Is there a way of validating a string type program option before it's stored in the map?  

add_options()

  ("arg" , value<std::string>()->default_value("Hi"), "Greeting");

boost::regex pattern("^(Hello | Good Morning | Hi)$");

I've used custom validation for types that program options doesn't recognize.  I'd like to check the string before it's stored in the map like the custom validation does for unknown types.  I'd rather the error be caught early.  Is there a way to use the regex against the program option?

Ryan