Remove problematic std::regex usage

Replace two occurrences of std::regex usage that leads to huge freezes on startup on Windows with certain locales, due to the upstream bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98723, by porting them to either Glib::Regex or a non-regex-based solution.

A preferred longer-term solution is to switch both Glib::Regex and std::regex to boost::regex, after adding that library to the CI image; for now this is just a quick solution that minimally fixes #4147 (closed).

Unrelatedly, I also threw in a warning-silencer for a warning introduced in !5216 (merged) on non-Mac builds.