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 in this section. 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.
   Leading whitespace before a valid delimiter is silently ignored.
  </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</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\H</emphasis></term>
     <listitem><simpara>any character that is not a horizontal whitespace character</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</simpara></listitem>
    </varlistentry>
    <varlistentry>
     <term><emphasis>\V</emphasis></term>
     <listitem><simpara>any character that is not a vertical whitespace character</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>.$.</literal> and anchored at the end of
   the string. Note that this does not change the behavior of 
   delimiters; for instance the pattern <literal>#\Q#\E#$</literal>
   is not valid, because the second <literal>#</literal> marks the end
   of the pattern, and the <literal>\E#</literal> is interpreted as invalid
   modifiers.
  </para>

  <para>
   <literal>\K</literal> can be used to reset the match start. 
   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>Includes emojis</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 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. There are 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 <literal>Saturday</literal> 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.

  </para>
  <simpara>
   Prior to PHP 8.4.0, 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, <literal>{,6}</literal>
   is not a quantifier, but a literal string of four characters.

   As of PHP 8.4.0, the PCRE extension is bundled with PCRE2 version 10.44,
   which allows patterns such as <literal>\d{,8}</literal> and they are
   interpreted as <literal>\d{0,8}</literal>.

   Further, as of PHP 8.4.0, space characters around quantifiers such as
   <literal>\d{0 , 8}</literal> and <literal>\d{ 0 , 8 }</literal> are allowed.
  </simpara>
  <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
   <link linkend="regexp.reference.escape">escape sequences</link> 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>
   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>,
   <literal>\k&lt;name&gt;</literal>, <literal>\k'name'</literal>,
   <literal>\k{name}</literal>, <literal>\g{name}</literal>,
   <literal>\g&lt;name&gt;</literal> or <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>
  <para>
   <example>
    <title>Usage of comments in PCRE pattern</title>
    <programlisting role="php">
<![CDATA[
<?php

$subject = 'test';

/* (?# can be used to add comments without enabling PCRE_EXTENDED */
$match = preg_match('/te(?# this is a comment)st/', $subject);
var_dump($match);

/* Whitespace and # is treated as part of the pattern unless PCRE_EXTENDED is enabled */
$match = preg_match('/te   #~~~~
st/', $subject);
var_dump($match);

/* When PCRE_EXTENDED is enabled, all whitespace data characters and anything
   that follows an unescaped # on the same line is ignored */
$match = preg_match('/te    #~~~~
st/x', $subject);
var_dump($match);
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
int(1)
int(0)
int(1)
]]>
    </screen>
   </example>
  </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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