What should be the behavior of a regex::smatch whenever the regex::expression you are trying to match is not a well formed regex expression? With the current implementation is crush and burn. I do not know what is boost policy on this matter, but I think this is not desirable.
1) If you try and compile an invalid expression you get a boost::bad_expression exception thrown, it's up to you to handle it. 2) If you pass an invalid (or empty) regular expression object to one of the regex algorithms, then that's a (documented) precondition failure, and you get a std::invalid_argument exception thrown, and it's up to you to handle it. 3) The smatch object should be untouched after the above exception is thrown, that's hardly "crash and burn". John.