[boost] [Boost.Regex] Minor documentation issue

I believe the function validate_card_format should take a const std::string& instead of a const std::string http://www.boost.org/libs/regex/doc/introduction.html bool validate_card_format(const std::string s) { static const boost::regex e("(\\d{4}[- ]){3}\\d{4}"); return regex_match(s, e); }