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    <p>The c++ (0x, 11, tr1) regular expressions <a href="https://stackoverflow.com/q/4716680/170194">do not really work (stackoverflow)</a> in every case (look up the <a href="http://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.200x" rel="nofollow noreferrer">phrase <strong>regex</strong> on <strong>this page</strong></a> for gcc), so it is better to <a href="http://www.boost.org/libs/regex/" rel="nofollow noreferrer">use boost</a> for a while. </p> <p>You may try if your compiler supports the regular expressions needed:</p> <pre><code>#include &lt;string&gt; #include &lt;iostream&gt; #include &lt;regex&gt; using namespace std; int main(int argc, char * argv[]) { string test = "test replacing \"these characters\""; regex reg("[^\\w]+"); test = regex_replace(test, reg, "_"); cout &lt;&lt; test &lt;&lt; endl; } </code></pre> <p>The above works in Visual Studio 2012Rc.</p> <p><strong>Edit 1</strong>: To replace by <strong>two different strings</strong> in one pass (depending on the match), I'd think this won't work here. In Perl, this could easily be done within evaluated replacement expressions (<code>/e</code> switch).</p> <p>Therefore, you'll need two passes, as you already suspected:</p> <pre><code> ... string test = "test replacing \"these characters\""; test = regex_replace(test, regex("\\s+"), "_"); test = regex_replace(test, regex("\\W+"), ""); ... </code></pre> <p><strong>Edit 2</strong>:</p> <p>If it would be possible to use a <strong>callback function</strong> <code>tr()</code> in <code>regex_replace</code>, then you could modify the substitution there, like:</p> <pre><code> string output = regex_replace(test, regex("\\s+|\\W+"), tr); </code></pre> <p>with <code>tr()</code> doing the replacement work:</p> <pre><code> string tr(const smatch &amp;m) { return m[0].str()[0] == ' ' ? "_" : ""; } </code></pre> <p>the problem would have been solved. Unfortunately, there's <strong>no such overload</strong> in some C++11 regex implementations, but Boost <strong><a href="http://boost.2283326.n4.nabble.com/regex-Context-sensitive-replacement-of-text-using-regex-replace-tp2589269p2589275.html" rel="nofollow noreferrer">has one</a></strong>. The following would work with boost and use one pass:</p> <pre><code>... #include &lt;boost/regex.hpp&gt; using namespace boost; ... string tr(const smatch &amp;m) { return m[0].str()[0] == ' ' ? "_" : ""; } ... string test = "test replacing \"these characters\""; test = regex_replace(test, regex("\\s+|\\W+"), tr); // &lt;= works in Boost ... </code></pre> <p>Maybe some day this will work with C++<strong>11</strong> or whatever number comes next.</p> <p>Regards</p> <p>rbo</p>
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    1. COI don't want to replace " with underscore, it should be replaced with nothing. That is the crux of my issue, I want to replace the first match group with _ and the second match group with empty string. I should also have mentioned that I cannot use boost.
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    2. COYour second edit that runs in VS2012 still does not solve my issue. The whitespace must be replaced with _ and all other non letter and number characters must be replaced with empty string
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    3. COThe two-pass version does this on my system, the result is `test_replacing_these_characters`.
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