[Bug libstdc++/61425] std::regex ignores backslash when using basic or grep grammars

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61425

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Hmm, I guess I shouldn't trust man pages, a POSIX BRE doesn't allow \|

Both GNU and BSD grep do allow 'a\|b' in a BRE though ... still trying to
determine if it should be allowed for the grep grammar or f this bug's just
invalid...