[Boost-users] [qi::phrase_parse] how to retain minus as string

I am trying to store minus as the string in the key. I think its getting conflict with to(a-z) symbol key = +qi::char_(".a-zA-Z0-9") | qi::lexeme[('[' >> +qi::char_("@ + - * . ( , ) $ a-zA-Z0-9 ") >> ']')]; value = +qi::char_("a-zA-Z0-9") ; I have the string like below: std::string input = “[(@vala - @valb) * @valc]:real"; Can someone help me to resolve this? Regards, UJ