[Bug libstdc++/124000] std::regex::extended is too strict about escaped chars

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124000

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> So we should relax our implementation to accept \] and \} as equivalent to ]
> and } respectively.

This should also apply to the regex::awk and regex::egrep grammars, which use
EREs.