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  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="boost_regex.captures"></a><a class="link" href="captures.html" title="Understanding Marked Sub-Expressions and Captures">Understanding Marked Sub-Expressions
  28     and Captures</a>
  29 </h2></div></div></div>
  30 <p>
  31       Captures are the iterator ranges that are "captured" by marked sub-expressions
  32       as a regular expression gets matched. Each marked sub-expression can result
  33       in more than one capture, if it is matched more than once. This document explains
  34       how captures and marked sub-expressions in Boost.Regex are represented and
  35       accessed.
  36     </p>
  37 <h5>
  38 <a name="boost_regex.captures.h0"></a>
  39       <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
  40       sub-expressions</a>
  41     </h5>
  42 <p>
  43       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
  44       marked sub-expression, for example the expression:
  45     </p>
  46 <pre class="programlisting">(\w+)\W+(\w+)</pre>
  47 <p>
  48       Has two marked sub-expressions (known as $1 and $2 respectively), in addition
  49       the complete match is known as $&amp;, everything before the first match as
  50       $`, and everything after the match as $'. So if the above expression is searched
  51       for within <code class="computeroutput"><span class="string">"@abc def--"</span></code>,
  52       then we obtain:
  53     </p>
  54 <div class="informaltable"><table class="table">
  55 <colgroup>
  56 <col>
  57 <col>
  58 </colgroup>
  59 <thead><tr>
  60 <th>
  61               <p>
  62                 Sub-expression
  63               </p>
  64             </th>
  65 <th>
  66               <p>
  67                 Text found
  68               </p>
  69             </th>
  70 </tr></thead>
  71 <tbody>
  72 <tr>
  73 <td>
  74               <p>
  75                 $`
  76               </p>
  77             </td>
  78 <td>
  79               <p>
  80                 "@"
  81               </p>
  82             </td>
  83 </tr>
  84 <tr>
  85 <td>
  86               <p>
  87                 $&amp;
  88               </p>
  89             </td>
  90 <td>
  91               <p>
  92                 "abc def"
  93               </p>
  94             </td>
  95 </tr>
  96 <tr>
  97 <td>
  98               <p>
  99                 $1
 100               </p>
 101             </td>
 102 <td>
 103               <p>
 104                 "abc"
 105               </p>
 106             </td>
 107 </tr>
 108 <tr>
 109 <td>
 110               <p>
 111                 $2
 112               </p>
 113             </td>
 114 <td>
 115               <p>
 116                 "def"
 117               </p>
 118             </td>
 119 </tr>
 120 <tr>
 121 <td>
 122               <p>
 123                 $'
 124               </p>
 125             </td>
 126 <td>
 127               <p>
 128                 "--"
 129               </p>
 130             </td>
 131 </tr>
 132 </tbody>
 133 </table></div>
 134 <p>
 135       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
 136       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:
 137     </p>
 138 <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">match_results</span><span class="special">&lt;</span><span class="identifier">IteratorType</span><span class="special">&gt;</span> <span class="identifier">m</span><span class="special">;</span>
 139 </pre>
 140 <p>
 141       The Perl and Boost.Regex equivalents are as follows:
 142     </p>
 143 <div class="informaltable"><table class="table">
 144 <colgroup>
 145 <col>
 146 <col>
 147 </colgroup>
 148 <thead><tr>
 149 <th>
 150               <p>
 151                 Perl
 152               </p>
 153             </th>
 154 <th>
 155               <p>
 156                 Boost.Regex
 157               </p>
 158             </th>
 159 </tr></thead>
 160 <tbody>
 161 <tr>
 162 <td>
 163               <p>
 164                 $`
 165               </p>
 166             </td>
 167 <td>
 168               <p>
 169                 <code class="computeroutput"><span class="identifier">m</span><span class="special">.</span><span class="identifier">prefix</span><span class="special">()</span></code>
 170               </p>
 171             </td>
 172 </tr>
 173 <tr>
 174 <td>
 175               <p>
 176                 $&amp;
 177               </p>
 178             </td>
 179 <td>
 180               <p>
 181                 <code class="computeroutput"><span class="identifier">m</span><span class="special">[</span><span class="number">0</span><span class="special">]</span></code>
 182               </p>
 183             </td>
 184 </tr>
 185 <tr>
 186 <td>
 187               <p>
 188                 $n
 189               </p>
 190             </td>
 191 <td>
 192               <p>
 193                 <code class="computeroutput"><span class="identifier">m</span><span class="special">[</span><span class="identifier">n</span><span class="special">]</span></code>
 194               </p>
 195             </td>
 196 </tr>
 197 <tr>
 198 <td>
 199               <p>
 200                 $'
 201               </p>
 202             </td>
 203 <td>
 204               <p>
 205                 <code class="computeroutput"><span class="identifier">m</span><span class="special">.</span><span class="identifier">suffix</span><span class="special">()</span></code>
 206               </p>
 207             </td>
 208 </tr>
 209 </tbody>
 210 </table></div>
 211 <p>
 212       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
 213       just a pair of iterators denoting the start and end position of the sub-expression
 214       match, but there are some additional operators provided so that objects of
 215       type <a class="link" href="ref/sub_match.html" title="sub_match"><code class="computeroutput"><span class="identifier">sub_match</span></code></a>
 216       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
 217       convertible to a <code class="computeroutput"><span class="identifier">basic_string</span></code>,
 218       they can be compared to a string, added to a string, or streamed out to an
 219       output stream.
 220     </p>
 221 <h5>
 222 <a name="boost_regex.captures.h1"></a>
 223       <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
 224       Sub-Expressions</a>
 225     </h5>
 226 <p>
 227       When a regular expression match is found there is no need for all of the marked
 228       sub-expressions to have participated in the match, for example the expression:
 229     </p>
 230 <pre class="programlisting">(abc)|(def)</pre>
 231 <p>
 232       can match either $1 or $2, but never both at the same time. In Boost.Regex
 233       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.
 234     </p>
 235 <h5>
 236 <a name="boost_regex.captures.h2"></a>
 237       <span class="phrase"><a name="boost_regex.captures.repeated_captures"></a></span><a class="link" href="captures.html#boost_regex.captures.repeated_captures">Repeated
 238       Captures</a>
 239     </h5>
 240 <p>
 241       When a marked sub-expression is repeated, then the sub-expression gets "captured"
 242       multiple times, however normally only the final capture is available, for example
 243       if
 244     </p>
 245 <pre class="programlisting">(?:(\w+)\W+)+</pre>
 246 <p>
 247       is matched against
 248     </p>
 249 <pre class="programlisting">one fine day</pre>
 250 <p>
 251       Then $1 will contain the string "day", and all the previous captures
 252       will have been forgotten.
 253     </p>
 254 <p>
 255       However, Boost.Regex has an experimental feature that allows all the capture
 256       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
 257       return a container that contains a sequence of all the captures obtained during
 258       the regular expression matching. The following example program shows how this
 259       information may be used:
 260     </p>
 261 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
 262 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
 263
 264 <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">&amp;</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">&amp;</span> <span class="identifier">text</span><span class="special">)</span>
 265 <span class="special">{</span>
 266    <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>
 267    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">smatch</span> <span class="identifier">what</span><span class="special">;</span>
 268    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Expression:  \""</span> <span class="special">&lt;&lt;</span> <span class="identifier">regx</span> <span class="special">&lt;&lt;</span> <span class="string">"\"\n"</span><span class="special">;</span>
 269    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Text:        \""</span> <span class="special">&lt;&lt;</span> <span class="identifier">text</span> <span class="special">&lt;&lt;</span> <span class="string">"\"\n"</span><span class="special">;</span>
 270    <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>
 271    <span class="special">{</span>
 272       <span class="keyword">unsigned</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">j</span><span class="special">;</span>
 273       <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"** Match found **\n   Sub-Expressions:\n"</span><span class="special">;</span>
 274       <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">&lt;</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>
 275          <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"      $"</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">" = \""</span> <span class="special">&lt;&lt;</span> <span class="identifier">what</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="string">"\"\n"</span><span class="special">;</span>
 276       <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"   Captures:\n"</span><span class="special">;</span>
 277       <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">&lt;</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>
 278       <span class="special">{</span>
 279          <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"      $"</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">" = {"</span><span class="special">;</span>
 280          <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">&lt;</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>
 281          <span class="special">{</span>
 282             <span class="keyword">if</span><span class="special">(</span><span class="identifier">j</span><span class="special">)</span>
 283                <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">", "</span><span class="special">;</span>
 284             <span class="keyword">else</span>
 285                <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">" "</span><span class="special">;</span>
 286             <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\""</span> <span class="special">&lt;&lt;</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">&lt;&lt;</span> <span class="string">"\""</span><span class="special">;</span>
 287          <span class="special">}</span>
 288          <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">" }\n"</span><span class="special">;</span>
 289       <span class="special">}</span>
 290    <span class="special">}</span>
 291    <span class="keyword">else</span>
 292    <span class="special">{</span>
 293       <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"** No Match found **\n"</span><span class="special">;</span>
 294    <span class="special">}</span>
 295 <span class="special">}</span>
 296
 297 <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>
 298 <span class="special">{</span>
 299    <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>
 300    <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>
 301    <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>
 302    <span class="identifier">print_captures</span><span class="special">(</span><span class="string">"^(?:(\\w+)|(?&gt;\\W+))*$"</span><span class="special">,</span>
 303       <span class="string">"now is the time for all good men to come to the aid of the party"</span><span class="special">);</span>
 304    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 305 <span class="special">}</span>
 306 </pre>
 307 <p>
 308       Which produces the following output:
 309     </p>
 310 <pre class="programlisting">Expression:  "(([[:lower:]]+)|([[:upper:]]+))+"
 311 Text:        "aBBcccDDDDDeeeeeeee"
 312 ** Match found **
 313    Sub-Expressions:
 314       $0 = "aBBcccDDDDDeeeeeeee"
 315       $1 = "eeeeeeee"
 316       $2 = "eeeeeeee"
 317       $3 = "DDDDD"
 318    Captures:
 319       $0 = { "aBBcccDDDDDeeeeeeee" }
 320       $1 = { "a", "BB", "ccc", "DDDDD", "eeeeeeee" }
 321       $2 = { "a", "ccc", "eeeeeeee" }
 322       $3 = { "BB", "DDDDD" }
 323 Expression:  "(.*)bar|(.*)bah"
 324 Text:        "abcbar"
 325 ** Match found **
 326    Sub-Expressions:
 327       $0 = "abcbar"
 328       $1 = "abc"
 329       $2 = ""
 330    Captures:
 331       $0 = { "abcbar" }
 332       $1 = { "abc" }
 333       $2 = { }
 334 Expression:  "(.*)bar|(.*)bah"
 335 Text:        "abcbah"
 336 ** Match found **
 337    Sub-Expressions:
 338       $0 = "abcbah"
 339       $1 = ""
 340       $2 = "abc"
 341    Captures:
 342       $0 = { "abcbah" }
 343       $1 = { }
 344       $2 = { "abc" }
 345 Expression:  "^(?:(\w+)|(?&gt;\W+))*$"
 346 Text:        "now is the time for all good men to come to the aid of the party"
 347 ** Match found **
 348    Sub-Expressions:
 349       $0 = "now is the time for all good men to come to the aid of the party"
 350       $1 = "party"
 351    Captures:
 352       $0 = { "now is the time for all good men to come to the aid of the party" }
 353       $1 = { "now", "is", "the", "time", "for", "all", "good", "men", "to",
 354          "come", "to", "the", "aid", "of", "the", "party" }
 355 </pre>
 356 <p>
 357       Unfortunately enabling this feature has an impact on performance (even if you
 358       don't use it), and a much bigger impact if you do use it, therefore to use
 359       this feature you need to:
 360     </p>
 361 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 362 <li class="listitem">
 363           Define BOOST_REGEX_MATCH_EXTRA for all translation units including the
 364           library source (the best way to do this is to uncomment this define in
 365           boost/regex/user.hpp and then rebuild everything.
 366         </li>
 367 <li class="listitem">
 368           Pass the match_extra flag to the particular algorithms where you actually
 369           need the captures information (regex_search, regex_match, or regex_iterator).
 370         </li>
 371 </ul></div>
 372 </div>
 373 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 374 <td align="left"></td>
 375 <td align="right"><div class="copyright-footer">Copyright &#169; 1998-2013 John Maddock<p>
 376         Distributed under the Boost Software License, Version 1.0. (See accompanying
 377         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>)
 378       </p>
 379 </div></td>
 380 </tr></table>
 381 <hr>
 382 <div class="spirit-nav">
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