title: Compiletime build up of std::regex
body: <p>Since I know the regexes at compiletime, and building up a regex is in O(2^m) where m is the length of the regex, I would love to build up the regex at compiletime.</p>
  
  <p>Is this possible with <code>std::regex</code>? (I don't think so, because I don't see any <code>constexpr</code> constructor for <code>basic_regex</code>)</p>
  
  <p>And if not, is there a regex library which can buildup my regexes at compiletime?</p>