https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115524
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> --- >From 104587: I believe I have analyzed a bug report from reddit (https://redd.it/sp52gq) to be caused by the way libstdc++ builds on Cygwin. The reported problem in a nutshell is that regex_match(L"a", std::wregex(L"[[:xdigit:]]")) erroneously evaluates to zero on current Cygwin, using GCC 11.2.0. AFAICS this happens because the cygwin target uses the generic version of ctype_members.cc, when it almost certainly should be picking up the newlib edition instead. As-is, the generic version of _M_initialize_ctype tries to set up its 16-bit character class masks, but the type of the array it stores those in, std::ctype_base::mask, is just a plain 8-bit char in config/os/newlib/ctype_base.h. In terms of configuration options, Cygwin currently combines os_include_dir=os/newlib with --enable-clocale-generic (by default) and that fails to work.
