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<title>Partial Matches</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_regex.partial_matches"></a><a class="link" href="partial_matches.html" title="Partial Matches">Partial Matches</a>
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</h2></div></div></div>
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<p>
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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>
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<code class="computeroutput"><span class="identifier">match_partial</span></code> can be passed
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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
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<a class="link" href="ref/regex_iterator.html" title="regex_iterator"><code class="computeroutput"><span class="identifier">regex_iterator</span></code></a>.
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When used it indicates that partial as well as full matches should be found.
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A partial match is one that matched one or more characters at the end of the
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text input, but did not match all of the regular expression (although it may
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have done so had more input been available). Partial matches are typically
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used when either validating data input (checking each character as it is entered
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on the keyboard), or when searching texts that are either too long to load
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into memory (or even into a memory mapped file), or are of indeterminate length
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(for example the source may be a socket or similar). Partial and full matches
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can be differentiated as shown in the following table (the variable M represents
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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>,
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<a class="link" href="ref/regex_search.html" title="regex_search"><code class="computeroutput"><span class="identifier">regex_search</span></code></a>
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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>):
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</p>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col>
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<col>
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<col>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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</th>
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<th>
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<p>
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Result
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</p>
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</th>
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<th>
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<p>
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M[0].matched
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</p>
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</th>
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<th>
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<p>
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M[0].first
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</p>
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</th>
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<th>
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<p>
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M[0].second
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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No match
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</p>
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</td>
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<td>
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<p>
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False
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</p>
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</td>
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<td>
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<p>
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Undefined
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</p>
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</td>
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<td>
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<p>
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Undefined
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</p>
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</td>
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<td>
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<p>
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Undefined
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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Partial match
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</p>
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</td>
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<td>
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<p>
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True
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</p>
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</td>
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<td>
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<p>
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False
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</p>
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</td>
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<td>
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<p>
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Start of partial match.
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</p>
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</td>
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<td>
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<p>
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End of partial match (end of text).
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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Full match
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</p>
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</td>
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<td>
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<p>
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True
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</p>
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</td>
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<td>
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<p>
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True
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</p>
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</td>
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<td>
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<p>
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Start of full match.
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</p>
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</td>
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<td>
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<p>
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End of full match.
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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<p>
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Be aware that using partial matches can sometimes result in somewhat imperfect
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behavior:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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There are some expressions, such as ".*abc" that will always
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produce a partial match. This problem can be reduced by careful construction
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of the regular expressions used, or by setting flags like match_not_dot_newline
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so that expressions like .* can't match past line boundaries.
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</li>
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<li class="listitem">
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Boost.Regex currently prefers leftmost matches to full matches, so for
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example matching "abc|b" against "ab" produces a partial
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match against the "ab" rather than a full match against "b".
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It's more efficient to work this way, but may not be the behavior you want
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in all situations.
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</li>
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<li class="listitem">
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There are situations where full matches are found even though partial matches
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are also possible: for example if the partial string terminates with "abc"
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and the regular expression is "\w+", then a full match is found
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even though there may be more alphabetical characters to come. This particular
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case can be detected by checking if the match found terminates at the end
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of current input string. However, there are situations where that is not
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possible: for example an expression such as "abc.*123" may always
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have longer matches available since it could conceivably match the entire
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input string (no matter how long it may be).
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</li>
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</ul></div>
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<p>
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The following example tests to see whether the text could be a valid credit
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card number, as the user presses a key, the character entered would be added
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to the string being built up, and passed to <code class="computeroutput"><span class="identifier">is_possible_card_number</span></code>.
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If this returns true then the text could be a valid card number, so the user
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interface's OK button would be enabled. If it returns false, then this is not
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yet a valid card number, but could be with more input, so the user interface
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would disable the OK button. Finally, if the procedure throws an exception
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the input could never become a valid number, and the inputted character must
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be discarded, and a suitable error indication displayed to the user.
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
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<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>
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<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>
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<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>
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<span class="special">{</span>
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<span class="comment">//</span>
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<span class="comment">// return false for partial match, true for full match, or throw for</span>
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<span class="comment">// impossible match based on what we have so far...</span>
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<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>
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<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>
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<span class="special">{</span>
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<span class="comment">// the input so far could not possibly be valid so reject it:</span>
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<span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span>
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<span class="string">"Invalid data entered - this could not possibly be a valid card number"</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="comment">// OK so far so good, but have we finished?</span>
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<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>
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<span class="special">{</span>
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<span class="comment">// excellent, we have a result:</span>
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<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="comment">// what we have so far is only a partial match...</span>
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<span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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<p>
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In the following example, text input is taken from a stream containing an unknown
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amount of text; this example simply counts the number of html tags encountered
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in the stream. The text is loaded into a buffer and searched a part at a time,
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if a partial match was encountered, then the partial match gets searched a
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second time as the start of the next batch of text:
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">fstream</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">sstream</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span>
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<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>
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<span class="comment">// match some kind of html tag:</span>
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<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>
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<span class="comment">// count how many:</span>
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<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>
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<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>
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<span class="special">{</span>
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<span class="comment">// buffer we'll be searching in:</span>
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<span class="keyword">char</span> <span class="identifier">buf</span><span class="special">[</span><span class="number">4096</span><span class="special">];</span>
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<span class="comment">// saved position of end of partial match:</span>
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<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>
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<span class="comment">// flag to indicate whether there is more input to come:</span>
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<span class="keyword">bool</span> <span class="identifier">have_more</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span>
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<span class="keyword">while</span><span class="special">(</span><span class="identifier">have_more</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="comment">// how much do we copy forward from last try:</span>
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<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>
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<span class="comment">// and how much is left to fill:</span>
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<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>
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<span class="comment">// copy forward whatever we have left:</span>
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<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>
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<span class="comment">// fill the rest from the stream:</span>
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<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>
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<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>
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<span class="comment">// check to see if we've run out of text:</span>
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<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>
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<span class="comment">// reset next_pos:</span>
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<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>
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<span class="comment">// and then iterate:</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">cregex_iterator</span> <span class="identifier">a</span><span class="special">(</span>
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<span class="identifier">buf</span><span class="special">,</span>
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<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>
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<span class="identifier">e</span><span class="special">,</span>
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<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>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">cregex_iterator</span> <span class="identifier">b</span><span class="special">;</span>
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<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>
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<span class="special">{</span>
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<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>
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<span class="special">{</span>
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<span class="comment">// Partial match, save position and break:</span>
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<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>
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<span class="keyword">break</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="keyword">else</span>
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<span class="special">{</span>
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<span class="comment">// full match:</span>
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<span class="special">++</span><span class="identifier">tags</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="comment">// move to next match:</span>
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<span class="special">++</span><span class="identifier">a</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="special">}</span>
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<span class="special">}</span>
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</pre>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 1998-2013 John Maddock<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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</div></td>
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</tr></table>
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<hr>
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<div class="spirit-nav">
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