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| <div class="section" lang="en"> |
| <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> |
| <a name="boost_regex.captures.marked_sub_expressions"></a><h5> |
| <a name="id905654"></a> |
| <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> |
| <a name="boost_regex.captures.unmatched_sub_expressions"></a><h5> |
| <a name="id906123"></a> |
| <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> |
| <a name="boost_regex.captures.repeated_captures"></a><h5> |
| <a name="id906162"></a> |
| <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 type="disc"> |
| <li> |
| 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> |
| 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 -2010 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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