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| 3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
| 4 | <title>Partial Matches</title> | |
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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
| 27 | <a name="boost_regex.partial_matches"></a><a class="link" href="partial_matches.html" title="Partial Matches">Partial Matches</a> | |
| 28 | </h2></div></div></div> | |
| 29 | <p> | |
| 30 | The <a class="link" href="ref/match_flag_type.html" title="match_flag_type"><code class="computeroutput"><span class="identifier">match_flag_type</span></code></a> | |
| 31 | <code class="computeroutput"><span class="identifier">match_partial</span></code> can be passed | |
| 32 | to the following algorithms: <a class="link" href="ref/regex_match.html" title="regex_match"><code class="computeroutput"><span class="identifier">regex_match</span></code></a>, <a class="link" href="ref/regex_search.html" title="regex_search"><code class="computeroutput"><span class="identifier">regex_search</span></code></a>, and <a class="link" href="ref/deprecated_interfaces/regex_grep.html" title="regex_grep (Deprecated)"><code class="computeroutput"><span class="identifier">regex_grep</span></code></a>, and used with the iterator | |
| 33 | <a class="link" href="ref/regex_iterator.html" title="regex_iterator"><code class="computeroutput"><span class="identifier">regex_iterator</span></code></a>. | |
| 34 | When used it indicates that partial as well as full matches should be found. | |
| 35 | A partial match is one that matched one or more characters at the end of the | |
| 36 | text input, but did not match all of the regular expression (although it may | |
| 37 | have done so had more input been available). Partial matches are typically | |
| 38 | used when either validating data input (checking each character as it is entered | |
| 39 | on the keyboard), or when searching texts that are either too long to load | |
| 40 | into memory (or even into a memory mapped file), or are of indeterminate length | |
| 41 | (for example the source may be a socket or similar). Partial and full matches | |
| 42 | can be differentiated as shown in the following table (the variable M represents | |
| 43 | an instance of <a class="link" href="ref/match_results.html" title="match_results"><code class="computeroutput"><span class="identifier">match_results</span></code></a> as filled in by <a class="link" href="ref/regex_match.html" title="regex_match"><code class="computeroutput"><span class="identifier">regex_match</span></code></a>, | |
| 44 | <a class="link" href="ref/regex_search.html" title="regex_search"><code class="computeroutput"><span class="identifier">regex_search</span></code></a> | |
| 45 | or <a class="link" href="ref/deprecated_interfaces/regex_grep.html" title="regex_grep (Deprecated)"><code class="computeroutput"><span class="identifier">regex_grep</span></code></a>): | |
| 46 | </p> | |
| 47 | <div class="informaltable"><table class="table"> | |
| 48 | <colgroup> | |
| 49 | <col> | |
| 50 | <col> | |
| 51 | <col> | |
| 52 | <col> | |
| 53 | <col> | |
| 54 | </colgroup> | |
| 55 | <thead><tr> | |
| 56 | <th> | |
| 57 | </th> | |
| 58 | <th> | |
| 59 | <p> | |
| 60 | Result | |
| 61 | </p> | |
| 62 | </th> | |
| 63 | <th> | |
| 64 | <p> | |
| 65 | M[0].matched | |
| 66 | </p> | |
| 67 | </th> | |
| 68 | <th> | |
| 69 | <p> | |
| 70 | M[0].first | |
| 71 | </p> | |
| 72 | </th> | |
| 73 | <th> | |
| 74 | <p> | |
| 75 | M[0].second | |
| 76 | </p> | |
| 77 | </th> | |
| 78 | </tr></thead> | |
| 79 | <tbody> | |
| 80 | <tr> | |
| 81 | <td> | |
| 82 | <p> | |
| 83 | No match | |
| 84 | </p> | |
| 85 | </td> | |
| 86 | <td> | |
| 87 | <p> | |
| 88 | False | |
| 89 | </p> | |
| 90 | </td> | |
| 91 | <td> | |
| 92 | <p> | |
| 93 | Undefined | |
| 94 | </p> | |
| 95 | </td> | |
| 96 | <td> | |
| 97 | <p> | |
| 98 | Undefined | |
| 99 | </p> | |
| 100 | </td> | |
| 101 | <td> | |
| 102 | <p> | |
| 103 | Undefined | |
| 104 | </p> | |
| 105 | </td> | |
| 106 | </tr> | |
| 107 | <tr> | |
| 108 | <td> | |
| 109 | <p> | |
| 110 | Partial match | |
| 111 | </p> | |
| 112 | </td> | |
| 113 | <td> | |
| 114 | <p> | |
| 115 | True | |
| 116 | </p> | |
| 117 | </td> | |
| 118 | <td> | |
| 119 | <p> | |
| 120 | False | |
| 121 | </p> | |
| 122 | </td> | |
| 123 | <td> | |
| 124 | <p> | |
| 125 | Start of partial match. | |
| 126 | </p> | |
| 127 | </td> | |
| 128 | <td> | |
| 129 | <p> | |
| 130 | End of partial match (end of text). | |
| 131 | </p> | |
| 132 | </td> | |
| 133 | </tr> | |
| 134 | <tr> | |
| 135 | <td> | |
| 136 | <p> | |
| 137 | Full match | |
| 138 | </p> | |
| 139 | </td> | |
| 140 | <td> | |
| 141 | <p> | |
| 142 | True | |
| 143 | </p> | |
| 144 | </td> | |
| 145 | <td> | |
| 146 | <p> | |
| 147 | True | |
| 148 | </p> | |
| 149 | </td> | |
| 150 | <td> | |
| 151 | <p> | |
| 152 | Start of full match. | |
| 153 | </p> | |
| 154 | </td> | |
| 155 | <td> | |
| 156 | <p> | |
| 157 | End of full match. | |
| 158 | </p> | |
| 159 | </td> | |
| 160 | </tr> | |
| 161 | </tbody> | |
| 162 | </table></div> | |
| 163 | <p> | |
| 164 | Be aware that using partial matches can sometimes result in somewhat imperfect | |
| 165 | behavior: | |
| 166 | </p> | |
| 167 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
| 168 | <li class="listitem"> | |
| 169 | There are some expressions, such as ".*abc" that will always | |
| 170 | produce a partial match. This problem can be reduced by careful construction | |
| 171 | of the regular expressions used, or by setting flags like match_not_dot_newline | |
| 172 | so that expressions like .* can't match past line boundaries. | |
| 173 | </li> | |
| 174 | <li class="listitem"> | |
| 175 | Boost.Regex currently prefers leftmost matches to full matches, so for | |
| 176 | example matching "abc|b" against "ab" produces a partial | |
| 177 | match against the "ab" rather than a full match against "b". | |
| 178 | It's more efficient to work this way, but may not be the behavior you want | |
| 179 | in all situations. | |
| 180 | </li> | |
| 181 | <li class="listitem"> | |
| 182 | There are situations where full matches are found even though partial matches | |
| 183 | are also possible: for example if the partial string terminates with "abc" | |
| 184 | and the regular expression is "\w+", then a full match is found | |
| 185 | even though there may be more alphabetical characters to come. This particular | |
| 186 | case can be detected by checking if the match found terminates at the end | |
| 187 | of current input string. However, there are situations where that is not | |
| 188 | possible: for example an expression such as "abc.*123" may always | |
| 189 | have longer matches available since it could conceivably match the entire | |
| 190 | input string (no matter how long it may be). | |
| 191 | </li> | |
| 192 | </ul></div> | |
| 193 | <p> | |
| 194 | The following example tests to see whether the text could be a valid credit | |
| 195 | card number, as the user presses a key, the character entered would be added | |
| 196 | to the string being built up, and passed to <code class="computeroutput"><span class="identifier">is_possible_card_number</span></code>. | |
| 197 | If this returns true then the text could be a valid card number, so the user | |
| 198 | interface's OK button would be enabled. If it returns false, then this is not | |
| 199 | yet a valid card number, but could be with more input, so the user interface | |
| 200 | would disable the OK button. Finally, if the procedure throws an exception | |
| 201 | the input could never become a valid number, and the inputted character must | |
| 202 | be discarded, and a suitable error indication displayed to the user. | |
| 203 | </p> | |
| 204 | <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span> | |
| 205 | <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> | |
| 206 | <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> | |
| 207 | ||
| 208 | <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="string">"(\\d{3,4})[- ]?(\\d{4})[- ]?(\\d{4})[- ]?(\\d{4})"</span><span class="special">);</span> | |
| 209 | ||
| 210 | <span class="keyword">bool</span> <span class="identifier">is_possible_card_number</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">input</span><span class="special">)</span> | |
| 211 | <span class="special">{</span> | |
| 212 | <span class="comment">//</span> | |
| 213 | <span class="comment">// return false for partial match, true for full match, or throw for</span> | |
| 214 | <span class="comment">// impossible match based on what we have so far...</span> | |
| 215 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">match_results</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">const_iterator</span><span class="special">></span> <span class="identifier">what</span><span class="special">;</span> | |
| 216 | <span class="keyword">if</span><span class="special">(</span><span class="number">0</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">input</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_default</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">match_partial</span><span class="special">))</span> | |
| 217 | <span class="special">{</span> | |
| 218 | <span class="comment">// the input so far could not possibly be valid so reject it:</span> | |
| 219 | <span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span> | |
| 220 | <span class="string">"Invalid data entered - this could not possibly be a valid card number"</span><span class="special">);</span> | |
| 221 | <span class="special">}</span> | |
| 222 | <span class="comment">// OK so far so good, but have we finished?</span> | |
| 223 | <span class="keyword">if</span><span class="special">(</span><span class="identifier">what</span><span class="special">[</span><span class="number">0</span><span class="special">].</span><span class="identifier">matched</span><span class="special">)</span> | |
| 224 | <span class="special">{</span> | |
| 225 | <span class="comment">// excellent, we have a result:</span> | |
| 226 | <span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span> | |
| 227 | <span class="special">}</span> | |
| 228 | <span class="comment">// what we have so far is only a partial match...</span> | |
| 229 | <span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span> | |
| 230 | <span class="special">}</span> | |
| 231 | </pre> | |
| 232 | <p> | |
| 233 | In the following example, text input is taken from a stream containing an unknown | |
| 234 | amount of text; this example simply counts the number of html tags encountered | |
| 235 | in the stream. The text is loaded into a buffer and searched a part at a time, | |
| 236 | if a partial match was encountered, then the partial match gets searched a | |
| 237 | second time as the start of the next batch of text: | |
| 238 | </p> | |
| 239 | <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> | |
| 240 | <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">fstream</span><span class="special">></span> | |
| 241 | <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">sstream</span><span class="special">></span> | |
| 242 | <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span> | |
| 243 | <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> | |
| 244 | ||
| 245 | <span class="comment">// match some kind of html tag:</span> | |
| 246 | <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="string">"<[^>]*>"</span><span class="special">);</span> | |
| 247 | <span class="comment">// count how many:</span> | |
| 248 | <span class="keyword">unsigned</span> <span class="keyword">int</span> <span class="identifier">tags</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> | |
| 249 | ||
| 250 | <span class="keyword">void</span> <span class="identifier">search</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span><span class="special">&</span> <span class="identifier">is</span><span class="special">)</span> | |
| 251 | <span class="special">{</span> | |
| 252 | <span class="comment">// buffer we'll be searching in:</span> | |
| 253 | <span class="keyword">char</span> <span class="identifier">buf</span><span class="special">[</span><span class="number">4096</span><span class="special">];</span> | |
| 254 | <span class="comment">// saved position of end of partial match:</span> | |
| 255 | <span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">next_pos</span> <span class="special">=</span> <span class="identifier">buf</span> <span class="special">+</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">buf</span><span class="special">);</span> | |
| 256 | <span class="comment">// flag to indicate whether there is more input to come:</span> | |
| 257 | <span class="keyword">bool</span> <span class="identifier">have_more</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> | |
| 258 | ||
| 259 | <span class="keyword">while</span><span class="special">(</span><span class="identifier">have_more</span><span class="special">)</span> | |
| 260 | <span class="special">{</span> | |
| 261 | <span class="comment">// how much do we copy forward from last try:</span> | |
| 262 | <span class="keyword">unsigned</span> <span class="identifier">leftover</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">buf</span> <span class="special">+</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">buf</span><span class="special">))</span> <span class="special">-</span> <span class="identifier">next_pos</span><span class="special">;</span> | |
| 263 | <span class="comment">// and how much is left to fill:</span> | |
| 264 | <span class="keyword">unsigned</span> <span class="identifier">size</span> <span class="special">=</span> <span class="identifier">next_pos</span> <span class="special">-</span> <span class="identifier">buf</span><span class="special">;</span> | |
| 265 | <span class="comment">// copy forward whatever we have left:</span> | |
| 266 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">memmove</span><span class="special">(</span><span class="identifier">buf</span><span class="special">,</span> <span class="identifier">next_pos</span><span class="special">,</span> <span class="identifier">leftover</span><span class="special">);</span> | |
| 267 | <span class="comment">// fill the rest from the stream:</span> | |
| 268 | <span class="identifier">is</span><span class="special">.</span><span class="identifier">read</span><span class="special">(</span><span class="identifier">buf</span> <span class="special">+</span> <span class="identifier">leftover</span><span class="special">,</span> <span class="identifier">size</span><span class="special">);</span> | |
| 269 | <span class="keyword">unsigned</span> <span class="identifier">read</span> <span class="special">=</span> <span class="identifier">is</span><span class="special">.</span><span class="identifier">gcount</span><span class="special">();</span> | |
| 270 | <span class="comment">// check to see if we've run out of text:</span> | |
| 271 | <span class="identifier">have_more</span> <span class="special">=</span> <span class="identifier">read</span> <span class="special">==</span> <span class="identifier">size</span><span class="special">;</span> | |
| 272 | <span class="comment">// reset next_pos:</span> | |
| 273 | <span class="identifier">next_pos</span> <span class="special">=</span> <span class="identifier">buf</span> <span class="special">+</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">buf</span><span class="special">);</span> | |
| 274 | <span class="comment">// and then iterate:</span> | |
| 275 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">cregex_iterator</span> <span class="identifier">a</span><span class="special">(</span> | |
| 276 | <span class="identifier">buf</span><span class="special">,</span> | |
| 277 | <span class="identifier">buf</span> <span class="special">+</span> <span class="identifier">read</span> <span class="special">+</span> <span class="identifier">leftover</span><span class="special">,</span> | |
| 278 | <span class="identifier">e</span><span class="special">,</span> | |
| 279 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">match_default</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">match_partial</span><span class="special">);</span> | |
| 280 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">cregex_iterator</span> <span class="identifier">b</span><span class="special">;</span> | |
| 281 | ||
| 282 | <span class="keyword">while</span><span class="special">(</span><span class="identifier">a</span> <span class="special">!=</span> <span class="identifier">b</span><span class="special">)</span> | |
| 283 | <span class="special">{</span> | |
| 284 | <span class="keyword">if</span><span class="special">((*</span><span class="identifier">a</span><span class="special">)[</span><span class="number">0</span><span class="special">].</span><span class="identifier">matched</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">)</span> | |
| 285 | <span class="special">{</span> | |
| 286 | <span class="comment">// Partial match, save position and break:</span> | |
| 287 | <span class="identifier">next_pos</span> <span class="special">=</span> <span class="special">(*</span><span class="identifier">a</span><span class="special">)[</span><span class="number">0</span><span class="special">].</span><span class="identifier">first</span><span class="special">;</span> | |
| 288 | <span class="keyword">break</span><span class="special">;</span> | |
| 289 | <span class="special">}</span> | |
| 290 | <span class="keyword">else</span> | |
| 291 | <span class="special">{</span> | |
| 292 | <span class="comment">// full match:</span> | |
| 293 | <span class="special">++</span><span class="identifier">tags</span><span class="special">;</span> | |
| 294 | <span class="special">}</span> | |
| 295 | ||
| 296 | <span class="comment">// move to next match:</span> | |
| 297 | <span class="special">++</span><span class="identifier">a</span><span class="special">;</span> | |
| 298 | <span class="special">}</span> | |
| 299 | <span class="special">}</span> | |
| 300 | <span class="special">}</span> | |
| 301 | </pre> | |
| 302 | </div> | |
| 303 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 304 | <td align="left"></td> | |
| 305 | <td align="right"><div class="copyright-footer">Copyright © 1998-2013 John Maddock<p> | |
| 306 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 307 | 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>) | |
| 308 | </p> | |
| 309 | </div></td> | |
| 310 | </tr></table> | |
| 311 | <hr> | |
| 312 | <div class="spirit-nav"> | |
| 313 | <a accesskey="p" href="captures.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="syntax.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> | |
| 314 | </div> | |
| 315 | </body> | |
| 316 | </html> |