title: In C++ STL, how do I remove non-numeric characters from std::string with regex_replace?
body: <p>Using the C++ Standard Template Library function <code>regex_replace()</code>, how do I remove non-numeric characters from a <code>std::string</code> and return a <code>std::string</code>?</p>
  
  <blockquote>
    <p>This question is not a duplicate of <a href="https://stackoverflow.com/questions/747735/how-do-i-use-regex-replace">question
    747735</a>
    because that question requests how to use TR1/regex, and I'm
    requesting how to use standard STL regex, and because the answer given
    is merely some very complex documentation links. The C++ regex
    documentation is extremely hard to understand and poorly documented,
    in my opinion, so even if a question pointed out the <a href="http://www.cplusplus.com/reference/regex/regex_replace/" rel="nofollow">standard C++
    <code>regex_replace</code>
    documentation</a>,
    it still wouldn't be very useful to new coders.</p>
  </blockquote>