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- <title>Partial Matches</title>
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- <div class="section">
- <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/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/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>
- </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 class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- 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 class="listitem">
- 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>
- <li class="listitem">
- There are situations where full matches are found even though partial matches
- are also possible: for example if the partial string terminates with "abc"
- and the regular expression is "\w+", then a full match is found
- even though there may be more alphabetical characters to come. This particular
- case can be detected by checking if the match found terminates at the end
- of current input string. However, there are situations where that is not
- possible: for example an expression such as "abc.*123" may always
- have longer matches available since it could conceivably match the entire
- input string (no matter how long it may be).
- </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 © 1998-2013 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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