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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_regex.partial_matches"></a><a class="link" href="partial_matches.html" title="Partial Matches"> Partial Matches</a> |
| </h2></div></div></div> |
| <p> |
| 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> |
| <code class="computeroutput"><span class="identifier">match_partial</span></code> can be passed |
| 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 |
| <a class="link" href="ref/regex_iterator.html" title="regex_iterator"><code class="computeroutput"><span class="identifier">regex_iterator</span></code></a>. |
| When used it indicates that partial as well as full matches should be found. |
| A partial match is one that matched one or more characters at the end of the |
| text input, but did not match all of the regular expression (although it may |
| have done so had more input been available). Partial matches are typically |
| used when either validating data input (checking each character as it is entered |
| on the keyboard), or when searching texts that are either too long to load |
| into memory (or even into a memory mapped file), or are of indeterminate length |
| (for example the source may be a socket or similar). Partial and full matches |
| can be differentiated as shown in the following table (the variable M represents |
| 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>, |
| <a class="link" href="ref/regex_search.html" title="regex_search"><code class="computeroutput"><span class="identifier">regex_search</span></code></a> |
| 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>): |
| </p> |
| <div class="informaltable"><table class="table"> |
| <colgroup> |
| <col> |
| <col> |
| <col> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| </p> |
| </th> |
| <th> |
| <p> |
| Result |
| </p> |
| </th> |
| <th> |
| <p> |
| M[0].matched |
| </p> |
| </th> |
| <th> |
| <p> |
| M[0].first |
| </p> |
| </th> |
| <th> |
| <p> |
| M[0].second |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| No match |
| </p> |
| </td> |
| <td> |
| <p> |
| False |
| </p> |
| </td> |
| <td> |
| <p> |
| Undefined |
| </p> |
| </td> |
| <td> |
| <p> |
| Undefined |
| </p> |
| </td> |
| <td> |
| <p> |
| Undefined |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Partial match |
| </p> |
| </td> |
| <td> |
| <p> |
| True |
| </p> |
| </td> |
| <td> |
| <p> |
| False |
| </p> |
| </td> |
| <td> |
| <p> |
| Start of partial match. |
| </p> |
| </td> |
| <td> |
| <p> |
| End of partial match (end of text). |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Full match |
| </p> |
| </td> |
| <td> |
| <p> |
| True |
| </p> |
| </td> |
| <td> |
| <p> |
| True |
| </p> |
| </td> |
| <td> |
| <p> |
| Start of full match. |
| </p> |
| </td> |
| <td> |
| <p> |
| End of full match. |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| <p> |
| Be aware that using partial matches can sometimes result in somewhat imperfect |
| behavior: |
| </p> |
| <div class="itemizedlist"><ul type="disc"> |
| <li> |
| There are some expressions, such as ".*abc" that will always |
| produce a partial match. This problem can be reduced by careful construction |
| of the regular expressions used, or by setting flags like match_not_dot_newline |
| so that expressions like .* can't match past line boundaries. |
| </li> |
| <li> |
| Boost.Regex currently prefers leftmost matches to full matches, so for |
| example matching "abc|b" against "ab" produces a partial |
| match against the "ab" rather than a full match against "b". |
| It's more efficient to work this way, but may not be the behavior you want |
| in all situations. |
| </li> |
| </ul></div> |
| <p> |
| The following example tests to see whether the text could be a valid credit |
| card number, as the user presses a key, the character entered would be added |
| to the string being built up, and passed to <code class="computeroutput"><span class="identifier">is_possible_card_number</span></code>. |
| If this returns true then the text could be a valid card number, so the user |
| interface's OK button would be enabled. If it returns false, then this is not |
| yet a valid card number, but could be with more input, so the user interface |
| would disable the OK button. Finally, if the procedure throws an exception |
| the input could never become a valid number, and the inputted character must |
| be discarded, and a suitable error indication displayed to the user. |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</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="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="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> |
| |
| <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> |
| <span class="special">{</span> |
| <span class="comment">// |
| </span> <span class="comment">// return false for partial match, true for full match, or throw for |
| </span> <span class="comment">// impossible match based on what we have so far... |
| </span> <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> |
| <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> |
| <span class="special">{</span> |
| <span class="comment">// the input so far could not possibly be valid so reject it: |
| </span> <span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span> |
| <span class="string">"Invalid data entered - this could not possibly be a valid card number"</span><span class="special">);</span> |
| <span class="special">}</span> |
| <span class="comment">// OK so far so good, but have we finished? |
| </span> <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> |
| <span class="special">{</span> |
| <span class="comment">// excellent, we have a result: |
| </span> <span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="comment">// what we have so far is only a partial match... |
| </span> <span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| In the following example, text input is taken from a stream containing an unknown |
| amount of text; this example simply counts the number of html tags encountered |
| in the stream. The text is loaded into a buffer and searched a part at a time, |
| if a partial match was encountered, then the partial match gets searched a |
| second time as the start of the next batch of text: |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">fstream</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">sstream</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span> |
| <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="comment">// match some kind of html tag: |
| </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="string">"<[^>]*>"</span><span class="special">);</span> |
| <span class="comment">// count how many: |
| </span><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> |
| |
| <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> |
| <span class="special">{</span> |
| <span class="comment">// buffer we'll be searching in: |
| </span> <span class="keyword">char</span> <span class="identifier">buf</span><span class="special">[</span><span class="number">4096</span><span class="special">];</span> |
| <span class="comment">// saved position of end of partial match: |
| </span> <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> |
| <span class="comment">// flag to indicate whether there is more input to come: |
| </span> <span class="keyword">bool</span> <span class="identifier">have_more</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> |
| |
| <span class="keyword">while</span><span class="special">(</span><span class="identifier">have_more</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="comment">// how much do we copy forward from last try: |
| </span> <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> |
| <span class="comment">// and how much is left to fill: |
| </span> <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> |
| <span class="comment">// copy forward whatever we have left: |
| </span> <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> |
| <span class="comment">// fill the rest from the stream: |
| </span> <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> |
| <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> |
| <span class="comment">// check to see if we've run out of text: |
| </span> <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> |
| <span class="comment">// reset next_pos: |
| </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> |
| <span class="comment">// and then iterate: |
| </span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">cregex_iterator</span> <span class="identifier">a</span><span class="special">(</span> |
| <span class="identifier">buf</span><span class="special">,</span> |
| <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> |
| <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> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">cregex_iterator</span> <span class="identifier">b</span><span class="special">;</span> |
| |
| <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> |
| <span class="special">{</span> |
| <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> |
| <span class="special">{</span> |
| <span class="comment">// Partial match, save position and break: |
| </span> <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> |
| <span class="keyword">break</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="keyword">else</span> |
| <span class="special">{</span> |
| <span class="comment">// full match: |
| </span> <span class="special">++</span><span class="identifier">tags</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="comment">// move to next match: |
| </span> <span class="special">++</span><span class="identifier">a</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">}</span> |
| <span class="special">}</span> |
| </pre> |
| </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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