[Bug libstdc++/102480] std::regex fails to match ^ when match_prev_avail is used

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102480

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #3)
> The original report was invalid, but I've now implemented the multiline
> option which does support the behaviour in the original testcase.

Backported for 11.4 now too.