/lib/win32/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,
- PCRE must be built to include UTF-8 support, and you must call
- <b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There
- is also a special sequence that can be given at the start of a pattern:
- <pre>
- (*UTF8)
- </pre>
- Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
- option. This feature is not Perl-compatible. How setting UTF-8 mode 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> or
- <b>pcre_compile2()</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 often 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
- <a href="#subpatternsassubroutines">subroutine</a>
- 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. Note that this also affects \b, because it is defined in terms of \w
- and \W.
- </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> or
- <b>pcre_compile2()</b>, but they can be overridden by options given to
- <b>pcre_exec()</b> or <b>pcre_dfa_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). Perl does not support the Cs property.
- </P>
- <P>
- The long synonyms for property names 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. Neither PCRE nor Perl has a
- separte "start of word" or "end of word" metasequence. However, whatever
- follows \b normally determines which it is. For example, the fragment
- \ba matches "a" at the start of a word.
- </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 by default.
- However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
- bracket causes a compile-time error. 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 be more than one byte long. 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 in UTF8-mode 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 one of these option changes occurs at top level (that is, not inside
- subpattern parentheses), the change applies to the remainder of the pattern
- that follows. If the change is placed right at the start of a pattern, PCRE
- extracts it into the global options (and it will therefore show up in data
- extracted by the <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 such as (*CRLF) 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. There is also the (*UTF8) leading sequence that can be used to set UTF-8
- mode; this is equivalent to setting the PCRE_UTF8 option.
- <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".
- <a name="dupsubpatternnumber"></a></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 to a numbered subpattern uses the most recent value that is set
- for that number by any subpattern. The following pattern matches "abcabc" or
- "defdef":
- <pre>
- /(?|(abc)|(def))\1/
- </pre>
- In contrast, a recursive or "subroutine" call to a numbered subpattern always
- refers to the first one in the pattern with the given number. The following
- pattern matches "abcabc" or "defabc":
- <pre>
- /(?|(abc)|(def))(?1)/
- </pre>
- If a
- <a href="#conditions">condition test</a>
- for a subpattern's having matched refers to a non-unique number, the test is
- true if any of the subpatterns of that number have matched.
- </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 suppo