Due to an implementation detail, the <regex> header will never throw an exception corresponding to std::regex_constants::error_ctype. This can be exhibited from the below code sample: #include <regex> int main(void) {   try {     std::regex e("[[:not_a_class:]]", std::regex_constants::ECMAScript);     std::regex_match("a", e);   }   catch(std::regex_error e){     // e.code() will not be std::regex_constants::error_ctype   }   return 0; }