string_view support

[OlafvdSpek]

regex doesn't appear to support string_view

  {
    std::string s;
    boost::match_results<std::string::const_iterator> m;
    boost::regex_search(s, m, boost::regex(R"(\[quote.*?\])", boost::regex::icase), boost::match_default);
  }
  {
    std::string_view sv;
    boost::match_results<std::string_view::const_iterator> m;
    boost::regex_search(sv, m, boost::regex(R"(\[quote.*?\])", boost::regex::icase), boost::match_default); 
  // error: no matching function for call to 'regex_search(std::string_view&, boost::match_results<const char*>&, boost::regex, boost::regex_constants::_match_flags)'
  }