https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124004
--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
What about word boundary instead of line-begin, i.e.
std::regex_search("ab", std::regex("a(?=\\b)b")
We also accept this which seems wrong. libc++ accepts it too though (but it
rejects the line-begin version)
