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

[schwab@linux-m68k.org]

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

--- Comment #4 from Andreas Schwab <schwab@linux-m68k.org> ---
\| is just undefined in BRE, so an implementation can do whatever it wants.