore.
- </P>
- <P>
- The only metacharacters that are recognized in character classes are backslash,
- hyphen (only where it can be interpreted as specifying a range), circumflex
- (only at the start), opening square bracket (only when it can be interpreted as
- introducing a POSIX class name - see the next section), and the terminating
- closing square bracket. However, escaping other non-alphanumeric characters
- does no harm.
- </P>
- <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
- <P>
- 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,
- <pre>
- [01[:alpha:]%]
- </pre>
- matches "0", "1", any alphabetic character, or "%". The supported class names
- are
- <pre>
- alnum letters and digits
- alpha letters
- ascii character codes 0 - 127
- blank space or tab only
- cntrl control characters
- digit decimal digits (same as \d)
- graph printing characters, excluding space
- lower lower case letters
- print printing characters, including space
- punct printing characters, excluding letters and digits
- space white space (not quite the same as \s)
- upper upper case letters
- word "word" characters (same as \w)
- xdigit hexadecimal digits
- </pre>
- The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
- space (32). Notice that this list includes the VT character (code 11). This
- makes "space" different to \s, which does not include VT (for Perl
- compatibility).
- </P>
- <P>
- The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
- 5.8. Another Perl extension is negation, which is indicated by a ^ character
- after the colon. For example,
- <pre>
- [12[:^digit:]]
- </pre>
- 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>
- <P>
- In UTF-8 mode, characters with values greater than 128 do not match any of
- the POSIX character classes.
- </P>
- <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
- <P>
- Vertical bar characters are used to separate alternative patterns. For example,
- the pattern
- <pre>
- gilbert|sullivan
- </pre>
- matches either "gilbert" or "sullivan". Any number of alternatives may appear,
- and an empty alternative is permitted (matching the empty string). The matching
- process tries each alternative in turn, from left to right, and the first one
- that succeeds is used. If the alternatives are within a subpattern
- <a href="#subpattern">(defined below),</a>
- "succeeds" means matching the rest of the main pattern as well as the
- alternative in the subpattern.
- </P>
- <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
- <P>
- The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
- PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
- the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
- The option letters are
- <pre>
- i for PCRE_CASELESS
- m for PCRE_MULTILINE
- s for PCRE_DOTALL
- x for PCRE_EXTENDED
- </pre>
- 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>
- <P>
- The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
- changed in the same way as the Perl-compatible options by using the characters
- J, U and X respectively.
- </P>
- <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
- <b>pcre_fullinfo()</b> function).
- </P>
- <P>
- An option change within a subpattern (see below for a description of
- subpatterns) affects only that part of the current pattern that follows it, so
- <pre>
- (a(?i)b)c
- </pre>
- 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,
- <pre>
- (a(?i)b|c)
- </pre>
- 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>
- <P>
- <b>Note:</b> There are other PCRE-specific options that can be set by the
- application when the compile or match functions are called. In some cases the
- pattern can contain special leading sequences to override what the application
- has set or what has been defaulted. Details are given in the section entitled
- <a href="#newlineseq">"Newline sequences"</a>
- above.
- <a name="subpattern"></a></P>
- <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
- <P>
- Subpatterns are delimited by parentheses (round brackets), which can be nested.
- Turning part of a pattern into a subpattern does two things:
- <br>
- <br>
- 1. It localizes a set of alternatives. For example, the pattern
- <pre>
- cat(aract|erpillar|)
- </pre>
- matches one of the words "cat", "cataract", or "caterpillar". Without the
- parentheses, it would match "cataract", "erpillar" or an empty string.
- <br>
- <br>
- 2. It sets up the subpattern as a capturing subpattern. This means that, when
- the whole pattern matches, that portion of the subject string that matched the
- subpattern is passed back to the caller via the <i>ovector</i> argument of
- <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
- from 1) to obtain numbers for the capturing subpatterns.
- </P>
- <P>
- For example, if the string "the red king" is matched against the pattern
- <pre>
- the ((red|white) (king|queen))
- </pre>
- the captured substrings are "red king", "red", and "king", and are numbered 1,
- 2, and 3, respectively.
- </P>
- <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
- <pre>
- the ((?:red|white) (king|queen))
- </pre>
- the captured substrings are "white queen" and "queen", and are numbered 1 and
- 2. The maximum number of capturing subpatterns is 65535.
- </P>
- <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
- <pre>
- (?i:saturday|sunday)
- (?:(?i)saturday|sunday)
- </pre>
- 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".
- </P>
- <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
- <P>
- Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
- the same numbers for its capturing parentheses. Such a subpattern starts with
- (?| and is itself a non-capturing subpattern. For example, consider this
- pattern:
- <pre>
- (?|(Sat)ur|(Sun))day
- </pre>
- Because the two alternatives are inside a (?| group, both sets of capturing
- parentheses are numbered one. Thus, when the pattern matches, you can look
- at captured substring number one, whichever alternative matched. This construct
- is useful when you want to capture part, but not all, of one of a number of
- alternatives. Inside a (?| group, parentheses are numbered as usual, but the
- number is reset at the start of each branch. The numbers of any capturing
- buffers that follow the subpattern start after the highest number used in any
- branch. The following example is taken from the Perl documentation.
- The numbers underneath show in which buffer the captured content will be
- stored.
- <pre>
- # before ---------------branch-reset----------- after
- / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
- # 1 2 2 3 2 3 4
- </pre>
- A backreference or a recursive call to a numbered subpattern always refers to
- the first one in the pattern with the given number.
- </P>
- <P>
- An alternative approach to using this "branch reset" feature is to use
- duplicate named subpatterns, as described in the next section.
- </P>
- <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
- <P>
- Identifying capturing parentheses by number is simple, but it can be very hard
- to keep track of the numbers in complicated regular expressions. Furthermore,
- if an expression is modified, the numbers may change. To help with this
- difficulty, PCRE supports the naming of subpatterns. This feature was not
- added to Perl until release 5.10. Python had the feature earlier, and PCRE
- introduced it at release 4.0, using the Python syntax. PCRE now supports both
- the Perl and the Python syntax.
- </P>
- <P>
- In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
- (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
- parentheses from other parts of the pattern, such as
- <a href="#backreferences">backreferences,</a>
- <a href="#recursion">recursion,</a>
- and
- <a href="#conditions">conditions,</a>
- can be made by name as well as by number.
- </P>
- <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>
- <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:
- <pre>
- (?<DN>Mon|Fri|Sun)(?:day)?|
- (?<DN>Tue)(?:sday)?|
- (?<DN>Wed)(?:nesday)?|
- (?<DN>Thu)(?:rsday)?|
- (?<DN>Sat)(?:urday)?
- </pre>
- There are five capturing substrings, but only one is ever set after a match.
- (An alternative way of solving this problem is to use a "branch reset"
- subpattern, as described in the previous section.)
- </P>
- <P>
- 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
- <a href="pcreapi.html"><b>pcreapi</b></a>
- documentation.
- </P>
- <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
- <P>
- Repetition is specified by quantifiers, which can follow any of the following
- items:
- <pre>
- a literal data character
- the dot metacharacter
- the \C escape sequence
- the \X escape sequence (in UTF-8 mode with Unicode properties)
- the \R escape sequence
- an escape such as \d that matches a single character
- a character class
- a back reference (see next section)
- a parenthesized subpattern (unless it is an assertion)
- </pre>
- 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:
- <pre>
- z{2,4}
- </pre>
- 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
- <pre>
- [aeiou]{3,}
- </pre>
- matches at least 3 successive vowels, but may match many more, while
- <pre>
- \d{8}
- </pre>
- 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>
- <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
- which is represented by a two-byte sequence. Similarly, when Unicode property
- support is available, \X{3} matches three Unicode extended sequences, each of
- which may be several bytes long (and they may be of different lengths).
- </P>
- <P>
- The quantifier {0} is permitted, causing the expression to behave as if the
- previous item and the quantifier were not present. This may be useful for
- subpatterns that are referenced as
- <a href="#subpatternsassubroutines">subroutines</a>
- from elsewhere in the pattern. Items other than subpatterns that have a {0}
- quantifier are omitted from the compiled pattern.
- </P>
- <P>
- For convenience, the three most common quantifiers have single-character
- abbreviations:
- <pre>
- * is equivalent to {0,}
- + is equivalent to {1,}
- ? is equivalent to {0,1}
- </pre>
- 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:
- <pre>
- (a?)*
- </pre>
- 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>
- <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 /* and */
- and within the comment, individual * and / characters may appear. An attempt to
- match C comments by applying the pattern
- <pre>
- /\*.*\*/
- </pre>
- to the string
- <pre>
- /* first comment */ not comment /* second comment */
- </pre>
- fails, because it matches the entire string owing to the greediness of the .*
- item.
- </P>
- <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
- <pre>
- /\*.*?\*/
- </pre>
- 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
- <pre>
- \d??\d
- </pre>
- which matches one digit by preference, but can match two if that is the only
- way the rest of the pattern matches.
- </P>
- <P>
- 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>
- <P>
- When a parenthesized subpattern is quantified with a minimum repeat count that
- is greater than 1 or with a limited maximum, more memory is required for the
- compiled pattern, in proportion to the size of the minimum or maximum.
- </P>
- <P>
- If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
- to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
- implicitly anchored, because whatever follows will be tried against every
- character position in the subject string, so there is no point in retrying the
- overall match at any position after the first. PCRE normally treats such a
- pattern as though it were preceded by \A.
- </P>
- <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>
- <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 where a later one
- succeeds. Consider, for example:
- <pre>
- (.*)abc\1
- </pre>
- If the subject is "xyz123abc123" the match point is the fourth character. For
- this reason, such a pattern is not implicitly anchored.
- </P>
- <P>
- When a capturing subpattern is repeated, the value captured is the substring
- that matched the final iteration. For example, after
- <pre>
- (tweedle[dume]{3}\s*)+
- </pre>
- 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
- <pre>
- /(a|(b))+/
- </pre>
- matches "aba" the value of the second captured substring is "b".
- <a name="atomicgroup"></a></P>
- <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
- <P>
- With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
- repetition, failure of what follows normally causes the repeated item to be
- re-evaluated to see if a different number of repeats allows the rest of the
- pattern to match. Sometimes it is useful to prevent this, either to change the
- nature of the match, or to cause it fail earlier than it otherwise might, when
- the author of the pattern knows there is no point in carrying on.
- </P>
- <P>
- Consider, for example, the pattern \d+foo when applied to the subject line
- <pre>
- 123456bar
- </pre>
- After matching all 6 digits and then failing to match "foo", the normal
- action of the matcher is to try again with only 5 digits matching the \d+
- item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
- (a term taken from Jeffrey Friedl's book) provides the means for specifying
- that once a subpattern has matched, it is not to be re-evaluated in this way.
- </P>
- <P>
- 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:
- <pre>
- (?>\d+)foo
- </pre>
- 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>
- <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>
- <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
- number of digits they match in order to make the rest of the pattern match,
- (?>\d+) can only match an entire sequence of digits.
- </P>
- <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
- <pre>
- \d++foo
- </pre>
- Note that a possessive quantifier can be used with an entire group, for
- example:
- <pre>
- (abc|xyz){2,3}+
- </pre>
- Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
- option is ignored. They are a convenient notation for the simpler forms of
- atomic group. However, there is no difference in the meaning of a possessive
- quantifier and the equivalent atomic group, though there may be a performance
- difference; possessive quantifiers should be slightly faster.
- </P>
- <P>
- 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>
- <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>
- <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
- <pre>
- (\D+|<\d+>)*[!?]
- </pre>
- 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
- <pre>
- aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
- </pre>
- it takes a long time before reporting failure. This is because the string can
- be divided between the internal \D+ repeat and the external * repeat in a
- large number of ways, and all have to be tried. (The example uses [!?] rather
- than a single character at the end, because both PCRE and Perl have an
- optimization that allows for fast failure when a single character is used. They
- remember the last single character that is required for a match, and fail early
- if it is not present in the string.) If the pattern is changed so that it uses
- an atomic group, like this:
- <pre>
- ((?>\D+)|<\d+>)*[!?]
- </pre>
- sequences of non-digits cannot be broken, and failure happens quickly.
- <a name="backreferences"></a></P>
- <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
- <P>
- 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>
- <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. A "forward back reference" of this type can make sense
- when a repetition is involved and the subpattern to the right has participated
- in an earlier iteration.
- </P>
- <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 \50 is
- interpreted as a character defined in octal. See the subsection entitled
- "Non-printing characters"
- <a href="#digitsafterbackslash">above</a>
- 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>
- <P>
- Another way of avoiding the ambiguity inherent in the use of digits following a
- backslash is to use the \g escape sequence, which is a feature introduced in
- Perl 5.10. This escape must be followed by an unsigned number or a negative
- number, optionally enclosed in braces. These examples are all identical:
- <pre>
- (ring), \1
- (ring), \g1
- (ring), \g{1}
- </pre>
- An unsigned 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:
- <pre>
- (abc(def)ghi)\g{-1}
- </pre>
- The sequence \g{-1} is a reference to the most recently started capturing
- subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
- would be equivalent to \1. The use of relative references can be helpful in
- long patterns, and also in patterns that are created by joining together
- fragments that contain references within themselves.
- </P>
- <P>
- A back reference matches whatever actually matched the capturing subpattern in
- the current subject string, rather than anything matching the subpattern
- itself (see
- <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
- below for a way of doing that). So the pattern
- <pre>
- (sens|respons)e and \1ibility
- </pre>
- 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,
- <pre>
- ((?i)rah)\s+\1
- </pre>
- matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
- capturing subpattern is matched caselessly.
- </P>
- <P>
- There are several different ways of writing back references to named
- subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
- \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
- back reference syntax, in which \g can be used for both numeric and named
- references, is also supported. We could rewrite the above example in any of
- the following ways:
- <pre>
- (?<p1>(?i)rah)\s+\k<p1>
- (?'p1'(?i)rah)\s+\k{p1}
- (?P<p1>(?i)rah)\s+(?P=p1)
- (?<p1>(?i)rah)\s+\g{p1}
- </pre>
- A subpattern that is referenced by name may appear in the pattern before or
- after the reference.
- </P>
- <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
- <pre>
- (a|(bc))\2
- </pre>
- always fails if it starts to match "a" rather than "bc". Because there may be
- many capturing parentheses in a pattern, all digits following the backslash are
- taken as part of a potential back reference number. If the pattern continues
- with a digit character, some delimiter must be used to terminate the back
- reference. If the PCRE_EXTENDED option is set, this can be whitespace.
- Otherwise an empty comment (see
- <a href="#comments">"Comments"</a>
- below) can be used.
- </P>
- <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.
- However, such references can be useful inside repeated subpatterns. For
- example, the pattern
- <pre>
- (a|b\1)+
- </pre>
- 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.
- <a name="bigassertions"></a></P>
- <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
- <P>
- 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
- <a href="#smallassertions">above.</a>
- </P>
- <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. An assertion subpattern is matched in the normal way,
- except that it does not cause the current matching position to be changed.
- </P>
- <P>
- Assertion subpatterns are not capturing subpatterns, and may not be repeated,
- because it makes no sense to assert the same thing several times. If any kind
- of assertion contains capturing subpatterns within it, these are counted for
- the purposes of numbering the capturing subpatterns in the whole pattern.
- However, substring capturing is carried out only for positive assertions,
- because it does not make sense for negative assertions.
- </P>
- <br><b>
- Lookahead assertions
- </b><br>
- <P>
- Lookahead assertions start with (?= for positive assertions and (?! for
- negative assertions. For example,
- <pre>
- \w+(?=;)
- </pre>
- matches a word followed by a semicolon, but does not include the semicolon in
- the match, and
- <pre>
- foo(?!bar)
- </pre>
- matches any occurrence of "foo" that is not followed by "bar". Note that the
- apparently similar pattern
- <pre>
- (?!foo)bar
- </pre>
- does not find an occurrence of "bar" that is preceded by something other than
- "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
- (?!foo) is always true when the next three characters are "bar". A
- lookbehind assertion is needed to achieve the other effect.
- </P>
- <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.
- <a name="lookbehind"></a></P>
- <br><b>
- Lookbehind assertions
- </b><br>
- <P>
- Lookbehind assertions start with (?<= for positive assertions and (?<! for
- negative assertions. For example,
- <pre>
- (?<!foo)bar
- </pre>
- 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 top-level alternatives, they
- do not all have to have the same fixed length. Thus
- <pre>
- (?<=bullock|donkey)
- </pre>
- is permitted, but
- <pre>
- (?<!dogs?|cats?)
- </pre>
- 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
- <pre>
- (?<=ab(c|de))
- </pre>
- 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:
- <pre>
- (?<=abc|abde)
- </pre>
- In some cases, the Perl 5.10 escape sequence \K
- <a href="#resetmatchstart">(see above)</a>
- can be used instead of a lookbehind assertion; this is not restricted to a
- fixed-length.
- </P>
- <P>
- The implementation of lookbehind assertions is, for each alternative, to
- temporarily move the current position back by the fixed length and then try to
- match. If there are insufficient characters before the current position, the
- assertion fails.
- </P>
- <P>
- PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
- to appear in lookbehind assertions, because it makes it impossible to calculate
- the length of the lookbehind. The \X and \R escapes, which can match
- different numbers of bytes, are also not permitted.
- </P>
- <P>
- Possessive quantifiers can be used in conjunction with lookbehind assertions to
- specify efficient matching at the end of the subject string. Consider a simple
- pattern such as
- <pre>
- abcd$
- </pre>
- 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
- <pre>
- ^.*abcd$
- </pre>
- 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
- <pre>
- ^.*+(?<=abcd)
- </pre>
- 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.
- </P>
- <br><b>
- Using multiple assertions
- </b><br>
- <P>
- Several assertions (of any sort) may occur in succession. For example,
- <pre>
- (?<=\d{3})(?<!999)foo
- </pre>
- 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 <i>not</i> 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
- <pre>
- (?<=\d{3}...)(?<!999)foo
- </pre>
- 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>
- <P>
- Assertions can be nested in any combination. For example,
- <pre>
- (?<=(?<!foo)bar)baz
- </pre>
- matches an occurrence of "baz" that is preceded by "bar" which in turn is not
- preceded by "foo", while
- <pre>
- (?<=\d{3}(?!999)...)foo
- </pre>
- is another pattern that matches "foo" preceded by three digits and any three
- characters that are not "999".
- <a name="conditions"></a></P>
- <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
- <P>
- 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
- <pre>
- (?(condition)yes-pattern)
- (?(condition)yes-pattern|no-pattern)
- </pre>
- 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>
- <P>
- There are four kinds of condition: references to subpatterns, references to
- recursion, a pseudo-condition called DEFINE, and assertions.
- </P>
- <br><b>
- Checking for a used subpattern by number
- </b><br>
- <P>
- If the text between the parentheses consists of a sequence of digits, the
- condition is true if the capturing subpattern of that number has previously
- matched. An alternative notation is to precede the digits with a plus or minus
- sign. In this case, the subpattern number is relative rather than absolute.
- The most recently opened parentheses can be referenced by (?(-1), the next most
- recent by (?(-2), and so on. In looping constructs it can also make sense to
- refer to subsequent groups with constructs such as (?(+2).
- </P>
- <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:
- <pre>
- ( \( )? [^()]+ (?(1) \) )
- </pre>
- 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
- conditional subpattern that tests whether the first set of parentheses matched
- or not. If they did, that is, if subject started with an opening parenthesis,
- the condition is true, and so the yes-pattern is executed and a closing
- parenthesis is required. Otherwise, since no-pattern is not present, the
- subpattern matches nothing. In other words, this pattern matches a sequence of
- non-parentheses, optionally enclosed in parentheses.
- </P>
- <P>
- If you were embedding this pattern in a larger one, you could use a relative
- reference:
- <pre>
- ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
- </pre>
- This makes the fragment independent of the parentheses in the larger pattern.
- </P>
- <br><b>
- Checking for a used subpattern by name
- </b><br>
- <P>
- Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
- subpattern by name. For compatibility with earlier versions of PCRE, which had
- this facility before Perl, the syntax (?(name)...) is also recognized. However,
- there is a possible ambiguity with this syntax, because subpattern names may
- consist entirely of digits. PCRE looks first for a named subpattern; if it
- cannot find one and the name consists entirely of digits, PCRE looks for a
- subpattern of that number, which must be greater than zero. Using subpattern
- names that consist entirely of digits is not recommended.
- </P>
- <P>
- Rewriting the above example to use a named subpattern gives this:
- <pre>
- (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
- </PRE>
- </P>
- <br><b>
- Checking for pattern recursion
- </b><br>
- <P>
- 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:
- <pre>
- (?(R3)...) or (?(R&name)...)
- </pre>
- 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>
- <P>
- At "top level", all these recursion test conditions are false. Recursive
- patterns are described below.
- </P>
- <br><b>
- Defining subpatterns for use by reference only
- </b><br>
- <P>
- 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):
- <pre>
- (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
- \b (?&byte) (\.(?&byte)){3} \b
- </pre>
- 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>
- <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.
- </P>
- <br><b>
- Assertion conditions
- </b><br>
- <P>
- 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:
- <pre>
- (?(?=[^a-z]*[a-z])
- \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
- </pre>
- 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.
- <a name="comments"></a></P>
- <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
- <P>
- 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>
- <P>
- If the PCRE_EXTENDED option is set, an unescaped # character outside a
- character class introduces a comment that continues to immediately after the
- next newline in the pattern.
- <a name="recursion"></a></P>
- <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
- <P>
- 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.
- </P>
- <P>
- For some time, Perl has provided a facility that allows regular expressions to
- recurse (amongst other things). It does this by interpolating Perl code in the
- expression at run time, and the code can refer to the expression itself. A Perl
- pattern using code interpolation to solve the parentheses problem can be
- created like this:
- <pre>
- $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
- </pre>
- The (?p{...}) item interpolates Perl code at run time, and in this case refers
- recursively to the pattern in which it appears.
- </P>
- <P>
- Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
- supports special syntax for recursion of the entire pattern, and also for
- individual subpattern recursion. After its introduction in PCRE and Python,
- this kind of recursion was introduced into Perl at release 5.10.
- </P>
- <P>
- A special item that consists of (? followed by a number greater than zero and a
- closing parenthesis is a recursive call of the subpattern of the given number,
- provided that it occurs inside that subpattern. (If not, it is a "subroutine"
- call, which is described in the next section.) The special item (?R) or (?0) is
- a recursive call of the entire regular expression.
- </P>
- <P>
- 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>
- <P>
- This PCRE pattern solves the nested parentheses problem (assume the
- PCRE_EXTENDED option is set so that white space is ignored):
- <pre>
- \( ( (?>[^()]+) | (?R) )* \)
- </pre>
- 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).
- Finally there is a closing parenthesis.
- </P>
- <P>
- If this were part of a larger pattern, you would not want to recurse the entire
- pattern, so instead you could use this:
- <pre>
- ( \( ( (?>[^()]+) | (?1) )* \) )
- </pre>
- We have put the pattern into parentheses, and caused the recursion to refer to
- them instead of the whole pattern.
- </P>
- <P>
- In a larger pattern, keeping track of parenthesis numbers can be tricky. This
- is made easier by the use of relative references. (A Perl 5.10 feature.)
- Instead of (?1) in the pattern above you can write (?-2) to refer to the second
- most recently opened parentheses preceding the recursion. In other words, a
- negative number counts capturing parentheses leftwards from the point at which
- it is encountered.
- </P>
- <P>
- It is also possible to refer to subsequently opened parentheses, by writing
- references such as (?+2). However, these cannot be recursive because the
- reference is not inside the parentheses that are referenced. They are always
- "subroutine" calls, as described in the next section.
- </P>
- <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:
- <pre>
- (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
- </pre>
- If there is more than one subpattern with the same name, the earliest one is
- used.
- </P>
- <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
- <pre>
- (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
- </pre>
- 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>
- <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
- <a href="pcrecallout.html"><b>pcrecallout</b></a>
- documentation). If the pattern above is matched against
- <pre>
- (ab(cd)ef)
- </pre>
- 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
- <pre>
- \( ( ( (?>[^()]+) | (?R) )* ) \)
- ^ ^
- ^ ^
- </pre>
- 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 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
- memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
- </P>
- <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.
- <pre>
- < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
- </pre>
- 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.
- <a name="subpatternsassubroutines"></a></P>
- <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
- <P>
- If the syntax for a recursive subpattern reference (either by number or by
- name) is used outside the parentheses to which it refers, it operates like a
- subroutine in a programming language. The "called" subpattern may be defined
- before or after the reference. A numbered reference can be absolute or
- relative, as in these examples:
- <pre>
- (...(absolute)...)...(?2)...
- (...(relative)...)...(?-1)...
- (...(?+1)...(relative)...
- </pre>
- An earlier example pointed out that the pattern
- <pre>
- (sens|respons)e and \1ibility
- </pre>
- matches "sense and sensibility" and "response and responsibility", but not
- "sense and responsibility". If instead the pattern
- <pre>
- (sens|respons)e and (?1)ibility
- </pre>
- is used, it does match "sense and responsibility" as well as the other two
- strings. Another example is given in the discussion of DEFINE above.
- </P>
- <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>
- <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:
- <pre>
- (abc)(?i:(?-1))
- </pre>
- It matches "abcabc". It does not match "abcABC" because the change of
- processing option does not affect the called subpattern.
- <a name="onigurumasubroutines"></a></P>
- <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
- <P>
- For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
- a number enclosed either in angle brackets or single quotes, is an alternative
- syntax for referencing a subpattern as a subroutine, possibly recursively. Here
- are two of the examples used above, rewritten using this syntax:
- <pre>
- (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
- (sens|respons)e and \g'1'ibility
- </pre>
- PCRE supports an extension to Oniguruma: if a number is preceded by a
- plus or a minus sign it is taken as a relative reference. For example:
- <pre>
- (abc)(?i:\g<-1>)
- </pre>
- Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
- synonymous. The former is a back reference; the latter is a subroutine call.
- </P>
- <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
- <P>
- Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
- code to be obeyed in the middle of matching a regular expression. This makes it
- possible, amongst other things, to extract different substrings that match the
- same pair of parentheses when there is a repetition.
- </P>
- <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 <i>pcre_callout</i>.
- By default, this variable contains NULL, which disables all calling out.
- </P>
- <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:
- <pre>
- (?C1)abc(?C2)def
- </pre>
- If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
- automatically installed before each item in the pattern. They are all numbered
- 255.
- </P>
- <P>
- During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
- set), the external function is called. It is provided with the number of the
- callout, the position in the pattern, and, optionally, one item of data
- originally supplied by the caller of <b>pcre_exec()</b>. The callout function
- may cause matching to proceed, to backtrack, or to fail altogether. A complete
- description of the interface to the callout function is given in the
- <a href="pcrecallout.html"><b>pcrecallout</b></a>
- documentation.
- </P>
- <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
- <P>
- Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
- are described in the Perl documentation as "experimental and subject to change
- or removal in a future version of Perl". It goes on to say: "Their usage in
- production code should be noted to avoid problems during upgrades." The same
- remarks apply to the PCRE features described in this section.
- </P>
- <P>
- Since these verbs are specifically related to backtracking, most of them can be
- used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
- a backtracking algorithm. With the exception of (*FAIL), which behaves like a
- failing negative assertion, they cause an error if encountered by
- <b>pcre_dfa_exec()</b>.
- </P>
- <P>
- The new verbs make use of what was previously invalid syntax: an opening
- parenthesis followed by an asterisk. In Perl, they are generally of the form
- (*VERB:ARG) but PCRE does not support the use of arguments, so its general
- form is just (*VERB). Any number of these verbs may occur in a pattern. There
- are two kinds:
- </P>
- <br><b>
- Verbs that act immediately
- </b><br>
- <P>
- The following verbs act as soon as they are encountered:
- <pre>
- (*ACCEPT)
- </pre>
- This verb causes the match to end successfully, skipping the remainder of the
- pattern. When inside a recursion, only the innermost pattern is ended
- immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
- capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
- captured. For example:
- <pre>
- A(A|B(*ACCEPT)|C)D
- </pre>
- This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
- captured.
- <pre>
- (*FAIL) or (*F)
- </pre>
- This verb causes the match to fail, forcing backtracking to occur. It is
- equivalent to (?!) but easier to read. The Perl documentation notes that it is
- probably useful only when combined with (?{}) or (??{}). Those are, of course,
- Perl features that are not present in PCRE. The nearest equivalent is the
- callout feature, as for example in this pattern:
- <pre>
- a+(?C)(*FAIL)
- </pre>
- A match with the string "aaaa" always fails, but the callout is taken before
- each backtrack happens (in this example, 10 times).
- </P>
- <br><b>
- Verbs that act after backtracking
- </b><br>
- <P>
- The following verbs do nothing when they are encountered. Matching continues
- with what follows, but if there is no subsequent match, a failure is forced.
- The verbs differ in exactly what kind of failure occurs.
- <pre>
- (*COMMIT)
- </pre>
- This verb causes the whole match to fail outright if the rest of the pattern
- does not match. Even if the pattern is unanchored, no further attempts to find
- a match by advancing the start point take place. Once (*COMMIT) has been
- passed, <b>pcre_exec()</b> is committed to finding a match at the current
- starting point, or not at all. For example:
- <pre>
- a+(*COMMIT)b
- </pre>
- This matches "xxaab" but not "aacaab". It can be thought of as a kind of
- dynamic anchor, or "I've started, so I must finish."
- <pre>
- (*PRUNE)
- </pre>
- This verb causes the match to fail at the current position if the rest of the
- pattern does not match. If the pattern is unanchored, the normal "bumpalong"
- advance to the next starting character then happens. Backtracking can occur as
- usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
- if there is no match to the right, backtracking cannot cross (*PRUNE).
- In simple cases, the use of (*PRUNE) is just an alternative to an atomic
- group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
- be expressed in any other way.
- <pre>
- (*SKIP)
- </pre>
- This verb is like (*PRUNE), except that if the pattern is unanchored, the
- "bumpalong" advance is not to the next character, but to the position in the
- subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
- was matched leading up to it cannot be part of a successful match. Consider:
- <pre>
- a+(*SKIP)b
- </pre>
- If the subject is "aaaac...", after the first match attempt fails (starting at
- the first character in the string), the starting point skips on to start the
- next attempt at "c". Note that a possessive quantifer does not have the same
- effect in this example; although it would suppress backtracking during the
- first match attempt, the second attempt would start at the second character
- instead of skipping on to "c".
- <pre>
- (*THEN)
- </pre>
- This verb causes a skip to the next alternation if the rest of the pattern does
- not match. That is, it cancels pending backtracking, but only within the
- current alternation. Its name comes from the observation that it can be used
- for a pattern-based if-then-else block:
- <pre>
- ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
- </pre>
- If the COND1 pattern matches, FOO is tried (and possibly further items after
- the end of the group if FOO succeeds); on failure the matcher skips to the
- second alternative and tries COND2, without backtracking into COND1. If (*THEN)
- is used outside of any alternation, it acts exactly like (*PRUNE).
- </P>
- <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
- <P>
- <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
- </P>
- <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
- <P>
- Philip Hazel
- <br>
- University Computing Service
- <br>
- Cambridge CB2 3QH, England.
- <br>
- </P>
- <br><a name="SEC28" href="#TOC1">REVISION</a><br>
- <P>
- Last updated: 19 April 2008
- <br>
- Copyright © 1997-2008 University of Cambridge.
- <br>
- <p>
- Return to the <a href="index.html">PCRE index page</a>.
- </p>