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<head>
<title>pcrepattern specification</title>
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<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&#60;...&#62; (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>
  (?&#62;\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>
  (?&#62;\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: (?&#60;name&#62;...) or
(?'name'...) as in Perl, or (?P&#60;name&#62;...) 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>
  (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
  (?&#60;DN&#62;Tue)(?:sday)?|
  (?&#60;DN&#62;Wed)(?:nesday)?|
  (?&#60;DN&#62;Thu)(?:rsday)?|
  (?&#60;DN&#62;Sat)(?:urday)?
</pre>
There are five capturing substrings, but only one is…