https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82366
--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> --- My best guess for what's happening in the original report is that the shared library is being loaded into a running process which uses an older version of libstdc++, and so the locale facets needed by the shared library were never created at startup. When std::regex tried to use those facets, it fails to find it. But without a way to reproduce the problem, this will never get resolved.