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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_regex.syntax.perl_syntax"></a><a class="link" href="perl_syntax.html" title="Perl Regular Expression Syntax">Perl Regular Expression
      Syntax</a>
</h3></div></div></div>
<h4>
<a name="boost_regex.syntax.perl_syntax.h0"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.synopsis"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.synopsis">Synopsis</a>
      </h4>
<p>
        The Perl regular expression syntax is based on that used by the programming
        language Perl . Perl regular expressions are the default behavior in Boost.Regex
        or you can pass the flag <code class="literal">perl</code> to the <a class="link" href="../ref/basic_regex.html" title="basic_regex"><code class="computeroutput"><span class="identifier">basic_regex</span></code></a> constructor, for example:
      </p>
<pre class="programlisting"><span class="comment">// e1 is a case sensitive Perl regular expression: </span>
<span class="comment">// since Perl is the default option there's no need to explicitly specify the syntax used here:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e1</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">);</span>
<span class="comment">// e2 a case insensitive Perl regular expression:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e2</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">perl</span><span class="special">|</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">icase</span><span class="special">);</span>
</pre>
<h4>
<a name="boost_regex.syntax.perl_syntax.h1"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.perl_regular_expression_syntax"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.perl_regular_expression_syntax">Perl
        Regular Expression Syntax</a>
      </h4>
<p>
        In Perl regular expressions, all characters match themselves except for the
        following special characters:
      </p>
<pre class="programlisting">.[{}()\*+?|^$</pre>
<p>
        Other characters are special only in certain situations - for example <code class="computeroutput"><span class="special">]</span></code> is special only after an opening <code class="computeroutput"><span class="special">[</span></code>.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h2"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.wildcard"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.wildcard">Wildcard</a>
      </h5>
<p>
        The single character '.' when used outside of a character set will match
        any single character except:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            The NULL character when the <a class="link" href="../ref/match_flag_type.html" title="match_flag_type">flag
            <code class="literal">match_not_dot_null</code></a> is passed to the matching
            algorithms.
          </li>
<li class="listitem">
            The newline character when the <a class="link" href="../ref/match_flag_type.html" title="match_flag_type">flag
            <code class="literal">match_not_dot_newline</code></a> is passed to the matching
            algorithms.
          </li>
</ul></div>
<h5>
<a name="boost_regex.syntax.perl_syntax.h3"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.anchors"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.anchors">Anchors</a>
      </h5>
<p>
        A '^' character shall match the start of a line.
      </p>
<p>
        A '$' character shall match the end of a line.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h4"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.marked_sub_expressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.marked_sub_expressions">Marked sub-expressions</a>
      </h5>
<p>
        A section beginning <code class="literal">(</code> and ending <code class="literal">)</code>
        acts as a marked sub-expression. Whatever matched the sub-expression is split
        out in a separate field by the matching algorithms. Marked sub-expressions
        can also repeated, or referred to by a back-reference.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h5"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.non_marking_grouping"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.non_marking_grouping">Non-marking
        grouping</a>
      </h5>
<p>
        A marked sub-expression is useful to lexically group part of a regular expression,
        but has the side-effect of spitting out an extra field in the result. As
        an alternative you can lexically group part of a regular expression, without
        generating a marked sub-expression by using <code class="literal">(?:</code> and <code class="literal">)</code>
        , for example <code class="literal">(?:ab)+</code> will repeat <code class="literal">ab</code>
        without splitting out any separate sub-expressions.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h6"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.repeats"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.repeats">Repeats</a>
      </h5>
<p>
        Any atom (a single character, a marked sub-expression, or a character class)
        can be repeated with the <code class="literal">*</code>, <code class="literal">+</code>, <code class="literal">?</code>,
        and <code class="literal">{}</code> operators.
      </p>
<p>
        The <code class="literal">*</code> operator will match the preceding atom zero or more
        times, for example the expression <code class="literal">a*b</code> will match any of
        the following:
      </p>
<pre class="programlisting"><span class="identifier">b</span>
<span class="identifier">ab</span>
<span class="identifier">aaaaaaaab</span>
</pre>
<p>
        The <code class="literal">+</code> operator will match the preceding atom one or more
        times, for example the expression <code class="literal">a+b</code> will match any of
        the following:
      </p>
<pre class="programlisting"><span class="identifier">ab</span>
<span class="identifier">aaaaaaaab</span>
</pre>
<p>
        But will not match:
      </p>
<pre class="programlisting"><span class="identifier">b</span>
</pre>
<p>
        The <code class="literal">?</code> operator will match the preceding atom zero or one
        times, for example the expression ca?b will match any of the following:
      </p>
<pre class="programlisting"><span class="identifier">cb</span>
<span class="identifier">cab</span>
</pre>
<p>
        But will not match:
      </p>
<pre class="programlisting"><span class="identifier">caab</span>
</pre>
<p>
        An atom can also be repeated with a bounded repeat:
      </p>
<p>
        <code class="literal">a{n}</code> Matches 'a' repeated exactly n times.
      </p>
<p>
        <code class="literal">a{n,}</code> Matches 'a' repeated n or more times.
      </p>
<p>
        <code class="literal">a{n, m}</code> Matches 'a' repeated between n and m times inclusive.
      </p>
<p>
        For example:
      </p>
<pre class="programlisting">^a{2,3}$</pre>
<p>
        Will match either of:
      </p>
<pre class="programlisting"><span class="identifier">aa</span>
<span class="identifier">aaa</span>
</pre>
<p>
        But neither of:
      </p>
<pre class="programlisting"><span class="identifier">a</span>
<span class="identifier">aaaa</span>
</pre>
<p>
        Note that the "{" and "}" characters will treated as
        ordinary literals when used in a context that is not a repeat: this matches
        Perl 5.x behavior. For example in the expressions "ab{1", "ab1}"
        and "a{b}c" the curly brackets are all treated as literals and
        <span class="emphasis"><em>no error will be raised</em></span>.
      </p>
<p>
        It is an error to use a repeat operator, if the preceding construct can not
        be repeated, for example:
      </p>
<pre class="programlisting"><span class="identifier">a</span><span class="special">(*)</span>
</pre>
<p>
        Will raise an error, as there is nothing for the <code class="literal">*</code> operator
        to be applied to.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h7"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.non_greedy_repeats"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.non_greedy_repeats">Non
        greedy repeats</a>
      </h5>
<p>
        The normal repeat operators are "greedy", that is to say they will
        consume as much input as possible. There are non-greedy versions available
        that will consume as little input as possible while still producing a match.
      </p>
<p>
        <code class="literal">*?</code> Matches the previous atom zero or more times, while
        consuming as little input as possible.
      </p>
<p>
        <code class="literal">+?</code> Matches the previous atom one or more times, while
        consuming as little input as possible.
      </p>
<p>
        <code class="literal">??</code> Matches the previous atom zero or one times, while
        consuming as little input as possible.
      </p>
<p>
        <code class="literal">{n,}?</code> Matches the previous atom n or more times, while
        consuming as little input as possible.
      </p>
<p>
        <code class="literal">{n,m}?</code> Matches the previous atom between n and m times,
        while consuming as little input as possible.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h8"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.possessive_repeats"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.possessive_repeats">Possessive
        repeats</a>
      </h5>
<p>
        By default when a repeated pattern does not match then the engine will backtrack
        until a match is found. However, this behaviour can sometime be undesireble
        so there are also "possessive" repeats: these match as much as
        possible and do not then allow backtracking if the rest of the expression
        fails to match.
      </p>
<p>
        <code class="literal">*+</code> Matches the previous atom zero or more times, while
        giving nothing back.
      </p>
<p>
        <code class="literal">++</code> Matches the previous atom one or more times, while
        giving nothing back.
      </p>
<p>
        <code class="literal">?+</code> Matches the previous atom zero or one times, while
        giving nothing back.
      </p>
<p>
        <code class="literal">{n,}+</code> Matches the previous atom n or more times, while
        giving nothing back.
      </p>
<p>
        <code class="literal">{n,m}+</code> Matches the previous atom between n and m times,
        while giving nothing back.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h9"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.back_references"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.back_references">Back
        references</a>
      </h5>
<p>
        An escape character followed by a digit <span class="emphasis"><em>n</em></span>, where <span class="emphasis"><em>n</em></span>
        is in the range 1-9, matches the same string that was matched by sub-expression
        <span class="emphasis"><em>n</em></span>. For example the expression:
      </p>
<pre class="programlisting">^(a*)[^a]*\1$</pre>
<p>
        Will match the string:
      </p>
<pre class="programlisting"><span class="identifier">aaabbaaa</span>
</pre>
<p>
        But not the string:
      </p>
<pre class="programlisting"><span class="identifier">aaabba</span>
</pre>
<p>
        You can also use the \g escape for the same function, for example:
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Escape
                </p>
              </th>
<th>
                <p>
                  Meaning
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="literal">\g1</code>
                </p>
              </td>
<td>
                <p>
                  Match whatever matched sub-expression 1
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\g{1}</code>
                </p>
              </td>
<td>
                <p>
                  Match whatever matched sub-expression 1: this form allows for safer
                  parsing of the expression in cases like <code class="literal">\g{1}2</code>
                  or for indexes higher than 9 as in <code class="literal">\g{1234}</code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\g-1</code>
                </p>
              </td>
<td>
                <p>
                  Match whatever matched the last opened sub-expression
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\g{-2}</code>
                </p>
              </td>
<td>
                <p>
                  Match whatever matched the last but one opened sub-expression
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\g{one}</code>
                </p>
              </td>
<td>
                <p>
                  Match whatever matched the sub-expression named "one"
                </p>
              </td>
</tr>
</tbody>
</table></div>
<p>
        Finally the \k escape can be used to refer to named subexpressions, for example
        <code class="literal">\k&lt;two&gt;</code> will match whatever matched the subexpression
        named "two".
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h10"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.alternation"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.alternation">Alternation</a>
      </h5>
<p>
        The <code class="literal">|</code> operator will match either of its arguments, so
        for example: <code class="literal">abc|def</code> will match either "abc"
        or "def".
      </p>
<p>
        Parenthesis can be used to group alternations, for example: <code class="literal">ab(d|ef)</code>
        will match either of "abd" or "abef".
      </p>
<p>
        Empty alternatives are not allowed (these are almost always a mistake), but
        if you really want an empty alternative use <code class="literal">(?:)</code> as a
        placeholder, for example:
      </p>
<p>
        <code class="literal">|abc</code> is not a valid expression, but
      </p>
<p>
        <code class="literal">(?:)|abc</code> is and is equivalent, also the expression:
      </p>
<p>
        <code class="literal">(?:abc)??</code> has exactly the same effect.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h11"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.character_sets"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.character_sets">Character
        sets</a>
      </h5>
<p>
        A character set is a bracket-expression starting with <code class="literal">[] and ending
        with <code class="literal"></code></code>, it defines a set of characters, and matches
        any single character that is a member of that set.
      </p>
<p>
        A bracket expression may contain any combination of the following:
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h12"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.single_characters"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.single_characters">Single
        characters</a>
      </h6>
<p>
        For example <code class="literal">[abc]</code>, will match any of the characters 'a',
        'b', or 'c'.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h13"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.character_ranges"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.character_ranges">Character
        ranges</a>
      </h6>
<p>
        For example <code class="literal">[a-c]</code> will match any single character in the
        range 'a' to 'c'. By default, for Perl regular expressions, a character x
        is within the range y to z, if the code point of the character lies within
        the codepoints of the endpoints of the range. Alternatively, if you set the
        <a class="link" href="../ref/syntax_option_type/syntax_option_type_perl.html" title="Options for Perl Regular Expressions"><code class="literal">collate</code>
        flag</a> when constructing the regular expression, then ranges are locale
        sensitive.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h14"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.negation"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.negation">Negation</a>
      </h6>
<p>
        If the bracket-expression begins with the ^ character, then it matches the
        complement of the characters it contains, for example <code class="literal">[^a-c]</code>
        matches any character that is not in the range <code class="literal">a-c</code>.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h15"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.character_classes"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.character_classes">Character
        classes</a>
      </h6>
<p>
        An expression of the form <code class="literal">[[:name:]]</code> matches the named
        character class "name", for example <code class="literal">[[:lower:]]</code>
        matches any lower case character. See <a class="link" href="character_classes.html" title="Character Class Names">character
        class names</a>.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h16"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.collating_elements"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.collating_elements">Collating
        Elements</a>
      </h6>
<p>
        An expression of the form <code class="literal">[[.col.]]</code> matches the collating
        element <span class="emphasis"><em>col</em></span>. A collating element is any single character,
        or any sequence of characters that collates as a single unit. Collating elements
        may also be used as the end point of a range, for example: <code class="literal">[[.ae.]-c]</code>
        matches the character sequence "ae", plus any single character
        in the range "ae"-c, assuming that "ae" is treated as
        a single collating element in the current locale.
      </p>
<p>
        As an extension, a collating element may also be specified via it's <a class="link" href="collating_names.html" title="Collating Names">symbolic name</a>, for example:
      </p>
<pre class="programlisting"><span class="special">[[.</span><span class="identifier">NUL</span><span class="special">.]]</span>
</pre>
<p>
        matches a <code class="literal">\0</code> character.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h17"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.equivalence_classes"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.equivalence_classes">Equivalence
        classes</a>
      </h6>
<p>
        An expression of the form <code class="literal">[[=col=]]</code>, matches any character
        or collating element whose primary sort key is the same as that for collating
        element <span class="emphasis"><em>col</em></span>, as with collating elements the name <span class="emphasis"><em>col</em></span>
        may be a <a class="link" href="collating_names.html" title="Collating Names">symbolic name</a>.
        A primary sort key is one that ignores case, accentation, or locale-specific
        tailorings; so for example <code class="computeroutput"><span class="special">[[=</span><span class="identifier">a</span><span class="special">=]]</span></code> matches
        any of the characters: a, &#192;, &#193;, &#194;, &#195;, &#196;, &#197;, A, &#224;, &#225;, &#226;, &#227;, &#228; and &#229;. Unfortunately implementation
        of this is reliant on the platform's collation and localisation support;
        this feature can not be relied upon to work portably across all platforms,
        or even all locales on one platform.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h18"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.escaped_characters"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.escaped_characters">Escaped
        Characters</a>
      </h6>
<p>
        All the escape sequences that match a single character, or a single character
        class are permitted within a character class definition. For example <code class="computeroutput"><span class="special">[\[\]]</span></code> would match either of <code class="computeroutput"><span class="special">[</span></code> or <code class="computeroutput"><span class="special">]</span></code>
        while <code class="computeroutput"><span class="special">[\</span><span class="identifier">W</span><span class="special">\</span><span class="identifier">d</span><span class="special">]</span></code>
        would match any character that is either a "digit", <span class="emphasis"><em>or</em></span>
        is <span class="emphasis"><em>not</em></span> a "word" character.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h19"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.combinations"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.combinations">Combinations</a>
      </h6>
<p>
        All of the above can be combined in one character set declaration, for example:
        <code class="literal">[[:digit:]a-c[.NUL.]]</code>.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h20"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.escapes"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.escapes">Escapes</a>
      </h5>
<p>
        Any special character preceded by an escape shall match itself.
      </p>
<p>
        The following escape sequences are all synonyms for single characters:
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Escape
                </p>
              </th>
<th>
                <p>
                  Character
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="literal">\a</code>
                </p>
              </td>
<td>
                <p>
                  <code class="literal">\a</code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\e</code>
                </p>
              </td>
<td>
                <p>
                  <code class="literal">0x1B</code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\f</code>
                </p>
              </td>
<td>
                <p>
                  <code class="literal">\f</code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\n</code>
                </p>
              </td>
<td>
                <p>
                  <code class="literal">\n</code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\r</code>
                </p>
              </td>
<td>
                <p>
                  <code class="literal">\r</code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\t</code>
                </p>
              </td>
<td>
                <p>
                  <code class="literal">\t</code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\v</code>
                </p>
              </td>
<td>
                <p>
                  <code class="literal">\v</code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\b</code>
                </p>
              </td>
<td>
                <p>
                  <code class="literal">\b</code> (but only inside a character class declaration).
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\cX</code>
                </p>
              </td>
<td>
                <p>
                  An ASCII escape sequence - the character whose code point is X
                  % 32
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\xdd</code>
                </p>
              </td>
<td>
                <p>
                  A hexadecimal escape sequence - matches the single character whose
                  code point is 0xdd.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\x{dddd}</code>
                </p>
              </td>
<td>
                <p>
                  A hexadecimal escape sequence - matches the single character whose
                  code point is 0xdddd.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\0ddd</code>
                </p>
              </td>
<td>
                <p>
                  An octal escape sequence - matches the single character whose code
                  point is 0ddd.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">\N{name}</code>
                </p>
              </td>
<td>
                <p>
                  Matches the single character which has the <a class="link" href="collating_names.html" title="Collating Names">symbolic
                  name</a> <span class="emphasis"><em>name</em></span>. For example <code class="literal">\N{newline}</code>
                  matches the single character \n.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_regex.syntax.perl_syntax.h21"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.single_character_character_class"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.single_character_character_class">"Single
        character" character classes:</a>
      </h6>
<p>
        Any escaped character <span class="emphasis"><em>x</em></span>, if <span class="emphasis"><em>x</em></span> is
        the name of a character class shall match any character that is a member
        of that class, and any escaped character <span class="emphasis"><em>X</em></span>, if <span class="emphasis"><em>x</em></span>
        is the name of a character class, shall match any character not in that class.
      </p>
<p>
        The following are supported by default:
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Escape sequence
                </p>
              </th>
<th>
                <p>
                  Equivalent to
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">d</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">digit</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">l</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">lower</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">s</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">space</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">u</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">upper</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">w</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">word</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">h</span></code>
                </p>
              </td>
<td>
                <p>
                  Horizontal whitespace
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">v</span></code>
                </p>
              </td>
<td>
                <p>
                  Vertical whitespace
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">D</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">digit</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">L</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">lower</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">S</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">space</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">U</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">upper</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">W</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">word</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">H</span></code>
                </p>
              </td>
<td>
                <p>
                  Not Horizontal whitespace
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">V</span></code>
                </p>
              </td>
<td>
                <p>
                  Not Vertical whitespace
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_regex.syntax.perl_syntax.h22"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.character_properties"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.character_properties">Character
        Properties</a>
      </h6>
<p>
        The character property names in the following table are all equivalent to
        the <a class="link" href="character_classes.html" title="Character Class Names">names used in character
        classes</a>.
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Form
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
<th>
                <p>
                  Equivalent character set form
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">pX</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches any character that has the property X.
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">X</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">p</span><span class="special">{</span><span class="identifier">Name</span><span class="special">}</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches any character that has the property Name.
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">Name</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">PX</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches any character that does not have the property X.
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">X</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">P</span><span class="special">{</span><span class="identifier">Name</span><span class="special">}</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches any character that does not have the property Name.
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">Name</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
</tbody>
</table></div>
<p>
        For example <code class="literal">\pd</code> matches any "digit" character,
        as does <code class="literal">\p{digit}</code>.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h23"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.word_boundaries"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.word_boundaries">Word
        Boundaries</a>
      </h6>
<p>
        The following escape sequences match the boundaries of words:
      </p>
<p>
        <code class="literal">&lt;</code> Matches the start of a word.
      </p>
<p>
        <code class="literal">&gt;</code> Matches the end of a word.
      </p>
<p>
        <code class="literal">\b</code> Matches a word boundary (the start or end of a word).
      </p>
<p>
        <code class="literal">\B</code> Matches only when not at a word boundary.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h24"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.buffer_boundaries"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.buffer_boundaries">Buffer
        boundaries</a>
      </h6>
<p>
        The following match only at buffer boundaries: a "buffer" in this
        context is the whole of the input text that is being matched against (note
        that ^ and $ may match embedded newlines within the text).
      </p>
<p>
        \` Matches at the start of a buffer only.
      </p>
<p>
        \' Matches at the end of a buffer only.
      </p>
<p>
        \A Matches at the start of a buffer only (the same as <code class="literal">\`</code>).
      </p>
<p>
        \z Matches at the end of a buffer only (the same as <code class="literal">\'</code>).
      </p>
<p>
        \Z Matches a zero-width assertion consisting of an optional sequence of newlines
        at the end of a buffer: equivalent to the regular expression <code class="literal">(?=\v*\z)</code>.
        Note that this is subtly different from Perl which behaves as if matching
        <code class="literal">(?=\n?\z)</code>.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h25"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.continuation_escape"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.continuation_escape">Continuation
        Escape</a>
      </h6>
<p>
        The sequence <code class="literal">\G</code> matches only at the end of the last match
        found, or at the start of the text being matched if no previous match was
        found. This escape useful if you're iterating over the matches contained
        within a text, and you want each subsequence match to start where the last
        one ended.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h26"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.quoting_escape"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.quoting_escape">Quoting
        escape</a>
      </h6>
<p>
        The escape sequence <code class="literal">\Q</code> begins a "quoted sequence":
        all the subsequent characters are treated as literals, until either the end
        of the regular expression or \E is found. For example the expression: <code class="literal">\Q*+\Ea+</code>
        would match either of:
      </p>
<pre class="programlisting"><span class="special">\*+</span><span class="identifier">a</span>
<span class="special">\*+</span><span class="identifier">aaa</span>
</pre>
<h6>
<a name="boost_regex.syntax.perl_syntax.h27"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.unicode_escapes"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.unicode_escapes">Unicode
        escapes</a>
      </h6>
<p>
        <code class="literal">\C</code> Matches a single code point: in Boost regex this has
        exactly the same effect as a "." operator. <code class="literal">\X</code>
        Matches a combining character sequence: that is any non-combining character
        followed by a sequence of zero or more combining characters.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h28"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.matching_line_endings"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.matching_line_endings">Matching Line
        Endings</a>
      </h6>
<p>
        The escape sequence <code class="literal">\R</code> matches any line ending character
        sequence, specifically it is identical to the expression <code class="literal">(?&gt;\x0D\x0A?|[\x0A-\x0C\x85\x{2028}\x{2029}])</code>.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h29"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.keeping_back_some_text"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.keeping_back_some_text">Keeping back
        some text</a>
      </h6>
<p>
        <code class="literal">\K</code> Resets the start location of $0 to the current text
        position: in other words everything to the left of \K is "kept back"
        and does not form part of the regular expression match. $` is updated accordingly.
      </p>
<p>
        For example <code class="literal">foo\Kbar</code> matched against the text "foobar"
        would return the match "bar" for $0 and "foo" for $`.
        This can be used to simulate variable width lookbehind assertions.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h30"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.any_other_escape"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.any_other_escape">Any
        other escape</a>
      </h6>
<p>
        Any other escape sequence matches the character that is escaped, for example
        \@ matches a literal '@'.
      </p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h31"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.perl_extended_patterns"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.perl_extended_patterns">Perl Extended
        Patterns</a>
      </h5>
<p>
        Perl-specific extensions to the regular expression syntax all start with
        <code class="literal">(?</code>.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h32"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.named_subexpressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.named_subexpressions">Named
        Subexpressions</a>
      </h6>
<p>
        You can create a named subexpression using:
      </p>
<pre class="programlisting"><span class="special">(?&lt;</span><span class="identifier">NAME</span><span class="special">&gt;</span><span class="identifier">expression</span><span class="special">)</span>
</pre>
<p>
        Which can be then be referred to by the name <span class="emphasis"><em>NAME</em></span>. Alternatively
        you can delimit the name using 'NAME' as in:
      </p>
<pre class="programlisting"><span class="special">(?</span><span class="char">'NAME'</span><span class="identifier">expression</span><span class="special">)</span>
</pre>
<p>
        These named subexpressions can be referred to in a backreference using either
        <code class="literal">\g{NAME}</code> or <code class="literal">\k&lt;NAME&gt;</code> and can
        also be referred to by name in a <a class="link" href="../format/perl_format.html" title="Perl Format String Syntax">Perl</a>
        format string for search and replace operations, or in the <a class="link" href="../ref/match_results.html" title="match_results"><code class="computeroutput"><span class="identifier">match_results</span></code></a> member functions.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h33"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.comments"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.comments">Comments</a>
      </h6>
<p>
        <code class="literal">(?# ... )</code> is treated as a comment, it's contents are ignored.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h34"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.modifiers"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.modifiers">Modifiers</a>
      </h6>
<p>
        <code class="literal">(?imsx-imsx ... )</code> alters which of the perl modifiers are
        in effect within the pattern, changes take effect from the point that the
        block is first seen and extend to any enclosing <code class="literal">)</code>. Letters
        before a '-' turn that perl modifier on, letters afterward, turn it off.
      </p>
<p>
        <code class="literal">(?imsx-imsx:pattern)</code> applies the specified modifiers to
        pattern only.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h35"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.non_marking_groups"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.non_marking_groups">Non-marking
        groups</a>
      </h6>
<p>
        <code class="literal">(?:pattern)</code> lexically groups pattern, without generating
        an additional sub-expression.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h36"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.branch_reset"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.branch_reset">Branch
        reset</a>
      </h6>
<p>
        <code class="literal">(?|pattern)</code> resets the subexpression count at the start
        of each "|" alternative within <span class="emphasis"><em>pattern</em></span>.
      </p>
<p>
        The sub-expression count following this construct is that of whichever branch
        had the largest number of sub-expressions. This construct is useful when
        you want to capture one of a number of alternative matches in a single sub-expression
        index.
      </p>
<p>
        In the following example the index of each sub-expression is shown below
        the expression:
      </p>
<pre class="programlisting"># before  ---------------branch-reset----------- after
/ ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1            2         2  3        2     3     4
</pre>
<h6>
<a name="boost_regex.syntax.perl_syntax.h37"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.lookahead"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.lookahead">Lookahead</a>
      </h6>
<p>
        <code class="literal">(?=pattern)</code> consumes zero characters, only if pattern
        matches.
      </p>
<p>
        <code class="literal">(?!pattern)</code> consumes zero characters, only if pattern
        does not match.
      </p>
<p>
        Lookahead is typically used to create the logical AND of two regular expressions,
        for example if a password must contain a lower case letter, an upper case
        letter, a punctuation symbol, and be at least 6 characters long, then the
        expression:
      </p>
<pre class="programlisting"><span class="special">(?=.*[[:</span><span class="identifier">lower</span><span class="special">:]])(?=.*[[:</span><span class="identifier">upper</span><span class="special">:]])(?=.*[[:</span><span class="identifier">punct</span><span class="special">:]]).{</span><span class="number">6</span><span class="special">,}</span>
</pre>
<p>
        could be used to validate the password.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h38"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.lookbehind"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.lookbehind">Lookbehind</a>
      </h6>
<p>
        <code class="literal">(?&lt;=pattern)</code> consumes zero characters, only if pattern
        could be matched against the characters preceding the current position (pattern
        must be of fixed length).
      </p>
<p>
        <code class="literal">(?&lt;!pattern)</code> consumes zero characters, only if pattern
        could not be matched against the characters preceding the current position
        (pattern must be of fixed length).
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h39"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.independent_sub_expressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.independent_sub_expressions">Independent
        sub-expressions</a>
      </h6>
<p>
        <code class="literal">(?&gt;pattern)</code> <span class="emphasis"><em>pattern</em></span> is matched
        independently of the surrounding patterns, the expression will never backtrack
        into <span class="emphasis"><em>pattern</em></span>. Independent sub-expressions are typically
        used to improve performance; only the best possible match for pattern will
        be considered, if this doesn't allow the expression as a whole to match then
        no match is found at all.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h40"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.recursive_expressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.recursive_expressions">Recursive
        Expressions</a>
      </h6>
<p>
        <code class="literal">(?<span class="emphasis"><em>N</em></span>) (?-<span class="emphasis"><em>N</em></span>) (?+<span class="emphasis"><em>N</em></span>)
        (?R) (?0) (?&amp;NAME)</code>
      </p>
<p>
        <code class="literal">(?R)</code> and <code class="literal">(?0)</code> recurse to the start
        of the entire pattern.
      </p>
<p>
        <code class="literal">(?<span class="emphasis"><em>N</em></span>)</code> executes sub-expression <span class="emphasis"><em>N</em></span>
        recursively, for example <code class="literal">(?2)</code> will recurse to sub-expression
        2.
      </p>
<p>
        <code class="literal">(?-<span class="emphasis"><em>N</em></span>)</code> and <code class="literal">(?+<span class="emphasis"><em>N</em></span>)</code>
        are relative recursions, so for example <code class="literal">(?-1)</code> recurses
        to the last sub-expression to be declared, and <code class="literal">(?+1)</code> recurses
        to the next sub-expression to be declared.
      </p>
<p>
        <code class="literal">(?&amp;NAME)</code> recurses to named sub-expression <span class="emphasis"><em>NAME</em></span>.
      </p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h41"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.conditional_expressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.conditional_expressions">Conditional
        Expressions</a>
      </h6>
<p>
        <code class="literal">(?(condition)yes-pattern|no-pattern)</code> attempts to match
        <span class="emphasis"><em>yes-pattern</em></span> if the <span class="emphasis"><em>condition</em></span> is
        true, otherwise attempts to match <span class="emphasis"><em>no-pattern</em></span>.
      </p>
<p>
        <code class="literal">(?(condition)yes-pattern)</code> attempts to match <span class="emphasis"><em>yes-pattern</em></span>
        if the <span class="emphasis"><em>condition</em></span> is true, otherwise matches the NULL
        string.
      </p>
<p>
        <span class="emphasis"><em>condition</em></span> may be either: a forward lookahead assert,
        the index of a marked sub-expression (the condition becomes true if the sub-expression
        has been matched), or an index of a recursion (the condition become true
        if we are executing directly inside the specified recursion).
      </p>
<p>
        Here is a summary of the possible predicates:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            <code class="literal">(?(?=assert)yes-pattern|no-pattern)</code> Executes <span class="emphasis"><em>yes-pattern</em></span>
            if the forward look-ahead assert matches, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
          </li>
<li class="listitem">
            <code class="literal">(?(?!assert)yes-pattern|no-pattern)</code> Executes <span class="emphasis"><em>yes-pattern</em></span>
            if the forward look-ahead assert does not match, otherwise executes
            <span class="emphasis"><em>no-pattern</em></span>.
          </li>
<li class="listitem">
            <code class="literal">(?(<span class="emphasis"><em>N</em></span>)yes-pattern|no-pattern)</code>
            Executes <span class="emphasis"><em>yes-pattern</em></span> if subexpression <span class="emphasis"><em>N</em></span>
            has been matched, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
          </li>
<li class="listitem">
            <code class="literal">(?(&lt;<span class="emphasis"><em>name</em></span>&gt;)yes-pattern|no-pattern)</code>
            Executes <span class="emphasis"><em>yes-pattern</em></span> if named subexpression <span class="emphasis"><em>name</em></span>
            has been matched, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
          </li>
<li class="listitem">
            <code class="literal">(?('<span class="emphasis"><em>name</em></span>')yes-pattern|no-pattern)</code>
            Executes <span class="emphasis"><em>yes-pattern</em></span> if named subexpression <span class="emphasis"><em>name</em></span>
            has been matched, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
          </li>
<li class="listitem">
            <code class="literal">(?(R)yes-pattern|no-pattern)</code> Executes <span class="emphasis"><em>yes-pattern</em></span>
            if we are executing inside a recursion, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
          </li>
<li class="listitem">
            <code class="literal">(?(R<span class="emphasis"><em>N</em></span>)yes-pattern|no-pattern)</code>
            Executes <span class="emphasis"><em>yes-pattern</em></span> if we are executing inside
            a recursion to sub-expression <span class="emphasis"><em>N</em></span>, otherwise executes
            <span class="emphasis"><em>no-pattern</em></span>.
          </li>
<li class="listitem">
            <code class="literal">(?(R&amp;<span class="emphasis"><em>name</em></span>)yes-pattern|no-pattern)</code>
            Executes <span class="emphasis"><em>yes-pattern</em></span> if we are executing inside
            a recursion to named sub-expression <span class="emphasis"><em>name</em></span>, otherwise
            executes <span class="emphasis"><em>no-pattern</em></span>.
          </li>
<li class="listitem">
            <code class="literal">(?(DEFINE)never-exectuted-pattern)</code> Defines a block
            of code that is never executed and matches no characters: this is usually
            used to define one or more named sub-expressions which are referred to
            from elsewhere in the pattern.
          </li>
</ul></div>
<h6>
<a name="boost_regex.syntax.perl_syntax.h42"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.backtracking_control_verbs"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.backtracking_control_verbs">Backtracking
        Control Verbs</a>
      </h6>
<p>
        This library has partial support for Perl's backtracking control verbs, in
        particular (*MARK) is not supported. There may also be detail differences
        in behaviour between this library and Perl, not least because Perl's behaviour
        is rather under-documented and often somewhat random in how it behaves in
        practice. The verbs supported are:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            <code class="literal">(*PRUNE)</code> Has no effect unless backtracked onto, in
            which case all the backtracking information prior to this point is discarded.
          </li>
<li class="listitem">
            <code class="literal">(*SKIP)</code> Behaves the same as <code class="literal">(*PRUNE)</code>
            except that it is assumed that no match can possibly occur prior to the
            current point in the string being searched. This can be used to optimize
            searches by skipping over chunks of text that have already been determined
            can not form a match.
          </li>
<li class="listitem">
            <code class="literal">(*THEN)</code> Has no effect unless backtracked onto, in
            which case all subsequent alternatives in a group of alternations are
            discarded.
          </li>
<li class="listitem">
            <code class="literal">(*COMMIT)</code> Has no effect unless backtracked onto, in
            which case all subsequent matching/searching attempts are abandoned.
          </li>
<li class="listitem">
            <code class="literal">(*FAIL)</code> Causes the match to fail unconditionally at
            this point, can be used to force the engine to backtrack.
          </li>
<li class="listitem">
            <code class="literal">(*ACCEPT)</code> Causes the pattern to be considered matched
            at the current point. Any half-open sub-expressions are closed at the
            current point.
          </li>
</ul></div>
<h5>
<a name="boost_regex.syntax.perl_syntax.h43"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.operator_precedence"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.operator_precedence">Operator
        precedence</a>
      </h5>
<p>
        The order of precedence for of operators is as follows:
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Collation-related bracket symbols <code class="computeroutput"><span class="special">[==]</span>
            <span class="special">[::]</span> <span class="special">[..]</span></code>
          </li>
<li class="listitem">
            Escaped characters <code class="literal">\</code>
          </li>
<li class="listitem">
            Character set (bracket expression) <code class="computeroutput"><span class="special">[]</span></code>
          </li>
<li class="listitem">
            Grouping <code class="literal">()</code>
          </li>
<li class="listitem">
            Single-character-ERE duplication <code class="literal">* + ? {m,n}</code>
          </li>
<li class="listitem">
            Concatenation
          </li>
<li class="listitem">
            Anchoring ^$
          </li>
<li class="listitem">
            Alternation |
          </li>
</ol></div>
<h4>
<a name="boost_regex.syntax.perl_syntax.h44"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.what_gets_matched"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.what_gets_matched">What
        gets matched</a>
      </h4>
<p>
        If you view the regular expression as a directed (possibly cyclic) graph,
        then the best match found is the first match found by a depth-first-search
        performed on that graph, while matching the input text.
      </p>
<p>
        Alternatively:
      </p>
<p>
        The best match found is the <a class="link" href="leftmost_longest_rule.html" title="The Leftmost Longest Rule">leftmost
        match</a>, with individual elements matched as follows;
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Construct
                </p>
              </th>
<th>
                <p>
                  What gets matched
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="literal">AtomA AtomB</code>
                </p>
              </td>
<td>
                <p>
                  Locates the best match for <span class="emphasis"><em>AtomA</em></span> that has
                  a following match for <span class="emphasis"><em>AtomB</em></span>.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">Expression1 | Expression2</code>
                </p>
              </td>
<td>
                <p>
                  If <span class="emphasis"><em>Expresion1</em></span> can be matched then returns
                  that match, otherwise attempts to match <span class="emphasis"><em>Expression2</em></span>.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">S{N}</code>
                </p>
              </td>
<td>
                <p>
                  Matches <span class="emphasis"><em>S</em></span> repeated exactly N times.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">S{N,M}</code>
                </p>
              </td>
<td>
                <p>
                  Matches S repeated between N and M times, and as many times as
                  possible.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">S{N,M}?</code>
                </p>
              </td>
<td>
                <p>
                  Matches S repeated between N and M times, and as few times as possible.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">S?, S*, S+</code>
                </p>
              </td>
<td>
                <p>
                  The same as <code class="literal">S{0,1}</code>, <code class="literal">S{0,UINT_MAX}</code>,
                  <code class="literal">S{1,UINT_MAX}</code> respectively.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">S??, S*?, S+?</code>
                </p>
              </td>
<td>
                <p>
                  The same as <code class="literal">S{0,1}?</code>, <code class="literal">S{0,UINT_MAX}?</code>,
                  <code class="literal">S{1,UINT_MAX}?</code> respectively.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">(?&gt;S)</code>
                </p>
              </td>
<td>
                <p>
                  Matches the best match for <span class="emphasis"><em>S</em></span>, and only that.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">(?=S), (?&lt;=S)</code>
                </p>
              </td>
<td>
                <p>
                  Matches only the best match for <span class="emphasis"><em>S</em></span> (this is
                  only visible if there are capturing parenthesis within <span class="emphasis"><em>S</em></span>).
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">(?!S), (?&lt;!S)</code>
                </p>
              </td>
<td>
                <p>
                  Considers only whether a match for S exists or not.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="literal">(?(condition)yes-pattern | no-pattern)</code>
                </p>
              </td>
<td>
                <p>
                  If condition is true, then only yes-pattern is considered, otherwise
                  only no-pattern is considered.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h4>
<a name="boost_regex.syntax.perl_syntax.h45"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.variations"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.variations">Variations</a>
      </h4>
<p>
        The <a class="link" href="../ref/syntax_option_type/syntax_option_type_perl.html" title="Options for Perl Regular Expressions">options
        <code class="literal">normal</code>, <code class="literal">ECMAScript</code>, <code class="literal">JavaScript</code>
        and <code class="literal">JScript</code></a> are all synonyms for <code class="literal">perl</code>.
      </p>
<h4>
<a name="boost_regex.syntax.perl_syntax.h46"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.options"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.options">Options</a>
      </h4>
<p>
        There are a <a class="link" href="../ref/syntax_option_type/syntax_option_type_perl.html" title="Options for Perl Regular Expressions">variety
        of flags</a> that may be combined with the <code class="literal">perl</code> option
        when constructing the regular expression, in particular note that the <code class="literal">newline_alt</code>
        option alters the syntax, while the <code class="literal">collate</code>, <code class="literal">nosubs</code>
        and <code class="literal">icase</code> options modify how the case and locale sensitivity
        are to be applied.
      </p>
<h4>
<a name="boost_regex.syntax.perl_syntax.h47"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.pattern_modifiers"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.pattern_modifiers">Pattern
        Modifiers</a>
      </h4>
<p>
        The perl <code class="literal">smix</code> modifiers can either be applied using a
        <code class="literal">(?smix-smix)</code> prefix to the regular expression, or with
        one of the <a class="link" href="../ref/syntax_option_type/syntax_option_type_perl.html" title="Options for Perl Regular Expressions">regex-compile
        time flags <code class="literal">no_mod_m</code>, <code class="literal">mod_x</code>, <code class="literal">mod_s</code>,
        and <code class="literal">no_mod_s</code></a>.
      </p>
<h4>
<a name="boost_regex.syntax.perl_syntax.h48"></a>
        <span class="phrase"><a name="boost_regex.syntax.perl_syntax.references"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.references">References</a>
      </h4>
<p>
        <a href="http://perldoc.perl.org/perlre.html" target="_top">Perl 5.8</a>.
      </p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 1998-2013 John Maddock<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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