pattern.syntax.xml

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<chapter xml:id="reference.pcre.pattern.syntax" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>Pattern Syntax</title>
 <titleabbrev>PCRE regex syntax</titleabbrev>

 <section xml:id="regexp.introduction">
  <title>Introduction</title>
  <para>
   The syntax and semantics of  the  regular  expressions
   supported  by PCRE are described below. Regular expressions are
   also described in the Perl documentation and in a number  of
   other  books,  some  of which have copious examples. Jeffrey
   Friedl's  "Mastering  Regular  Expressions",  published   by
   O'Reilly  (ISBN 1-56592-257-3), covers them in great detail.
   The description here is intended as reference documentation.
  </para>
  <para>
   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
   <literal>The quick brown fox</literal>
   matches a portion of a subject string that is  identical  to
   itself.
  </para>
 </section>
 <section xml:id="regexp.reference.delimiters">
  <title>Delimiters</title>
  <para>
   When using the PCRE functions, it is required that the pattern is enclosed
   by <emphasis>delimiters</emphasis>. A delimiter can be any non-alphanumeric,
   non-backslash, non-whitespace character.
  </para>
  <para>
   Often used delimiters are forward slashes (<literal>/</literal>), hash
   signs (<literal>#</literal>) and tildes (<literal>~</literal>). The
   following are all examples of valid delimited patterns.
   <informalexample>
    <programlisting>
<![CDATA[
/foo bar/
#^[^0-9]$#
+php+
%[a-zA-Z0-9_-]%
]]>
    </programlisting>
   </informalexample>
  </para>
  <para>
   It is also possible to use
   bracket style delimiters where the opening and closing brackets are the
   starting and ending delimiter, respectively. <literal>()</literal>,
   <literal>{}</literal>, <literal>[]</literal> and <literal>&lt;&gt;</literal>
   are all valid bracket style delimiter pairs.
   <informalexample>
    <programlisting>
<![CDATA[
(this [is] a (pattern))
{this [is] a (pattern)}
[this [is] a (pattern)]
<this [is] a (pattern)>
]]>
    </programlisting>
   </informalexample>
   Bracket style delimiters do not need to be escaped when they are used as meta
   characters within the pattern, but as with other delimiters they must be
   escaped when they are used as literal characters.
  </para>
  <para>
   If the delimiter needs to be matched inside the pattern it must be
   escaped using a backslash. If the delimiter appears often inside the
   pattern, it is a good idea to choose another delimiter in order to increase
   readability.
   <informalexample>
    <programlisting>
<![CDATA[
/http:\/\//
#http://#
]]>
    </programlisting>
   </informalexample>
   The <function>preg_quote</function> function may be used to escape a string
   for injection into a pattern and its optional second parameter may be used
   to specify the delimiter to be escaped.
  </para>
  <para>
   You may add <link linkend="reference.pcre.pattern.modifiers">pattern
   modifiers</link> after the ending delimiter. The following is an example
   of case-insensitive matching:
   <informalexample>
    <programlisting>
<![CDATA[
#[a-z]#i
]]>
    </programlisting>
   </informalexample>
  </para>
 </section>
 <section xml:id="regexp.reference.meta">
  <title>Meta-characters</title>
  <para>
   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
   <emphasis>meta-characters</emphasis>, which do not stand for  themselves  but  instead
   are interpreted in some special way.
  </para>
  <para>
   There are two different sets of meta-characters: those  that
   are  recognized anywhere in the pattern except within square
   brackets, and those that are recognized in square brackets.
   Outside square brackets, the meta-characters are as follows:

   <table>
     <title>Meta-characters outside square brackets</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Meta-character</entry><entry>Description</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>\</entry><entry>general escape character with several uses</entry>
      </row>
      <row>
       <entry>^</entry><entry>assert start of subject (or line, in multiline mode)</entry>
      </row>
      <row>
       <entry>$</entry><entry>assert end of subject or before a terminating newline (or end of line, in multiline mode)</entry>
      </row>
      <row>
       <entry>.</entry><entry>match any character except newline (by default)</entry>
      </row>
      <row>
       <entry>[</entry><entry>start character class definition</entry>
      </row>
      <row>
       <entry>]</entry><entry>end character class definition</entry>
      </row>
      <row>
       <entry>|</entry><entry>start of alternative branch</entry>
      </row>
      <row>
       <entry>(</entry><entry>start subpattern</entry>
      </row>
      <row>
       <entry>)</entry><entry>end subpattern</entry>
      </row>
      <row>
       <entry>?</entry><entry>extends the meaning of (, also 0 or 1 quantifier, also makes greedy
        quantifiers lazy (see <link linkend="regexp.reference.repetition">repetition</link>)</entry>
      </row>
      <row>
       <entry>*</entry><entry>0 or more quantifier</entry>
      </row>
      <row>
       <entry>+</entry><entry>1 or more quantifier</entry>
      </row>
      <row>
       <entry>{</entry><entry>start min/max quantifier</entry>
      </row>
      <row>
       <entry>}</entry><entry>end min/max quantifier</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   Part of a pattern that is in square brackets is called a
   <link linkend="regexp.reference.character-classes">character class</link>. In a character class the only
   meta-characters are:

   <table>
     <title>Meta-characters inside square brackets (<emphasis>character classes</emphasis>)</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Meta-character</entry><entry>Description</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>\</entry><entry>general escape character</entry>
      </row>
      <row>
       <entry>^</entry><entry>negate the class, but only if the first character</entry>
      </row>
      <row>
       <entry>-</entry><entry>indicates character range</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   The following sections describe the use of each of the
   meta-characters.
  </para>
 </section>

 <section xml:id="regexp.reference.escape">
  <title>Escape sequences</title>
  <para>
   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.
  </para>
  <para>
   For example, if you want to match a "*" character, you write
   "\*" in the pattern. This applies whether or not the
   following character would otherwise be interpreted as a
   meta-character, so it is always safe to precede a non-alphanumeric
   with "\" to specify that it stands for itself.  In
   particular, if you want to match a backslash, you write "\\".
  </para>
  <note>
   <para>
    Single and double quoted PHP <link
    linkend="language.types.string.syntax">strings</link> have special
    meaning of backslash. Thus if \ has to be matched with a regular
    expression \\, then "\\\\" or '\\\\' must be used in PHP code.
   </para>
  </note>
  <para>
   If a pattern is compiled with the
   <link linkend="reference.pcre.pattern.modifiers">PCRE_EXTENDED</link> option,
   whitespace in the pattern (other than in a character class) and
   characters between a "#" outside a character class and the next newline
   character are ignored. An escaping backslash can be used to include a
   whitespace or "#" character as part of the pattern.
  </para>
  <para>
   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:
  </para>
  <para>
   <variablelist>
    <varlistentry>
     <term><emphasis>\a</emphasis></term>
     <listitem>
      <simpara>alarm, that is, the BEL character (hex 07)</simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\cx</emphasis></term>
     <listitem>
      <simpara>"control-x", where x is any character</simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\e</emphasis></term>
     <listitem>
      <simpara>escape (hex 1B)</simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\f</emphasis></term>
     <listitem>
      <simpara>formfeed (hex 0C)</simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\n</emphasis></term>
     <listitem>
      <simpara>newline (hex 0A)</simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\p{xx}</emphasis></term>
     <listitem>
      <simpara>
       a character with the xx property, see
       <link linkend="regexp.reference.unicode">unicode properties</link>
       for more info
      </simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\P{xx}</emphasis></term>
     <listitem>
      <simpara>
       a character without the xx property, see
       <link linkend="regexp.reference.unicode">unicode properties</link>
       for more info
      </simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\r</emphasis></term>
     <listitem>
      <simpara>carriage return (hex 0D)</simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\R</emphasis></term>
     <listitem>
      <simpara>line break: matches \n, \r and \r\n</simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\t</emphasis></term>
     <listitem>
      <simpara>tab (hex 09)</simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\xhh</emphasis></term>
     <listitem>
      <simpara>
       character with hex code hh
      </simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\ddd</emphasis></term>
     <listitem>
      <simpara>character with octal code ddd, or backreference</simpara>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
  <para>
   The precise effect of "<literal>\cx</literal>" is as follows:
   if "<literal>x</literal>" is a lower case  letter, it is converted
   to upper case. Then bit 6 of the character (hex 40) is inverted.
   Thus "<literal>\cz</literal>" becomes  hex 1A, but
   "<literal>\c{</literal>" becomes hex 3B, while "<literal>\c;</literal>"
   becomes hex 7B.
  </para>
  <para>
   After "<literal>\x</literal>", up to two hexadecimal digits are
   read (letters can be in upper or lower case).
   In <emphasis>UTF-8 mode</emphasis>, "<literal>\x{...}</literal>" is
   allowed, where the contents of the braces is a string of hexadecimal
   digits. It is interpreted as a UTF-8 character whose code number is the
   given hexadecimal number. The original hexadecimal escape sequence,
   <literal>\xhh</literal>, matches a two-byte UTF-8 character if the value
   is greater than 127.
  </para>
  <para>
   After "<literal>\0</literal>" up to two further octal digits are read.
   In  both cases,  if  there are fewer than two digits, just those that
   are present are used. Thus the sequence "<literal>\0\x\07</literal>"
   specifies two binary zeros followed by a BEL character. Make sure you
   supply two digits after the initial zero if the character
   that follows is itself an octal digit.
  </para>
  <para>
   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 <emphasis>back reference</emphasis>. A description
   of how this works is given later, following  the  discussion
   of parenthesized subpatterns.
  </para>
  <para>
   Inside a character  class,  or  if  the  decimal  number  is
   greater than 9 and there have not been that many capturing
   subpatterns, PCRE re-reads up to three octal digits following
   the backslash, and generates a single byte from the
   least significant 8 bits of the value. Any subsequent digits
   stand for themselves.  For example:
  </para>
  <para>
   <variablelist>
    <varlistentry>
     <term><emphasis>\040</emphasis></term>
     <listitem><simpara>is another way of writing a space</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\40</emphasis></term>
     <listitem>
      <simpara>
       is the same, provided there are fewer than 40
       previous capturing subpatterns
      </simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\7</emphasis></term>
     <listitem><simpara>is always a back reference</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\11</emphasis></term>
     <listitem>
      <simpara>
       might be a back reference, or another way of
       writing a tab
      </simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\011</emphasis></term>
     <listitem><simpara>is always a tab</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\0113</emphasis></term>
     <listitem><simpara>is a tab followed by the character "3"</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\113</emphasis></term>
     <listitem>
      <simpara>
       is the character with octal code 113 (since there
       can be no more than 99 back references)
      </simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\377</emphasis></term>
     <listitem><simpara>is a byte consisting entirely of 1 bits</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\81</emphasis></term>
     <listitem>
      <simpara>
       is either a back reference, or a binary zero
       followed by the two characters "8" and "1"
      </simpara>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
  <para>
   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.
  </para>
  <para>
   All the sequences that define a single byte value can  be
   used both inside and outside character classes. In addition,
   inside a character class, the sequence "<literal>\b</literal>"
   is interpreted as the backspace character (hex 08). Outside a character
   class it has a different meaning (see below).
  </para>
  <para>
   The third use of backslash is for specifying generic
   character types:
  </para>
  <para>
   <variablelist>
    <varlistentry>
     <term><emphasis>\d</emphasis></term>
     <listitem><simpara>any decimal digit</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\D</emphasis></term>
     <listitem><simpara>any character that is not a decimal digit</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\h</emphasis></term>
     <listitem><simpara>any horizontal whitespace character (since PHP 5.2.4)</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\H</emphasis></term>
     <listitem><simpara>any character that is not a horizontal whitespace character (since PHP 5.2.4)</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\s</emphasis></term>
     <listitem><simpara>any whitespace character</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\S</emphasis></term>
     <listitem><simpara>any character that is not a whitespace character</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\v</emphasis></term>
     <listitem><simpara>any vertical whitespace character (since PHP 5.2.4)</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\V</emphasis></term>
     <listitem><simpara>any character that is not a vertical whitespace character (since PHP 5.2.4)</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\w</emphasis></term>
     <listitem><simpara>any "word" character</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\W</emphasis></term>
     <listitem><simpara>any "non-word" character</simpara></listitem>
    </varlistentry>
   </variablelist>
  </para>
  <para>
   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.
  </para>
  <para>
   The "whitespace" characters are HT (9), LF (10), FF (12), CR (13),
   and space (32). However, if locale-specific matching is happening,
   characters with code points in the range 128-255 may also be considered
   as whitespace characters, for instance, NBSP (A0).
  </para>
  <para>
   A "word" character is any letter or digit or the underscore
   character,  that  is,  any  character which can be part of a
   Perl "<emphasis>word</emphasis>". The definition of letters and digits is
   controlled by PCRE's character tables, and may vary if locale-specific
   matching is taking place. For example, in the "fr" (French) locale, some
   character codes greater than 128 are used for accented letters,
   and these are matched by <literal>\w</literal>.
  </para>
  <para>
   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.
  </para>
  <para>
   The fourth use of backslash is  for  certain  simple
   assertions. An assertion specifies a condition that has to be met
   at a particular point in  a match, without consuming any
   characters from the subject string. The use of subpatterns
   for more complicated assertions is described below. The
   backslashed assertions are
  </para>
  <para>
   <variablelist>
    <varlistentry>
     <term><emphasis>\b</emphasis></term>
     <listitem><simpara>word boundary</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\B</emphasis></term>
     <listitem><simpara>not a word boundary</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\A</emphasis></term>
     <listitem><simpara>start of subject (independent of multiline mode)</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\Z</emphasis></term>
     <listitem>
      <simpara>
       end of subject or newline at end (independent of
       multiline mode)
      </simpara>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\z</emphasis></term>
     <listitem><simpara>end of subject (independent of multiline mode)</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\G</emphasis></term>
     <listitem><simpara>first matching position in subject</simpara></listitem>
    </varlistentry>
   </variablelist>
  </para>
  <para>
   These assertions may not appear in  character  classes  (but
   note that "<literal>\b</literal>" has a different meaning, namely the backspace
   character, inside a character class).
  </para>
  <para>
   A word boundary is a position in the subject string where
   the current character and the previous character do not both
   match <literal>\w</literal> or <literal>\W</literal> (i.e. one matches
   <literal>\w</literal> and  the  other  matches
   <literal>\W</literal>), or the start or end of the string if the first
   or last character matches <literal>\w</literal>, respectively.
  </para>
  <para>
   The <literal>\A</literal>, <literal>\Z</literal>, and
   <literal>\z</literal> assertions differ  from  the  traditional
   circumflex  and  dollar  (described in <link linkend="regexp.reference.anchors">anchors</link> ) in that they only
   ever match at the very start and end of the subject  string,
   whatever  options  are  set.  They  are  not affected by the
   <link linkend="reference.pcre.pattern.modifiers">PCRE_MULTILINE</link> or
   <link linkend="reference.pcre.pattern.modifiers">PCRE_DOLLAR_ENDONLY</link>
   options. The  difference  between <literal>\Z</literal> and
   <literal>\z</literal>  is that <literal>\Z</literal> matches before a
   newline that is the last character of the string as well as at the end of
   the string, whereas <literal>\z</literal> matches only at the end.
  </para>
  <para>
   The <literal>\G</literal> assertion is true only when the current
   matching position is at the start point of the match, as specified by
   the <parameter>offset</parameter> argument of
   <function>preg_match</function>. It differs from <literal>\A</literal>
   when the value of <parameter>offset</parameter> is non-zero.
  </para>

  <para>
   <literal>\Q</literal> and <literal>\E</literal> can be used to ignore
   regexp metacharacters in the pattern. For example:
   <literal>\w+\Q.$.\E$</literal> will match one or more word characters,
   followed by literals <literal>.$.</literal> and anchored at the end of
   the string.
  </para>

  <para>
   <literal>\K</literal> can be used to reset the match start since
   PHP 5.2.4. For example, the pattern <literal>foo\Kbar</literal> matches
   "foobar", but reports that it has matched "bar". The use of
   <literal>\K</literal> does not interfere with the setting of captured
   substrings. For example, when the pattern <literal>(foo)\Kbar</literal>
   matches "foobar", the first substring is still set to "foo".
  </para>

 </section>

 <section xml:id="regexp.reference.unicode">
  <title>Unicode character properties</title>
  <para>
   Since 5.1.0, three
   additional escape sequences to match generic character types are available
   when <emphasis>UTF-8 mode</emphasis> is selected. They are:
  </para>
  <variablelist>
   <varlistentry>
    <term><emphasis>\p{xx}</emphasis></term>
    <listitem><simpara>a character with the xx property</simpara></listitem>
   </varlistentry>
   <varlistentry>
    <term><emphasis>\P{xx}</emphasis></term>
    <listitem><simpara>a character without the xx property</simpara></listitem>
   </varlistentry>
   <varlistentry>
    <term><emphasis>\X</emphasis></term>
    <listitem><simpara>an extended Unicode sequence</simpara></listitem>
   </varlistentry>
  </variablelist>
  <para>
   The property names represented by <literal>xx</literal> above are limited
   to the Unicode general category properties. Each character has exactly one
   such 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, <literal>\p{^Lu}</literal>
   is the same as <literal>\P{Lu}</literal>.
  </para>
  <para>
   If only one letter is specified with <literal>\p</literal> or
   <literal>\P</literal>, it includes all the 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:
  </para>
  <informalexample>
   <programlisting>
<![CDATA[
\p{L}
\pL
]]>
   </programlisting>
  </informalexample>
  <table>
   <title>Supported property codes</title>
   <tgroup cols="3">
    <thead>
     <row>
      <entry>Property</entry>
      <entry>Matches</entry>
      <entry>Notes</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry><literal>C</literal></entry>
      <entry>Other</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Cc</literal></entry>
      <entry>Control</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Cf</literal></entry>
      <entry>Format</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Cn</literal></entry>
      <entry>Unassigned</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Co</literal></entry>
      <entry>Private use</entry>
      <entry></entry>
     </row>
     <row rowsep="1">
      <entry><literal>Cs</literal></entry>
      <entry>Surrogate</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>L</literal></entry>
      <entry>Letter</entry>
      <entry>
       Includes the following properties: <literal>Ll</literal>,
       <literal>Lm</literal>, <literal>Lo</literal>, <literal>Lt</literal> and
       <literal>Lu</literal>.
      </entry>
     </row>
     <row>
      <entry><literal>Ll</literal></entry>
      <entry>Lower case letter</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Lm</literal></entry>
      <entry>Modifier letter</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Lo</literal></entry>
      <entry>Other letter</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Lt</literal></entry>
      <entry>Title case letter</entry>
      <entry></entry>
     </row>
     <row rowsep="1">
      <entry><literal>Lu</literal></entry>
      <entry>Upper case letter</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>M</literal></entry>
      <entry>Mark</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Mc</literal></entry>
      <entry>Spacing mark</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Me</literal></entry>
      <entry>Enclosing mark</entry>
      <entry></entry>
     </row>
     <row rowsep="1">
      <entry><literal>Mn</literal></entry>
      <entry>Non-spacing mark</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>N</literal></entry>
      <entry>Number</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Nd</literal></entry>
      <entry>Decimal number</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Nl</literal></entry>
      <entry>Letter number</entry>
      <entry></entry>
     </row>
     <row rowsep="1">
      <entry><literal>No</literal></entry>
      <entry>Other number</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>P</literal></entry>
      <entry>Punctuation</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Pc</literal></entry>
      <entry>Connector punctuation</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Pd</literal></entry>
      <entry>Dash punctuation</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Pe</literal></entry>
      <entry>Close punctuation</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Pf</literal></entry>
      <entry>Final punctuation</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Pi</literal></entry>
      <entry>Initial punctuation</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Po</literal></entry>
      <entry>Other punctuation</entry>
      <entry></entry>
     </row>
     <row rowsep="1">
      <entry><literal>Ps</literal></entry>
      <entry>Open punctuation</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>S</literal></entry>
      <entry>Symbol</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Sc</literal></entry>
      <entry>Currency symbol</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Sk</literal></entry>
      <entry>Modifier symbol</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Sm</literal></entry>
      <entry>Mathematical symbol</entry>
      <entry></entry>
     </row>
     <row rowsep="1">
      <entry><literal>So</literal></entry>
      <entry>Other symbol</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Z</literal></entry>
      <entry>Separator</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Zl</literal></entry>
      <entry>Line separator</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Zp</literal></entry>
      <entry>Paragraph separator</entry>
      <entry></entry>
     </row>
     <row>
      <entry><literal>Zs</literal></entry>
      <entry>Space separator</entry>
      <entry></entry>
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   Extended properties such as <literal>InMusicalSymbols</literal> are not
   supported by PCRE.
  </para>
  <para>
   Specifying case-insensitive (caseless) matching does not affect these escape sequences.
   For example, <literal>\p{Lu}</literal> always matches only upper case letters.
  </para>
  <para>
   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:
  </para>
  <itemizedlist>
   <listitem>
    <simpara><literal>\p{Greek}</literal></simpara>
   </listitem>
   <listitem>
    <simpara><literal>\P{Han}</literal></simpara>
   </listitem>
  </itemizedlist>
  <para>
   Those that are not part of an identified script are lumped together  as
   <literal>Common</literal>. The current list of scripts is:
  </para>
  <table>
   <title>Supported scripts</title>
   <tgroup cols="5">
    <tbody>
     <row>
      <entry><literal>Arabic</literal></entry>
      <entry><literal>Armenian</literal></entry>
      <entry><literal>Avestan</literal></entry>
      <entry><literal>Balinese</literal></entry>
      <entry><literal>Bamum</literal></entry>
     </row>
     <row>
      <entry><literal>Batak</literal></entry>
      <entry><literal>Bengali</literal></entry>
      <entry><literal>Bopomofo</literal></entry>
      <entry><literal>Brahmi</literal></entry>
      <entry><literal>Braille</literal></entry>
     </row>
     <row>
      <entry><literal>Buginese</literal></entry>
      <entry><literal>Buhid</literal></entry>
      <entry><literal>Canadian_Aboriginal</literal></entry>
      <entry><literal>Carian</literal></entry>
      <entry><literal>Chakma</literal></entry>
     </row>
     <row>
      <entry><literal>Cham</literal></entry>
      <entry><literal>Cherokee</literal></entry>
      <entry><literal>Common</literal></entry>
      <entry><literal>Coptic</literal></entry>
      <entry><literal>Cuneiform</literal></entry>
     </row>
     <row>
      <entry><literal>Cypriot</literal></entry>
      <entry><literal>Cyrillic</literal></entry>
      <entry><literal>Deseret</literal></entry>
      <entry><literal>Devanagari</literal></entry>
      <entry><literal>Egyptian_Hieroglyphs</literal></entry>
     </row>
     <row>
      <entry><literal>Ethiopic</literal></entry>
      <entry><literal>Georgian</literal></entry>
      <entry><literal>Glagolitic</literal></entry>
      <entry><literal>Gothic</literal></entry>
      <entry><literal>Greek</literal></entry>
     </row>
     <row>
      <entry><literal>Gujarati</literal></entry>
      <entry><literal>Gurmukhi</literal></entry>
      <entry><literal>Han</literal></entry>
      <entry><literal>Hangul</literal></entry>
      <entry><literal>Hanunoo</literal></entry>
     </row>
     <row>
      <entry><literal>Hebrew</literal></entry>
      <entry><literal>Hiragana</literal></entry>
      <entry><literal>Imperial_Aramaic</literal></entry>
      <entry><literal>Inherited</literal></entry>
      <entry><literal>Inscriptional_Pahlavi</literal></entry>
     </row>
     <row>
      <entry><literal>Inscriptional_Parthian</literal></entry>
      <entry><literal>Javanese</literal></entry>
      <entry><literal>Kaithi</literal></entry>
      <entry><literal>Kannada</literal></entry>
      <entry><literal>Katakana</literal></entry>
     </row>
     <row>
      <entry><literal>Kayah_Li</literal></entry>
      <entry><literal>Kharoshthi</literal></entry>
      <entry><literal>Khmer</literal></entry>
      <entry><literal>Lao</literal></entry>
      <entry><literal>Latin</literal></entry>
     </row>
     <row>
      <entry><literal>Lepcha</literal></entry>
      <entry><literal>Limbu</literal></entry>
      <entry><literal>Linear_B</literal></entry>
      <entry><literal>Lisu</literal></entry>
      <entry><literal>Lycian</literal></entry>
     </row>
     <row>
      <entry><literal>Lydian</literal></entry>
      <entry><literal>Malayalam</literal></entry>
      <entry><literal>Mandaic</literal></entry>
      <entry><literal>Meetei_Mayek</literal></entry>
      <entry><literal>Meroitic_Cursive</literal></entry>
     </row>
     <row>
      <entry><literal>Meroitic_Hieroglyphs</literal></entry>
      <entry><literal>Miao</literal></entry>
      <entry><literal>Mongolian</literal></entry>
      <entry><literal>Myanmar</literal></entry>
      <entry><literal>New_Tai_Lue</literal></entry>
     </row>
     <row>
      <entry><literal>Nko</literal></entry>
      <entry><literal>Ogham</literal></entry>
      <entry><literal>Old_Italic</literal></entry>
      <entry><literal>Old_Persian</literal></entry>
      <entry><literal>Old_South_Arabian</literal></entry>
     </row>
     <row>
      <entry><literal>Old_Turkic</literal></entry>
      <entry><literal>Ol_Chiki</literal></entry>
      <entry><literal>Oriya</literal></entry>
      <entry><literal>Osmanya</literal></entry>
      <entry><literal>Phags_Pa</literal></entry>
     </row>
     <row>
      <entry><literal>Phoenician</literal></entry>
      <entry><literal>Rejang</literal></entry>
      <entry><literal>Runic</literal></entry>
      <entry><literal>Samaritan</literal></entry>
      <entry><literal>Saurashtra</literal></entry>
     </row>
     <row>
      <entry><literal>Sharada</literal></entry>
      <entry><literal>Shavian</literal></entry>
      <entry><literal>Sinhala</literal></entry>
      <entry><literal>Sora_Sompeng</literal></entry>
      <entry><literal>Sundanese</literal></entry>
     </row>
     <row>
      <entry><literal>Syloti_Nagri</literal></entry>
      <entry><literal>Syriac</literal></entry>
      <entry><literal>Tagalog</literal></entry>
      <entry><literal>Tagbanwa</literal></entry>
      <entry><literal>Tai_Le</literal></entry>
     </row>
     <row>
      <entry><literal>Tai_Tham</literal></entry>
      <entry><literal>Tai_Viet</literal></entry>
      <entry><literal>Takri</literal></entry>
      <entry><literal>Tamil</literal></entry>
      <entry><literal>Telugu</literal></entry>
     </row>
     <row>
      <entry><literal>Thaana</literal></entry>
      <entry><literal>Thai</literal></entry>
      <entry><literal>Tibetan</literal></entry>
      <entry><literal>Tifinagh</literal></entry>
      <entry><literal>Ugaritic</literal></entry>
     </row>
     <row>
      <entry><literal>Vai</literal></entry>
      <entry><literal>Yi</literal></entry>
      <entry />
      <entry />
      <entry />
      <entry />
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   The <literal>\X</literal> escape matches a Unicode extended grapheme
   cluster. An extended grapheme cluster is one or more Unicode characters
   that combine to form a single glyph. In effect, this can be thought of as
   the Unicode equivalent of <literal>.</literal> as it will match one
   composed character, regardless of how many individual characters are
   actually used to render it.
  </para>
  <para>
   In versions of PCRE older than 8.32 (which corresponds to PHP versions
   before 5.4.14 when using the bundled PCRE library), <literal>\X</literal>
   is equivalent to <literal>(?>\PM\pM*)</literal>.  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 (see
   below). Characters with the "mark" property are typically accents that
   affect the preceding character.
  </para>
  <para>
   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
   <literal>\d</literal> and <literal>\w</literal> do not use Unicode properties
   in PCRE.
  </para>
 </section>

 <section xml:id="regexp.reference.anchors">
  <title>Anchors</title>
  <para>
   Outside a character class, in the default matching mode, the
   circumflex character (<literal>^</literal>) is an assertion which
   is true only if the current matching point is at the start  of
   the  subject string. Inside a character class, circumflex (<literal>^</literal>)
   has an entirely different meaning (see below).
  </para>
  <para>
   Circumflex (<literal>^</literal>) 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 (<literal>^</literal>), 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.)
  </para>
  <para>
   A dollar character (<literal>$</literal>) is an assertion which is
   &true; only if the current  matching point is at the end of the subject
   string, or immediately before a newline character that is  the  last
   character in the string (by default). Dollar (<literal>$</literal>)
   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.
  </para>
  <para>
   The meaning of dollar can be changed so that it matches only
   at the very end of the string, by setting the
   <link linkend="reference.pcre.pattern.modifiers">PCRE_DOLLAR_ENDONLY</link>
   option at compile or matching time. This does not affect the \Z assertion.
  </para>
  <para>
   The meanings of the circumflex and dollar characters are
   changed if the
   <link linkend="reference.pcre.pattern.modifiers">PCRE_MULTILINE</link> option
   is set. When this is the case, they match immediately after and
   immediately before an internal "\n" character, respectively, in addition
   to matching at the start and end of the subject string. For example, the
   pattern /^abc$/ matches the subject string "def\nabc" 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. The
   <link linkend="reference.pcre.pattern.modifiers">PCRE_DOLLAR_ENDONLY</link>
   option is ignored if
   <link linkend="reference.pcre.pattern.modifiers">PCRE_MULTILINE</link> is
   set.
  </para>
  <para>
   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 is it  always  anchored,
   whether <link linkend="reference.pcre.pattern.modifiers">PCRE_MULTILINE</link>
   is set or not.
  </para>
 </section>

 <section xml:id="regexp.reference.dot">
  <title>Dot</title>
  <para>
   Outside a character class, a dot in the pattern matches  any
   one  character  in  the  subject,  including  a non-printing
   character, but not (by default) newline.  If the
   <link linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link>
   option  is  set,  then dots match newlines as well. The
   handling of dot is entirely independent of the handling of
   circumflex  and  dollar,  the only relationship being that they
   both involve newline characters.  Dot has no special meaning
   in a character class.
  </para>
  <para>
   <emphasis>\C</emphasis> can be used to match single byte. It makes sense
   in <emphasis>UTF-8 mode</emphasis> where full stop matches the whole
   character which can consist of multiple bytes.
  </para>
 </section>

 <section xml:id="regexp.reference.character-classes">
  <title>Character classes</title>
  <para>
   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.
  </para>
  <para>
   A character class matches a single character in the subject;
   the  character  must  be in the set of characters defined by
   the class, unless the first character in the class 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.
  </para>
  <para>
   For example, the character class [aeiou] matches  any  lower
   case vowel, while [^aeiou] matches any character that is not
   a lower case vowel. Note that a circumflex is  just  a
   convenient  notation for specifying the characters which are in
   the class by enumerating those that are not. It  is  not  an
   assertion:  it  still  consumes a character from the subject
   string, and fails if the current pointer is at  the  end  of
   the string.
  </para>
  <para>
   When case-insensitive (caseless) matching is set, any letters
   in a class represent both their upper case and lower case
   versions, so for example, an insensitive [aeiou] matches "A"
   as well as "a", and an insensitive [^aeiou] does not match
   "A", whereas a sensitive (caseful) version would.
  </para>
  <para>
   The newline character is never treated in any special way in
   character  classes,  whatever the setting of the <link
   linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link>
   or <link linkend="reference.pcre.pattern.modifiers">PCRE_MULTILINE</link>
   options is. A class such as [^a] will always match a newline.
  </para>
  <para>
   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.
  </para>
  <para>
   It is not possible to have the literal character "]" as  the
   end  character  of  a  range.  A  pattern such as [W-]46] is
   interpreted as a class of two characters ("W" and "-")
   followed by a literal string "46]", so it would match "W46]" or
   "-46]". However, if the "]" is escaped with a  backslash  it
   is  interpreted  as  the end of range, so [W-\]46] is
   interpreted as a single class containing a range followed by  two
   separate characters. The octal or hexadecimal representation
   of "]" can also be used to end a range.
  </para>
  <para>
   Ranges operate in ASCII collating sequence. They can also be
   used  for  characters  specified  numerically,  for  example
   [\000-\037]. If a range that includes letters is  used  when
   case-insensitive (caseless)  matching  is set, it matches the
   letters in either case. For example, [W-c] is equivalent  to
   [][\^_`wxyzabc], matched case-insensitively, and if character
   tables for the "fr" locale are in use, [\xc8-\xcb] matches
   accented E characters in both cases.
  </para>
  <para>
   The character types \d, \D, \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.
  </para>
  <para>
   All non-alphanumeric characters other than \,  -,  ^  (at  the
   start)  and  the  terminating ] are non-special in character
   classes, but it does no harm if they are escaped. The pattern
   terminator is always special and must be escaped when used
   within an expression.
  </para>
  <para>
   Perl supports the POSIX notation for character classes. This uses names
   enclosed by <literal>[:</literal> and <literal>:]</literal> within the enclosing square brackets. PCRE also
   supports this notation. For example, <literal>[01[:alpha:]%]</literal>
   matches "0", "1", any alphabetic character, or "%". The supported class
   names are:
   <table>
    <title>Character classes</title>
    <tgroup cols="2">
     <tbody>
      <row><entry><literal>alnum</literal></entry><entry>letters and digits</entry></row>
      <row><entry><literal>alpha</literal></entry><entry>letters</entry></row>
      <row><entry><literal>ascii</literal></entry><entry>character codes 0 - 127</entry></row>
      <row><entry><literal>blank</literal></entry><entry>space or tab only</entry></row>
      <row><entry><literal>cntrl</literal></entry><entry>control characters</entry></row>
      <row><entry><literal>digit</literal></entry><entry>decimal digits (same as \d)</entry></row>
      <row><entry><literal>graph</literal></entry><entry>printing characters, excluding space</entry></row>
      <row><entry><literal>lower</literal></entry><entry>lower case letters</entry></row>
      <row><entry><literal>print</literal></entry><entry>printing characters, including space</entry></row>
      <row><entry><literal>punct</literal></entry><entry>printing characters, excluding letters and digits</entry></row>
      <row><entry><literal>space</literal></entry><entry>white space (not quite the same as \s)</entry></row>
      <row><entry><literal>upper</literal></entry><entry>upper case letters</entry></row>
      <row><entry><literal>word</literal></entry><entry>"word" characters (same as \w)</entry></row>
      <row><entry><literal>xdigit</literal></entry><entry>hexadecimal digits</entry></row>
     </tbody>
    </tgroup>
   </table>
   The <literal>space</literal> 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 <literal>\s</literal>, which does not include VT (for
   Perl compatibility).
  </para>
  <para>
   The name <literal>word</literal> is a Perl extension, and <literal>blank</literal> is a GNU extension
   from Perl 5.8. Another Perl extension is negation, which is indicated
   by a <literal>^</literal> character after the colon. For example,
   <literal>[12[:^digit:]]</literal> matches "1", "2", or any non-digit.
  </para>
  <para>
   In UTF-8 mode, characters with values greater than 128 do not match any
   of the POSIX character classes.
   As of PHP 5.3.0 and libpcre 8.10 some character classes are changed to use
   Unicode character properties, in which case the mentioned restriction does
   not apply. Refer to the <link xlink:href="&url.pcre.man;">PCRE(3) manual</link>
   for details.
  </para>
  <para>
   Unicode character properties can appear inside a character class. They can
   not be part of a range. The minus (hyphen) character after a Unicode
   character class will match literally. Trying to end a range with a Unicode
   character property will result in a warning.
  </para>
 </section>

 <section xml:id="regexp.reference.alternation">
  <title>Alternation</title>
  <para>
   Vertical bar characters are  used  to  separate  alternative
   patterns. For example, the pattern
   <literal>gilbert|sullivan</literal>
   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  (defined  below),  "succeeds" means matching the
   rest of the main pattern as well as the alternative  in  the
   subpattern.
  </para>
 </section>

 <section xml:id="regexp.reference.internal-options">
  <title>Internal option setting</title>
  <para>
   The settings of <link linkend="reference.pcre.pattern.modifiers">PCRE_CASELESS</link>,
   <link linkend="reference.pcre.pattern.modifiers">PCRE_MULTILINE</link>,
   <link linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link>,
   <link linkend="reference.pcre.pattern.modifiers">PCRE_UNGREEDY</link>,
   <link linkend="reference.pcre.pattern.modifiers">PCRE_EXTRA</link>,
   <link linkend="reference.pcre.pattern.modifiers">PCRE_EXTENDED</link>
   and PCRE_DUPNAMES can be changed from within the pattern by
   a sequence of Perl option letters enclosed between "(?"  and
   ")". The option letters are:

   <table>
    <title>Internal option letters</title>
    <tgroup cols="2">
     <tbody>
      <row>
       <entry><literal>i</literal></entry>
       <entry>for <link linkend="reference.pcre.pattern.modifiers">PCRE_CASELESS</link></entry>
      </row>
      <row>
       <entry><literal>m</literal></entry>
       <entry>for <link linkend="reference.pcre.pattern.modifiers">PCRE_MULTILINE</link></entry>
      </row>
      <row>
       <entry><literal>s</literal></entry>
       <entry>for <link linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link></entry>
      </row>
      <row>
       <entry><literal>x</literal></entry>
       <entry>for <link linkend="reference.pcre.pattern.modifiers">PCRE_EXTENDED</link></entry>
      </row>
      <row>
       <entry><literal>U</literal></entry>
       <entry>for <link linkend="reference.pcre.pattern.modifiers">PCRE_UNGREEDY</link></entry>
      </row>
      <row>
       <entry><literal>X</literal></entry>
       <entry>for <link linkend="reference.pcre.pattern.modifiers">PCRE_EXTRA</link> (no longer supported as of PHP 7.3.0)</entry>
      </row>
      <row>
       <entry><literal>J</literal></entry>
       <entry>for <link linkend="reference.pcre.pattern.modifiers">PCRE_INFO_JCHANGED</link></entry>
      </row>
     </tbody>
    </tgroup>
   </table>
  </para>
  <para>
   For example, (?im) sets case-insensitive (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 <link
   linkend="reference.pcre.pattern.modifiers">PCRE_CASELESS</link> and
   <link linkend="reference.pcre.pattern.modifiers">PCRE_MULTILINE</link>
   while unsetting <link
   linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link> and
   <link linkend="reference.pcre.pattern.modifiers">PCRE_EXTENDED</link>,
   is also  permitted. If  a  letter  appears both before and after the
   hyphen, the option is unset.
  </para>
  <para>
   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. So <literal>/ab(?i)c/</literal> matches only "abc"
   and "abC".
  </para>
  <para>
   If an option change occurs inside a subpattern,  the  effect
   is  different.  This is a change of behaviour in Perl 5.005.
   An option change inside a subpattern affects only that  part
   of the subpattern that follows it, so

   <literal>(a(?i)b)c</literal>

   matches  abc  and  aBc  and  no  other   strings   (assuming <link
   linkend="reference.pcre.pattern.modifiers">PCRE_CASELESS</link> 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,

   <literal>(a(?i)b|c)</literal>

   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.
  </para>
  <para>
   The PCRE-specific options <link
   linkend="reference.pcre.pattern.modifiers">PCRE_UNGREEDY</link>  and
   <link linkend="reference.pcre.pattern.modifiers">PCRE_EXTRA</link>   can
   be changed in the same way as the Perl-compatible options by
   using the characters U and X  respectively.  The  (?X)  flag
   setting  is  special in that it must always occur earlier in
   the pattern than any of the additional features it turns on,
   even when it is at top level. It is best put at the start.
  </para>
 </section>

 <section xml:id="regexp.reference.subpatterns">
  <title>Subpatterns</title>
  <para>
   Subpatterns are delimited by parentheses  (round  brackets),
   which can be nested.  Marking part of a pattern as a subpattern
   does two things:
  </para>
  <orderedlist>
   <listitem>
    <para>
     It localizes a set of alternatives. For example, the pattern
     <literal>cat(aract|erpillar|)</literal> matches one of the words "cat",
     "cataract", or "caterpillar". Without the parentheses, it would match
     "cataract", "erpillar" or the empty string.
    </para>
   </listitem>
   <listitem>
    <para>
     It sets up the subpattern as a capturing subpattern (as defined above).
     When the whole pattern matches, that portion of the subject string
     that matched the subpattern is passed back to the caller via the
     <emphasis>ovector</emphasis> argument of <function>pcre_exec</function>.
     Opening parentheses are counted from left to right (starting from 1) to
     obtain the numbers of the capturing subpatterns.
    </para>
   </listitem>
  </orderedlist>
  <para>
   For example, if the string "the red king" is matched against
   the pattern

   <literal>the ((red|white) (king|queen))</literal>

   the captured substrings are "red king", "red",  and  "king",
   and are numbered 1, 2, and 3.
  </para>
  <para>
   The fact that plain parentheses fulfill 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 "?:", 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

   <literal>the ((?:red|white) (king|queen))</literal>

   the captured substrings are "white queen" and  "queen",  and
   are  numbered  1  and 2. The maximum number of captured substrings
   is 65535. It may not be possible to compile such large patterns,
   however, depending on the configuration options of libpcre.
  </para>
  <para>
   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
  </para>

  <informalexample>
   <programlisting>
<![CDATA[
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
]]>
   </programlisting>
  </informalexample>

  <para>
   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".
  </para>

  <para>
   It is possible to name a subpattern using the syntax
   <literal>(?P&lt;name&gt;pattern)</literal>. This subpattern will then
   be indexed in the matches array by its normal numeric position and
   also by name. PHP 5.2.2 introduced two alternative syntaxes
   <literal>(?&lt;name&gt;pattern)</literal> and <literal>(?'name'pattern)</literal>.
  </para>

  <para>
   Sometimes it is necessary to have multiple matching, but alternating
   subgroups in a regular expression. Normally, each of these would be given
   their own backreference number even though only one of them would ever
   possibly match. To overcome this, the <literal>(?|</literal> syntax allows
   having duplicate numbers. Consider the following regex matched against the
   string <literal>Sunday</literal>:
  </para>

  <informalexample>
   <programlisting>
<![CDATA[(?:(Sat)ur|(Sun))day]]>
   </programlisting>
  </informalexample>

  <para>
   Here <literal>Sun</literal> is stored in backreference 2, while
   backreference 1 is empty. Matching yields <literal>Sat</literal> in
   backreference 1 while backreference 2 does not exist. Changing the pattern
   to use the <literal>(?|</literal> fixes this problem:
  </para>

  <informalexample>
   <programlisting>
<![CDATA[(?|(Sat)ur|(Sun))day]]>
   </programlisting>
  </informalexample>

  <para>
   Using this pattern, both <literal>Sun</literal> and <literal>Sat</literal>
   would be stored in backreference 1.
  </para>
 </section>

 <section xml:id="regexp.reference.repetition">
  <title>Repetition</title>
  <para>
   Repetition is specified by quantifiers, which can follow any
   of the following items:

   <itemizedlist>
    <listitem><simpara>a single character, possibly escaped</simpara></listitem>
    <listitem><simpara>the . metacharacter</simpara></listitem>
    <listitem><simpara>a character class</simpara></listitem>
    <listitem><simpara>a back reference (see next section)</simpara></listitem>
    <listitem><simpara>a parenthesized subpattern (unless it is  an  assertion  -
     see below)</simpara></listitem>
   </itemizedlist>
  </para>
  <para>
   The general repetition quantifier specifies  a  minimum  and
   maximum  number  of  permitted  matches,  by  giving the two
   numbers in curly brackets (braces), separated  by  a  comma.
   The  numbers  must be less than 65536, and the first must be
   less than or equal to the second. For example:

   <literal>z{2,4}</literal>

   matches "zz", "zzz", or "zzzz". A closing brace on  its  own
   is not a special character. If the second number is omitted,
   but the comma is present, there is no upper  limit;  if  the
   second number and the comma are both omitted, the quantifier
   specifies an exact number of required matches. Thus

   <literal>[aeiou]{3,}</literal>

   matches at least 3 successive vowels,  but  may  match  many
   more, while

   <literal>\d{8}</literal>

   matches exactly 8 digits.  An  opening  curly  bracket  that
   appears  in a position where a quantifier is not allowed, or
   one that does not match the syntax of a quantifier, is taken
   as  a literal character. For example, {,6} is not a quantifier,
   but a literal string of four characters.
  </para>
  <para>
   The quantifier {0} is permitted, causing the  expression  to
   behave  as  if the previous item and the quantifier were not
   present.
  </para>
  <para>
   For convenience (and  historical  compatibility)  the  three
   most common quantifiers have single-character abbreviations:

   <table>
    <title>Single-character quantifiers</title>
    <tgroup cols="2">
     <tbody>
      <row>
       <entry><literal>*</literal></entry>
       <entry>equivalent to <literal>{0,}</literal></entry>
      </row>
      <row>
       <entry><literal>+</literal></entry>
       <entry>equivalent to <literal>{1,}</literal></entry>
      </row>
      <row>
       <entry><literal>?</literal></entry>
       <entry>equivalent to <literal>{0,1}</literal></entry>
      </row>
     </tbody>
    </tgroup>
   </table>
  </para>
  <para>
   It is possible to construct infinite loops  by  following  a
   subpattern  that  can  match no characters with a quantifier
   that has no upper limit, for example:

   <literal>(a?)*</literal>
  </para>
  <para>
   Earlier versions of Perl and PCRE used to give an  error  at
   compile  time  for such patterns. However, because there are
   cases where this  can  be  useful,  such  patterns  are  now
   accepted,  but  if  any repetition of the subpattern does in
   fact match no characters, the loop is forcibly broken.
  </para>
  <para>
   By default, the quantifiers  are  "greedy",  that  is,  they
   match  as much as possible (up to the maximum number of permitted
   times), without causing the rest of  the  pattern  to
   fail. The classic example of where this gives problems is in
   trying to match comments in C programs. These appear between
   the  sequences /* and */ and within the sequence, individual
   * and / characters may appear. An attempt to  match  C  comments
   by applying the pattern

   <literal>/\*.*\*/</literal>

   to the string

   <literal>/* first comment */  not comment  /* second comment */</literal>

   fails, because it matches  the  entire  string  due  to  the
   greediness of the .*  item.
  </para>
  <para>
   However, if a quantifier is followed  by  a  question  mark,
   then it becomes lazy, and instead matches the minimum
   number of times possible, so the pattern

   <literal>/\*.*?\*/</literal>

   does the right thing with the C comments. The meaning of the
   various  quantifiers is not otherwise changed, just the preferred
   number of matches.  Do not confuse this use of
   question  mark  with  its  use as a quantifier in its own right.
   Because it has two uses, it can sometimes appear doubled, as
   in

   <literal>\d??\d</literal>

   which matches one digit by preference, but can match two  if
   that is the only way the rest of the pattern matches.
  </para>
  <para>
   If the <link linkend="reference.pcre.pattern.modifiers">PCRE_UNGREEDY</link>
   option is set (an option which  is  not
   available  in  Perl)  then the quantifiers are not greedy by
   default, but individual ones can be made greedy by following
   them  with  a  question mark. In other words, it inverts the
   default behaviour.
  </para>
  <para>
   Quantifiers followed by <literal>+</literal> are "possessive". They eat
   as many characters as possible and don't return to match the rest of the
   pattern. Thus <literal>.*abc</literal> matches "aabc" but
   <literal>.*+abc</literal> doesn't because <literal>.*+</literal> eats the
   whole string. Possessive quantifiers can be used to speed up processing.
  </para>
  <para>
   When a parenthesized subpattern is quantified with a minimum
   repeat  count  that is greater than 1 or with a limited maximum,
   more store is required for the  compiled  pattern,  in
   proportion to the size of the minimum or maximum.
  </para>
  <para>
   If a pattern starts with .* or  .{0,}  and  the  <link
   linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link>
   option (equivalent to Perl's /s) is set, thus allowing the .
   to match newlines, then the pattern is implicitly  anchored,
   because whatever follows will be tried against every character
   position in the subject string, so there is no point  in
   retrying  the overall match at any position after the first.
   PCRE treats such a pattern as though it were preceded by \A.
   In  cases where it is known that the subject string contains
   no newlines, it is worth setting <link
   linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link>  when  the
   pattern begins with .* in order to
   obtain this optimization, or
   alternatively using ^ to indicate anchoring explicitly.
  </para>
  <para>
   When a capturing subpattern is repeated, the value  captured
   is the substring that matched the final iteration. For example, after

   <literal>(tweedle[dume]{3}\s*)+</literal>

   has matched "tweedledum tweedledee" the value  of  the  captured
   substring  is  "tweedledee".  However,  if  there are
   nested capturing  subpatterns,  the  corresponding  captured
   values  may  have been set in previous iterations. For example,
   after

   <literal>/(a|(b))+/</literal>

   matches "aba" the value of the second captured substring  is
   "b".
  </para>
 </section>

 <section xml:id="regexp.reference.back-references">
  <title>Back references</title>
  <para>
   Outside a character class, a backslash followed by  a  digit
   greater  than  0  (and  possibly  further  digits) is a back
   reference to a capturing subpattern  earlier  (i.e.  to  its
   left)  in  the  pattern,  provided there have been that many
   previous capturing left parentheses.
  </para>
  <para>
   However, if the decimal number following  the  backslash  is
   less  than  10,  it is always taken as a back reference, and
   causes an error only if there are not  that  many  capturing
   left  parentheses in the entire pattern. In other words, the
   parentheses that are referenced need not be to the  left  of
   the  reference  for  numbers  less  than 10.
   A "forward back reference" can make sense when a repetition
   is involved and the subpattern to the right has participated
   in an earlier iteration. See the section
   entitled "Backslash" above for further details of  the  handling
   of digits following a backslash.
  </para>
  <para>
   A back reference matches whatever actually matched the  capturing
   subpattern in the current subject string, rather than
   anything matching the subpattern itself. So the pattern

   <literal>(sens|respons)e and \1ibility</literal>

   matches "sense and sensibility" and "response and  responsibility",
   but  not  "sense  and  responsibility". If case-sensitive (caseful)
   matching is in force at the time of the back reference, then
   the case of letters is relevant. For example,

   <literal>((?i)rah)\s+\1</literal>

   matches "rah rah" and "RAH RAH", but  not  "RAH  rah",  even
   though  the  original  capturing subpattern is matched
   case-insensitively (caselessly).
  </para>
  <para>
   There may be more than one back reference to the  same  subpattern.
   If  a  subpattern  has not actually been used in a
   particular match, then any  back  references  to  it  always
   fail. For example, the pattern

   <literal>(a|(bc))\2</literal>

   always fails if it starts to match  "a"  rather  than  "bc".
   Because  there  may  be up to 99 back references, all digits
   following the backslash are taken as  part  of  a  potential
   back reference number. If the pattern continues with a digit
   character, then some delimiter must be used to terminate the
   back reference. If the <link
   linkend="reference.pcre.pattern.modifiers">PCRE_EXTENDED</link>  option
   is set, this can be whitespace.  Otherwise an empty comment can be used.
  </para>
  <para>
   A back reference that occurs inside the parentheses to which
   it  refers  fails when the subpattern is first used, so, for
   example, (a\1) never matches.  However, such references  can
   be useful inside repeated subpatterns. For example, the pattern

   <literal>(a|b\1)+</literal>

   matches any number of "a"s and also "aba", "ababba" etc.  At
   each iteration of the subpattern, the back reference matches
   the character string corresponding to  the  previous  iteration.
   In order for this to work, the pattern must be such
   that the first iteration does not need  to  match  the  back
   reference.  This  can  be  done using alternation, as in the
   example above, or by a quantifier with a minimum of zero.
  </para>
  <para>
   As of PHP 5.2.2, the <literal>\g</literal> escape sequence can be
   used for absolute and relative referencing of subpatterns.
   This escape sequence must be followed by an unsigned number or a negative
   number, optionally enclosed in braces. The sequences <literal>\1</literal>,
   <literal>\g1</literal> and <literal>\g{1}</literal> are synonymous
   with one another. The use of this pattern with an unsigned number can
   help remove the ambiguity inherent when using digits following a
   backslash. The sequence helps to distinguish back references from octal
   characters and also makes it easier to have a back reference followed
   by a literal number, e.g. <literal>\g{2}1</literal>.
  </para>
  <para>
   The use of the <literal>\g</literal> sequence with a negative number
   signifies a relative reference. For example, <literal>(foo)(bar)\g{-1}</literal>
   would match the sequence "foobarbar" and <literal>(foo)(bar)\g{-2}</literal>
   matches "foobarfoo". This can be useful in long patterns as an alternative
   to keeping track of the number of subpatterns in order to reference
   a specific previous subpattern.
  </para>
  <para>
   Back references to the named subpatterns can be achieved by
   <literal>(?P=name)</literal> or, since PHP 5.2.2, also by
   <literal>\k&lt;name&gt;</literal> or <literal>\k'name'</literal>.
   Additionally PHP 5.2.4 added support for <literal>\k{name}</literal>
   and <literal>\g{name}</literal>, and PHP 5.2.7 for
   <literal>\g&lt;name&gt;</literal> and <literal>\g'name'</literal>.
  </para>
 </section>

 <section xml:id="regexp.reference.assertions">
  <title>Assertions</title>
  <para>
   An assertion is  a  test  on  the  characters  following  or
   preceding  the current matching point that does not actually
   consume any characters. The simple assertions coded  as  \b,
   \B,  \A,  \Z,  \z, ^ and $ are described in <link linkend="regexp.reference.escape">escape sequences</link>. More complicated
   assertions are coded as  subpatterns.  There  are  two
   kinds:  those that <emphasis>look ahead</emphasis> of the current position in the
   subject string, and those that <emphasis>look behind</emphasis> it.
  </para>
  <para>
   An assertion subpattern is matched in the normal way, except
   that  it  does not cause the current matching position to be
   changed. <emphasis>Lookahead</emphasis> assertions start with  (?=  for  positive
   assertions and (?! for negative assertions. For example,

   <literal>\w+(?=;)</literal>

   matches a word followed by a semicolon, but does not include
   the semicolon in the match, and

   <literal>foo(?!bar)</literal>

   matches any occurrence of "foo"  that  is  not  followed  by
   "bar". Note that the apparently similar pattern

   <literal>(?!foo)bar</literal>

   does not find an occurrence of "bar"  that  is  preceded  by
   something other than "foo"; it finds any occurrence of "bar"
   whatsoever, because the assertion  (?!foo)  is  always  &true;
   when  the  next  three  characters  are  "bar". A lookbehind
   assertion is needed to achieve this effect.
  </para>
  <para>
   <emphasis>Lookbehind</emphasis> assertions start with (?&lt;=  for  positive  assertions
   and (?&lt;! for negative assertions. For example,

   <literal>(?&lt;!foo)bar</literal>

   does find an occurrence of "bar" that  is  not  preceded  by
   "foo". The contents of a lookbehind assertion are restricted
   such that all the strings  it  matches  must  have  a  fixed
   length.  However, if there are several alternatives, they do
   not all have to have the same fixed length. Thus

   <literal>(?&lt;=bullock|donkey)</literal>

   is permitted, but

   <literal>(?&lt;!dogs?|cats?)</literal>

   causes an error at compile time. Branches  that  match  different
   length strings are permitted only at the top level of
   a lookbehind assertion. This is an extension  compared  with
   Perl  5.005,  which  requires all branches to match the same
   length of string. An assertion such as

   <literal>(?&lt;=ab(c|de))</literal>

   is not permitted, because its single  top-level  branch  can
   match two different lengths, but it is acceptable if rewritten
   to use two top-level branches:

   <literal>(?&lt;=abc|abde)</literal>

   The implementation of lookbehind  assertions  is,  for  each
   alternative,  to  temporarily move the current position back
   by the fixed width and then  try  to  match.  If  there  are
   insufficient  characters  before  the  current position, the
   match is deemed to fail.  Lookbehinds  in  conjunction  with
   once-only  subpatterns can be particularly useful for matching
   at the ends of strings; an example is given at  the  end
   of the section on once-only subpatterns.
  </para>
  <para>
   Several assertions (of any sort) may  occur  in  succession.
   For example,

   <literal>(?&lt;=\d{3})(?&lt;!999)foo</literal>

   matches "foo" preceded by three digits that are  not  "999".
   Notice  that each of the assertions is applied independently
   at the same point in the subject string. First  there  is  a
   check  that  the  previous  three characters are all digits,
   then there is a check that the same three characters are not
   "999".   This  pattern  does not match "foo" preceded by six
   characters, the first of which are digits and the last three
   of  which  are  not  "999".  For  example,  it doesn't match
   "123abcfoo". A pattern to do that is

   <literal>(?&lt;=\d{3}...)(?&lt;!999)foo</literal>
  </para>
  <para>
   This time the first assertion looks  at  the  preceding  six
   characters,  checking  that  the first three are digits, and
   then the second assertion checks that  the  preceding  three
   characters are not "999".
  </para>
  <para>
   Assertions can be nested in any combination. For example,

   <literal>(?&lt;=(?&lt;!foo)bar)baz</literal>

   matches an occurrence of "baz" that  is  preceded  by  "bar"
   which in turn is not preceded by "foo", while

   <literal>(?&lt;=\d{3}...(?&lt;!999))foo</literal>

   is another pattern which matches  "foo"  preceded  by  three
   digits and any three characters that are not "999".
  </para>
  <para>
   Assertion subpatterns are not capturing subpatterns, and may
   not  be  repeated,  because  it makes no sense to assert the
   same thing several times. If any kind of assertion  contains
   capturing  subpatterns  within it, these are counted for the
   purposes of numbering the capturing subpatterns in the whole
   pattern.   However,  substring capturing is carried out only
   for positive assertions, because it does not make sense  for
   negative assertions.
  </para>
  <para>
   Assertions count towards the maximum  of  200  parenthesized
   subpatterns.
  </para>
 </section>

 <section xml:id="regexp.reference.onlyonce">
  <title>Once-only subpatterns</title>
  <para>
   With both maximizing and minimizing repetition,  failure  of
   what  follows  normally  causes  the repeated item to be
   re-evaluated to see if a different number of repeats allows the
   rest  of  the  pattern  to  match. Sometimes it is useful to
   prevent this, either to change the nature of the  match,  or
   to  cause  it fail earlier than it otherwise might, when the
   author of the pattern knows there is no  point  in  carrying
   on.
  </para>
  <para>
   Consider, for example, the pattern \d+foo  when  applied  to
   the subject line

   <literal>123456bar</literal>
  </para>
  <para>
   After matching all 6 digits and then failing to match "foo",
   the normal action of the matcher is to try again with only 5
   digits matching the \d+ item, and then with 4,  and  so  on,
   before ultimately failing. Once-only subpatterns provide the
   means for specifying that once a portion of the pattern  has
   matched,  it  is  not to be re-evaluated in this way, so the
   matcher would give up immediately on failing to match  "foo"
   the  first  time.  The  notation  is another kind of special
   parenthesis, starting with (?&gt; as in this example:

   <literal>(?&gt;\d+)bar</literal>
  </para>
  <para>
   This kind of parenthesis "locks up" the  part of the pattern
   it  contains once it has matched, and a failure further into
   the pattern is prevented from backtracking  into  it.
   Backtracking  past  it to previous items, however, works as normal.
  </para>
  <para>
   An alternative description is that a subpattern of this type
   matches  the  string  of  characters that an identical standalone
   pattern would match, if anchored at the current point
   in the subject string.
  </para>
  <para>
   Once-only subpatterns are not capturing subpatterns.  Simple
   cases  such as the above example can be thought of as a maximizing
   repeat that must  swallow  everything  it  can.  So,
   while both \d+ and \d+? are prepared to adjust the number of
   digits they match in order to make the rest of  the  pattern
   match, (?&gt;\d+) can only match an entire sequence of digits.
  </para>
  <para>
   This construction can of course contain arbitrarily  complicated
   subpatterns, and it can be nested.
  </para>
  <para>
   Once-only subpatterns can be used in conjunction with
   lookbehind assertions  to specify efficient matching at the end
   of the subject string. Consider a simple pattern such as

   <literal>abcd$</literal>

   when applied to a long string which does not match.  Because
   matching  proceeds  from  left  to right, PCRE will look for
   each "a" in the subject and then see if what follows matches
   the rest of the pattern. If the pattern is specified as

   <literal>^.*abcd$</literal>

   then the initial .* matches the entire string at first,  but
   when  this  fails  (because  there  is no following "a"), it
   backtracks to match all but the last character, then all but
   the  last  two  characters, and so on. Once again the search
   for "a" covers the entire string, from right to left, so  we
   are no better off. However, if the pattern is written as

   <literal>^(?>.*)(?&lt;=abcd)</literal>

   then there can be no backtracking for the .*  item;  it  can
   match  only  the  entire  string.  The subsequent lookbehind
   assertion does a single test on the last four characters. If
   it  fails,  the  match  fails immediately. For long strings,
   this approach makes a significant difference to the processing time.
  </para>
  <para>
   When a pattern contains an unlimited repeat inside a subpattern
   that can itself be repeated an unlimited number of
   times, the use of a once-only subpattern is the only way  to
   avoid  some  failing matches taking a very long time indeed.
   The pattern

   <literal>(\D+|&lt;\d+>)*[!?]</literal>

   matches an unlimited number of substrings that  either  consist
   of  non-digits,  or digits enclosed in &lt;>, followed by
   either ! or ?. When it matches, it runs quickly. However, if
   it is applied to

   <literal>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</literal>

   it takes a long  time  before  reporting  failure.  This  is
   because the string can be divided between the two repeats in
   a large number of ways, and all have to be tried. (The example
   used  [!?]  rather  than a single character at the end,
   because both PCRE and Perl have an optimization that  allows
   for  fast  failure  when  a  single  character is used. They
   remember the last single character that is  required  for  a
   match,  and  fail early if it is not present in the string.)
   If the pattern is changed to

   <literal>((?>\D+)|&lt;\d+>)*[!?]</literal>

   sequences of non-digits cannot be broken, and  failure  happens quickly.
  </para>
 </section>

 <section xml:id="regexp.reference.conditional">
  <title>Conditional subpatterns</title>
  <para>
   It is possible to cause the matching process to obey a  subpattern
   conditionally  or to choose between two alternative
   subpatterns, depending on the result  of  an  assertion,  or
   whether  a previous capturing subpattern matched or not. The
   two possible forms of conditional subpattern are
  </para>

  <informalexample>
   <programlisting>
<![CDATA[
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
]]>
   </programlisting>
  </informalexample>
  <para>
   If the condition is satisfied, the yes-pattern is used; otherwise
   the  no-pattern  (if  present) is used. If there are
   more than two alternatives in the subpattern, a compile-time
   error occurs.
  </para>
  <para>
   There are two kinds of condition. If the  text  between  the
   parentheses  consists  of  a  sequence  of  digits, then the
   condition is satisfied if the capturing subpattern  of  that
   number  has  previously matched. Consider the following pattern,
   which contains non-significant white space to make  it
   more  readable  (assume  the  <link
   linkend="reference.pcre.pattern.modifiers">PCRE_EXTENDED</link>
   option)  and to divide it into three parts for ease of discussion:
  </para>
  <informalexample>
   <programlisting>
<![CDATA[
( \( )?    [^()]+    (?(1) \) )
]]>
   </programlisting>
  </informalexample>
  <para>
   The first part matches an optional opening parenthesis,  and
   if  that character is present, sets it as the first captured
   substring. The second part matches one  or  more  characters
   that  are  not  parentheses. The third part is a conditional
   subpattern that tests whether the first set  of  parentheses
   matched  or  not.  If  they did, that is, if subject started
   with an opening parenthesis, the condition is &true;,  and  so
   the  yes-pattern  is  executed  and a closing parenthesis is
   required. Otherwise, since no-pattern is  not  present,  the
   subpattern  matches  nothing.  In  other words, this pattern
   matches a sequence of non-parentheses,  optionally  enclosed
   in parentheses.
  </para>
  <para>
   If the condition is the string <literal>(R)</literal>, it is satisfied if
   a recursive call to the pattern or subpattern has been made. At "top
   level", the condition is false.
  </para>
  <para>
   If the condition is not a sequence of digits or (R), it must be  an
   assertion.  This  may be a positive or negative lookahead or
   lookbehind assertion. Consider this pattern, again  containing
   non-significant  white space, and with the two alternatives on
   the second line:
  </para>

  <informalexample>
   <programlisting>
<![CDATA[
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
]]>
   </programlisting>
  </informalexample>
  <para>
   The condition is a positive lookahead assertion that matches
   an optional sequence of non-letters followed by a letter. In
   other words, it tests for  the  presence  of  at  least  one
   letter  in the subject. If a letter is found, the subject is
   matched against  the  first  alternative;  otherwise  it  is
   matched  against the second. This pattern matches strings in
   one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are
   letters and dd are digits.
  </para>
 </section>

 <section xml:id="regexp.reference.comments">
  <title>Comments</title>
  <para>
   The  sequence  (?#  marks  the  start  of  a  comment  which
   continues   up  to  the  next  closing  parenthesis.  Nested
   parentheses are not permitted. The characters that make up a
   comment play no part in the pattern matching at all.
  </para>
  <para>
   If the <link linkend="reference.pcre.pattern.modifiers">PCRE_EXTENDED</link>
   option is set, an unescaped # character outside  a character class
   introduces a comment that continues up to the next newline character
   in the pattern.
  </para>
 </section>

 <section xml:id="regexp.reference.recursive">
  <title>Recursive patterns</title>
  <para>
   Consider the problem of matching a  string  in  parentheses,
   allowing  for  unlimited nested parentheses. Without the use
   of recursion, the best that can be done is to use a  pattern
   that  matches  up  to some fixed depth of nesting. It is not
   possible to handle an arbitrary nesting depth. Perl 5.6  has
   provided   an  experimental  facility  that  allows  regular
   expressions to recurse (among other things).  The  special
   item (?R) is  provided for  the specific  case of recursion.
   This PCRE  pattern  solves the  parentheses  problem (assume
   the <link linkend="reference.pcre.pattern.modifiers">PCRE_EXTENDED</link>
   option is set so that white space is
   ignored):

   <literal>\( ( (?>[^()]+) | (?R) )* \)</literal>
  </para>
  <para>
   First it matches an opening parenthesis. Then it matches any
   number  of substrings which can either be a sequence of
   non-parentheses, or a recursive  match  of  the  pattern  itself
   (i.e. a correctly parenthesized substring). Finally there is
   a closing parenthesis.
  </para>
  <para>
   This particular example pattern  contains  nested  unlimited
   repeats, and so the use of a once-only subpattern for matching
   strings of non-parentheses is  important  when  applying
   the  pattern to strings that do not match. For example, when
   it is applied to

   <literal>(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()</literal>

   it yields "no match" quickly. However, if a  once-only  subpattern
   is  not  used,  the match runs for a very long time
   indeed because there are so many different ways the + and  *
   repeats  can carve up the subject, and all have to be tested
   before failure can be reported.
  </para>
  <para>
   The values set for any capturing subpatterns are those  from
   the outermost level of the recursion at which the subpattern
   value is set. If the pattern above is matched against

   <literal>(ab(cd)ef)</literal>

   the value for the capturing parentheses is  "ef",  which  is
   the  last  value  taken  on  at the top level. If additional
   parentheses are added, giving

   <literal>\( ( ( (?>[^()]+) | (?R) )* ) \)</literal>
   then the string they capture
   is "ab(cd)ef", the contents of the top level parentheses. If
   there are more than 15 capturing parentheses in  a  pattern,
   PCRE  has  to  obtain  extra  memory  to store data during a
   recursion, which it does by using  pcre_malloc,  freeing  it
   via  pcre_free  afterwards. If no memory can be obtained, it
   saves data for the first 15 capturing parentheses  only,  as
   there is no way to give an out-of-memory error from within a
   recursion.
  </para>

  <para>
   <literal>(?1)</literal>, <literal>(?2)</literal> and so on
   can be used for recursive subpatterns too. It is also possible to use named
   subpatterns: <literal>(?P&gt;name)</literal> or
   <literal>(?&amp;name)</literal>.
  </para>
  <para>
   If the syntax for a recursive subpattern reference (either by number or
   by name) is used outside the parentheses to which it refers, it operates
   like a subroutine in a programming language. An earlier example
   pointed out that the pattern
   <literal>(sens|respons)e and \1ibility</literal>
   matches "sense and sensibility" and "response and responsibility", but
   not "sense and responsibility". If instead the pattern
   <literal>(sens|respons)e and (?1)ibility</literal>
   is used, it does match "sense and responsibility" as well as the other
   two strings. Such references must, however, follow the subpattern to
   which they refer.
  </para>

  <para>
   The maximum length of a subject string is the largest positive number
   that an integer variable can hold. However, PCRE uses recursion to
   handle subpatterns and indefinite repetition. This means that the
   available stack space may limit the size of a subject string that can be
   processed by certain patterns.
  </para>

 </section>

 <section xml:id="regexp.reference.performance">
  <title>Performance</title>
  <para>
   Certain items that may appear in patterns are more efficient
   than  others.  It is more efficient to use a character class
   like [aeiou] than a set of alternatives such as (a|e|i|o|u).
   In  general,  the  simplest  construction  that provides the
   required behaviour is usually the  most  efficient.  Jeffrey
   Friedl's  book contains a lot of discussion about optimizing
   regular expressions for efficient performance.
  </para>
  <para>
   When a pattern begins with .* and the <link
   linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link>  option  is
   set,  the  pattern  is implicitly anchored by PCRE, since it
   can match only at the start of a subject string. However, if
   <link linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link>
   is not set, PCRE cannot make this optimization,
   because the . metacharacter does not then match  a  newline,
   and if the subject string contains newlines, the pattern may
   match from the character immediately following one  of  them
   instead of from the very start. For example, the pattern

   <literal>(.*) second</literal>

   matches the subject "first\nand second" (where \n stands for
   a newline character) with the first captured substring being
   "and". In order to do this, PCRE  has  to  retry  the  match
   starting after every newline in the subject.
  </para>
  <para>
   If you are using such a pattern with subject strings that do
   not  contain  newlines,  the best performance is obtained by
   setting <link linkend="reference.pcre.pattern.modifiers">PCRE_DOTALL</link>,
   or starting the  pattern  with  ^.*  to
   indicate  explicit anchoring. That saves PCRE from having to
   scan along the subject looking for a newline to restart at.
  </para>
  <para>
   Beware of patterns that contain nested  indefinite  repeats.
   These  can  take a long time to run when applied to a string
   that does not match. Consider the pattern fragment

   <literal>(a+)*</literal>
  </para>
  <para>
   This can match "aaaa" in 33 different ways, and this  number
   increases  very  rapidly  as  the string gets longer. (The *
   repeat can match 0, 1, 2, 3, or 4 times,  and  for  each  of
   those  cases other than 0, the + repeats can match different
   numbers of times.) When the remainder of the pattern is such
   that  the entire match is going to fail, PCRE has in principle
   to try every possible variation, and this  can  take  an
   extremely long time.
  </para>
  <para>
   An optimization catches some of the more simple  cases  such
   as

   <literal>(a+)*b</literal>

   where a literal character follows. Before embarking  on  the
   standard matching procedure, PCRE checks that there is a "b"
   later in the subject string, and if there is not,  it  fails
   the  match  immediately. However, when there is no following
   literal this optimization cannot be used. You  can  see  the
   difference by comparing the behaviour of

   <literal>(a+)*\d</literal>

   with the pattern above. The former gives  a  failure  almost
   instantly  when  applied  to a whole line of "a" characters,
   whereas the latter takes an appreciable  time  with  strings
   longer than about 20 characters.
  </para>
 </section>
</chapter>

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