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<title>Understanding Marked Sub-Expressions and Captures</title>
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<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>
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