[Bug libstdc++/102447] std::regex incorrectly accepts invalid bracket expression

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102447

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The std::regex{"[\\w-a]"} case will throw a std::regex_error exception now. I'd
like to backport this, but I'm going to wait a while. I am not entirely
confident that my changes won't cause regressions elsewhere in the bracket
handling.