[Bug libstdc++/67015] "^[a-z0-9][a-z0-9-]*$", std::regex::extended is miscompiled

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67015

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
OK, let's backport to 4.9 too.