[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 #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Correction: it ignores it with -std=gnu++11, it throws regex_error with
-std=c++11

So it's treating \| as illegal, which is wrong for a POSIX BRE.