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- </div>
- <div class="section">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="boost_regex.captures"></a><a class="link" href="captures.html" title="Understanding Marked Sub-Expressions and Captures">Understanding Marked Sub-Expressions
- and Captures</a>
- </h2></div></div></div>
- <p>
- Captures are the iterator ranges that are "captured" by marked sub-expressions
- as a regular expression gets matched. Each marked sub-expression can result
- in more than one capture, if it is matched more than once. This document explains
- how captures and marked sub-expressions in Boost.Regex are represented and
- accessed.
- </p>
- <h5>
- <a name="boost_regex.captures.h0"></a>
- <span class="phrase"><a name="boost_regex.captures.marked_sub_expressions"></a></span><a class="link" href="captures.html#boost_regex.captures.marked_sub_expressions">Marked
- sub-expressions</a>
- </h5>
- <p>
- Every time a Perl regular expression contains a parenthesis group <code class="computeroutput"><span class="special">()</span></code>, it spits out an extra field, known as a
- marked sub-expression, for example the expression:
- </p>
- <pre class="programlisting">(\w+)\W+(\w+)</pre>
- <p>
- Has two marked sub-expressions (known as $1 and $2 respectively), in addition
- the complete match is known as $&, everything before the first match as
- $`, and everything after the match as $'. So if the above expression is searched
- for within <code class="computeroutput"><span class="string">"@abc def--"</span></code>,
- then we obtain:
- </p>
- <div class="informaltable"><table class="table">
- <colgroup>
- <col>
- <col>
- </colgroup>
- <thead><tr>
- <th>
- <p>
- Sub-expression
- </p>
- </th>
- <th>
- <p>
- Text found
- </p>
- </th>
- </tr></thead>
- <tbody>
- <tr>
- <td>
- <p>
- $`
- </p>
- </td>
- <td>
- <p>
- "@"
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- $&
- </p>
- </td>
- <td>
- <p>
- "abc def"
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- $1
- </p>
- </td>
- <td>
- <p>
- "abc"
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- $2
- </p>
- </td>
- <td>
- <p>
- "def"
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- $'
- </p>
- </td>
- <td>
- <p>
- "--"
- </p>
- </td>
- </tr>
- </tbody>
- </table></div>
- <p>
- In Boost.Regex all these are accessible via the <a class="link" href="ref/match_results.html" title="match_results"><code class="computeroutput"><span class="identifier">match_results</span></code></a> class that gets filled
- in when calling one of the regular expression matching algorithms ( <a class="link" href="ref/regex_search.html" title="regex_search"><code class="computeroutput"><span class="identifier">regex_search</span></code></a>, <a class="link" href="ref/regex_match.html" title="regex_match"><code class="computeroutput"><span class="identifier">regex_match</span></code></a>, or <a class="link" href="ref/regex_iterator.html" title="regex_iterator"><code class="computeroutput"><span class="identifier">regex_iterator</span></code></a>). So given:
- </p>
- <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">match_results</span><span class="special"><</span><span class="identifier">IteratorType</span><span class="special">></span> <span class="identifier">m</span><span class="special">;</span>
- </pre>
- <p>
- The Perl and Boost.Regex equivalents are as follows:
- </p>
- <div class="informaltable"><table class="table">
- <colgroup>
- <col>
- <col>
- </colgroup>
- <thead><tr>
- <th>
- <p>
- Perl
- </p>
- </th>
- <th>
- <p>
- Boost.Regex
- </p>
- </th>
- </tr></thead>
- <tbody>
- <tr>
- <td>
- <p>
- $`
- </p>
- </td>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">m</span><span class="special">.</span><span class="identifier">prefix</span><span class="special">()</span></code>
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- $&
- </p>
- </td>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">m</span><span class="special">[</span><span class="number">0</span><span class="special">]</span></code>
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- $n
- </p>
- </td>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">m</span><span class="special">[</span><span class="identifier">n</span><span class="special">]</span></code>
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- $'
- </p>
- </td>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">m</span><span class="special">.</span><span class="identifier">suffix</span><span class="special">()</span></code>
- </p>
- </td>
- </tr>
- </tbody>
- </table></div>
- <p>
- In Boost.Regex each sub-expression match is represented by a <a class="link" href="ref/sub_match.html" title="sub_match"><code class="computeroutput"><span class="identifier">sub_match</span></code></a> object, this is basically
- just a pair of iterators denoting the start and end position of the sub-expression
- match, but there are some additional operators provided so that objects of
- type <a class="link" href="ref/sub_match.html" title="sub_match"><code class="computeroutput"><span class="identifier">sub_match</span></code></a>
- behave a lot like a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_string</span></code>: for example they are implicitly
- convertible to a <code class="computeroutput"><span class="identifier">basic_string</span></code>,
- they can be compared to a string, added to a string, or streamed out to an
- output stream.
- </p>
- <h5>
- <a name="boost_regex.captures.h1"></a>
- <span class="phrase"><a name="boost_regex.captures.unmatched_sub_expressions"></a></span><a class="link" href="captures.html#boost_regex.captures.unmatched_sub_expressions">Unmatched
- Sub-Expressions</a>
- </h5>
- <p>
- When a regular expression match is found there is no need for all of the marked
- sub-expressions to have participated in the match, for example the expression:
- </p>
- <pre class="programlisting">(abc)|(def)</pre>
- <p>
- can match either $1 or $2, but never both at the same time. In Boost.Regex
- you can determine which sub-expressions matched by accessing the <code class="computeroutput"><span class="identifier">sub_match</span><span class="special">::</span><span class="identifier">matched</span></code> data member.
- </p>
- <h5>
- <a name="boost_regex.captures.h2"></a>
- <span class="phrase"><a name="boost_regex.captures.repeated_captures"></a></span><a class="link" href="captures.html#boost_regex.captures.repeated_captures">Repeated
- Captures</a>
- </h5>
- <p>
- When a marked sub-expression is repeated, then the sub-expression gets "captured"
- multiple times, however normally only the final capture is available, for example
- if
- </p>
- <pre class="programlisting">(?:(\w+)\W+)+</pre>
- <p>
- is matched against
- </p>
- <pre class="programlisting">one fine day</pre>
- <p>
- Then $1 will contain the string "day", and all the previous captures
- will have been forgotten.
- </p>
- <p>
- However, Boost.Regex has an experimental feature that allows all the capture
- information to be retained - this is accessed either via the <code class="computeroutput"><span class="identifier">match_results</span><span class="special">::</span><span class="identifier">captures</span></code> member function or the <code class="computeroutput"><span class="identifier">sub_match</span><span class="special">::</span><span class="identifier">captures</span></code> member function. These functions
- return a container that contains a sequence of all the captures obtained during
- the regular expression matching. The following example program shows how this
- information may be used:
- </p>
- <pre class="programlisting"><span class="preprocessor">#include</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">hpp</span><span class="special">></span>
- <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
- <span class="keyword">void</span> <span class="identifier">print_captures</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">regx</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
- <span class="special">{</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e</span><span class="special">(</span><span class="identifier">regx</span><span class="special">);</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">smatch</span> <span class="identifier">what</span><span class="special">;</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Expression: \""</span> <span class="special"><<</span> <span class="identifier">regx</span> <span class="special"><<</span> <span class="string">"\"\n"</span><span class="special">;</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Text: \""</span> <span class="special"><<</span> <span class="identifier">text</span> <span class="special"><<</span> <span class="string">"\"\n"</span><span class="special">;</span>
- <span class="keyword">if</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex_match</span><span class="special">(</span><span class="identifier">text</span><span class="special">,</span> <span class="identifier">what</span><span class="special">,</span> <span class="identifier">e</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">match_extra</span><span class="special">))</span>
- <span class="special">{</span>
- <span class="keyword">unsigned</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">j</span><span class="special">;</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"** Match found **\n Sub-Expressions:\n"</span><span class="special">;</span>
- <span class="keyword">for</span><span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">what</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">" $"</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">" = \""</span> <span class="special"><<</span> <span class="identifier">what</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special"><<</span> <span class="string">"\"\n"</span><span class="special">;</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">" Captures:\n"</span><span class="special">;</span>
- <span class="keyword">for</span><span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">what</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
- <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">" $"</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">" = {"</span><span class="special">;</span>
- <span class="keyword">for</span><span class="special">(</span><span class="identifier">j</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">j</span> <span class="special"><</span> <span class="identifier">what</span><span class="special">.</span><span class="identifier">captures</span><span class="special">(</span><span class="identifier">i</span><span class="special">).</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">j</span><span class="special">)</span>
- <span class="special">{</span>
- <span class="keyword">if</span><span class="special">(</span><span class="identifier">j</span><span class="special">)</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">", "</span><span class="special">;</span>
- <span class="keyword">else</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">" "</span><span class="special">;</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"\""</span> <span class="special"><<</span> <span class="identifier">what</span><span class="special">.</span><span class="identifier">captures</span><span class="special">(</span><span class="identifier">i</span><span class="special">)[</span><span class="identifier">j</span><span class="special">]</span> <span class="special"><<</span> <span class="string">"\""</span><span class="special">;</span>
- <span class="special">}</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">" }\n"</span><span class="special">;</span>
- <span class="special">}</span>
- <span class="special">}</span>
- <span class="keyword">else</span>
- <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"** No Match found **\n"</span><span class="special">;</span>
- <span class="special">}</span>
- <span class="special">}</span>
- <span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="special">,</span> <span class="keyword">char</span><span class="special">*</span> <span class="special">[])</span>
- <span class="special">{</span>
- <span class="identifier">print_captures</span><span class="special">(</span><span class="string">"(([[:lower:]]+)|([[:upper:]]+))+"</span><span class="special">,</span> <span class="string">"aBBcccDDDDDeeeeeeee"</span><span class="special">);</span>
- <span class="identifier">print_captures</span><span class="special">(</span><span class="string">"(.*)bar|(.*)bah"</span><span class="special">,</span> <span class="string">"abcbar"</span><span class="special">);</span>
- <span class="identifier">print_captures</span><span class="special">(</span><span class="string">"(.*)bar|(.*)bah"</span><span class="special">,</span> <span class="string">"abcbah"</span><span class="special">);</span>
- <span class="identifier">print_captures</span><span class="special">(</span><span class="string">"^(?:(\\w+)|(?>\\W+))*$"</span><span class="special">,</span>
- <span class="string">"now is the time for all good men to come to the aid of the party"</span><span class="special">);</span>
- <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
- <span class="special">}</span>
- </pre>
- <p>
- Which produces the following output:
- </p>
- <pre class="programlisting">Expression: "(([[:lower:]]+)|([[:upper:]]+))+"
- Text: "aBBcccDDDDDeeeeeeee"
- ** Match found **
- Sub-Expressions:
- $0 = "aBBcccDDDDDeeeeeeee"
- $1 = "eeeeeeee"
- $2 = "eeeeeeee"
- $3 = "DDDDD"
- Captures:
- $0 = { "aBBcccDDDDDeeeeeeee" }
- $1 = { "a", "BB", "ccc", "DDDDD", "eeeeeeee" }
- $2 = { "a", "ccc", "eeeeeeee" }
- $3 = { "BB", "DDDDD" }
- Expression: "(.*)bar|(.*)bah"
- Text: "abcbar"
- ** Match found **
- Sub-Expressions:
- $0 = "abcbar"
- $1 = "abc"
- $2 = ""
- Captures:
- $0 = { "abcbar" }
- $1 = { "abc" }
- $2 = { }
- Expression: "(.*)bar|(.*)bah"
- Text: "abcbah"
- ** Match found **
- Sub-Expressions:
- $0 = "abcbah"
- $1 = ""
- $2 = "abc"
- Captures:
- $0 = { "abcbah" }
- $1 = { }
- $2 = { "abc" }
- Expression: "^(?:(\w+)|(?>\W+))*$"
- Text: "now is the time for all good men to come to the aid of the party"
- ** Match found **
- Sub-Expressions:
- $0 = "now is the time for all good men to come to the aid of the party"
- $1 = "party"
- Captures:
- $0 = { "now is the time for all good men to come to the aid of the party" }
- $1 = { "now", "is", "the", "time", "for", "all", "good", "men", "to",
- "come", "to", "the", "aid", "of", "the", "party" }
- </pre>
- <p>
- Unfortunately enabling this feature has an impact on performance (even if you
- don't use it), and a much bigger impact if you do use it, therefore to use
- this feature you need to:
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- Define BOOST_REGEX_MATCH_EXTRA for all translation units including the
- library source (the best way to do this is to uncomment this define in
- boost/regex/user.hpp and then rebuild everything.
- </li>
- <li class="listitem">
- Pass the match_extra flag to the particular algorithms where you actually
- need the captures information (regex_search, regex_match, or regex_iterator).
- </li>
- </ul></div>
- </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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