1676 lines
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1676 lines
70 KiB
HTML
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<title>Perl Regular Expression Syntax</title>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_regex.syntax.perl_syntax"></a><a class="link" href="perl_syntax.html" title="Perl Regular Expression Syntax">Perl Regular Expression
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Syntax</a>
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</h3></div></div></div>
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<h4>
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<a name="boost_regex.syntax.perl_syntax.h0"></a>
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<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>
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</h4>
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<p>
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The Perl regular expression syntax is based on that used by the programming
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language Perl . Perl regular expressions are the default behavior in Boost.Regex
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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:
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</p>
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<pre class="programlisting"><span class="comment">// e1 is a case sensitive Perl regular expression: </span>
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<span class="comment">// since Perl is the default option there's no need to explicitly specify the syntax used here:</span>
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<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>
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<span class="comment">// e2 a case insensitive Perl regular expression:</span>
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<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>
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</pre>
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<h4>
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<a name="boost_regex.syntax.perl_syntax.h1"></a>
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<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
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Regular Expression Syntax</a>
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</h4>
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<p>
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In Perl regular expressions, all characters match themselves except for the
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following special characters:
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</p>
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<pre class="programlisting">.[{}()\*+?|^$</pre>
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<p>
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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>.
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</p>
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<h5>
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<a name="boost_regex.syntax.perl_syntax.h2"></a>
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<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>
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</h5>
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<p>
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The single character '.' when used outside of a character set will match
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any single character except:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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The NULL character when the <a class="link" href="../ref/match_flag_type.html" title="match_flag_type">flag
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<code class="literal">match_not_dot_null</code></a> is passed to the matching
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algorithms.
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</li>
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<li class="listitem">
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The newline character when the <a class="link" href="../ref/match_flag_type.html" title="match_flag_type">flag
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<code class="literal">match_not_dot_newline</code></a> is passed to the matching
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algorithms.
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</li>
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</ul></div>
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<h5>
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<a name="boost_regex.syntax.perl_syntax.h3"></a>
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<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>
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</h5>
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<p>
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A '^' character shall match the start of a line.
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</p>
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<p>
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A '$' character shall match the end of a line.
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</p>
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<h5>
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<a name="boost_regex.syntax.perl_syntax.h4"></a>
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<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>
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</h5>
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<p>
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A section beginning <code class="literal">(</code> and ending <code class="literal">)</code>
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acts as a marked sub-expression. Whatever matched the sub-expression is split
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out in a separate field by the matching algorithms. Marked sub-expressions
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can also repeated, or referred to by a back-reference.
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</p>
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<h5>
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<a name="boost_regex.syntax.perl_syntax.h5"></a>
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<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
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grouping</a>
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</h5>
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<p>
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A marked sub-expression is useful to lexically group part of a regular expression,
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but has the side-effect of spitting out an extra field in the result. As
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an alternative you can lexically group part of a regular expression, without
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generating a marked sub-expression by using <code class="literal">(?:</code> and <code class="literal">)</code>
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, for example <code class="literal">(?:ab)+</code> will repeat <code class="literal">ab</code>
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without splitting out any separate sub-expressions.
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</p>
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<h5>
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<a name="boost_regex.syntax.perl_syntax.h6"></a>
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<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>
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</h5>
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<p>
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Any atom (a single character, a marked sub-expression, or a character class)
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can be repeated with the <code class="literal">*</code>, <code class="literal">+</code>, <code class="literal">?</code>,
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and <code class="literal">{}</code> operators.
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</p>
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<p>
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The <code class="literal">*</code> operator will match the preceding atom zero or more
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times, for example the expression <code class="literal">a*b</code> will match any of
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the following:
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</p>
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<pre class="programlisting"><span class="identifier">b</span>
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<span class="identifier">ab</span>
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<span class="identifier">aaaaaaaab</span>
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</pre>
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<p>
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The <code class="literal">+</code> operator will match the preceding atom one or more
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times, for example the expression <code class="literal">a+b</code> will match any of
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the following:
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</p>
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<pre class="programlisting"><span class="identifier">ab</span>
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<span class="identifier">aaaaaaaab</span>
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</pre>
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<p>
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But will not match:
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</p>
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<pre class="programlisting"><span class="identifier">b</span>
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</pre>
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<p>
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The <code class="literal">?</code> operator will match the preceding atom zero or one
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times, for example the expression ca?b will match any of the following:
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</p>
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<pre class="programlisting"><span class="identifier">cb</span>
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<span class="identifier">cab</span>
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</pre>
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<p>
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But will not match:
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</p>
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<pre class="programlisting"><span class="identifier">caab</span>
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</pre>
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<p>
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An atom can also be repeated with a bounded repeat:
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</p>
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<p>
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<code class="literal">a{n}</code> Matches 'a' repeated exactly n times.
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</p>
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<p>
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<code class="literal">a{n,}</code> Matches 'a' repeated n or more times.
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</p>
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<p>
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<code class="literal">a{n, m}</code> Matches 'a' repeated between n and m times inclusive.
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</p>
|
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<p>
|
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For example:
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</p>
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<pre class="programlisting">^a{2,3}$</pre>
|
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<p>
|
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Will match either of:
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</p>
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<pre class="programlisting"><span class="identifier">aa</span>
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<span class="identifier">aaa</span>
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</pre>
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<p>
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But neither of:
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</p>
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<pre class="programlisting"><span class="identifier">a</span>
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<span class="identifier">aaaa</span>
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</pre>
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<p>
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Note that the "{" and "}" characters will treated as
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ordinary literals when used in a context that is not a repeat: this matches
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Perl 5.x behavior. For example in the expressions "ab{1", "ab1}"
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and "a{b}c" the curly brackets are all treated as literals and
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<span class="emphasis"><em>no error will be raised</em></span>.
|
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</p>
|
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<p>
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It is an error to use a repeat operator, if the preceding construct can not
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be repeated, for example:
|
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</p>
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<pre class="programlisting"><span class="identifier">a</span><span class="special">(*)</span>
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</pre>
|
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<p>
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Will raise an error, as there is nothing for the <code class="literal">*</code> operator
|
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to be applied to.
|
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</p>
|
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<h5>
|
|
<a name="boost_regex.syntax.perl_syntax.h7"></a>
|
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<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
|
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greedy repeats</a>
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</h5>
|
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<p>
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The normal repeat operators are "greedy", that is to say they will
|
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consume as much input as possible. There are non-greedy versions available
|
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that will consume as little input as possible while still producing a match.
|
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</p>
|
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<p>
|
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<code class="literal">*?</code> Matches the previous atom zero or more times, while
|
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consuming as little input as possible.
|
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</p>
|
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<p>
|
|
<code class="literal">+?</code> Matches the previous atom one or more times, while
|
|
consuming as little input as possible.
|
|
</p>
|
|
<p>
|
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<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
|
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repeats</a>
|
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</h5>
|
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<p>
|
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By default when a repeated pattern does not match then the engine will backtrack
|
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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
|
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fails to match.
|
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</p>
|
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<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
|
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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>
|
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An escape character followed by a digit <span class="emphasis"><em>n</em></span>, where <span class="emphasis"><em>n</em></span>
|
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is in the range 1-9, matches the same string that was matched by sub-expression
|
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<span class="emphasis"><em>n</em></span>. For example the expression:
|
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</p>
|
|
<pre class="programlisting">^(a*)[^a]*\1$</pre>
|
|
<p>
|
|
Will match the string:
|
|
</p>
|
|
<pre class="programlisting"><span class="identifier">aaabbaaa</span>
|
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</pre>
|
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<p>
|
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But not the string:
|
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</p>
|
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<pre class="programlisting"><span class="identifier">aaabba</span>
|
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</pre>
|
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<p>
|
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You can also use the \g escape for the same function, for example:
|
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</p>
|
|
<div class="informaltable"><table class="table">
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<colgroup>
|
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<col>
|
|
<col>
|
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</colgroup>
|
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<thead><tr>
|
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<th>
|
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<p>
|
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Escape
|
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</p>
|
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</th>
|
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<th>
|
|
<p>
|
|
Meaning
|
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</p>
|
|
</th>
|
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</tr></thead>
|
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<tbody>
|
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<tr>
|
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<td>
|
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<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<two></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, À, Á, Â, Ã, Ä, Å, A, à, á, â, ã, ä and å. 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"><</code> Matches the start of a word.
|
|
</p>
|
|
<p>
|
|
<code class="literal">></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">(?>\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">(?<</span><span class="identifier">NAME</span><span class="special">></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<NAME></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">(?<=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">(?<!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">(?>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) (?&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">(?&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">(?(<<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">(?('<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&<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">(?>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), (?<=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), (?<!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>
|
|
</div>
|
|
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
|
<td align="left"></td>
|
|
<td align="right"><div class="copyright-footer">Copyright © 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>)
|
|
</p>
|
|
</div></td>
|
|
</tr></table>
|
|
<hr>
|
|
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