There are two remaining reasons for failing libcxx tests related to regex:

• libc++'s implementation of match_prev_avail is broken, see [libc++] std::regex match_prev_avail implementation is regressed llvm/llvm-project#74838.
• the remaining failing tests are related to the recognition of collating elements, so are covered by issue <regex>: What names can and should regex_traits::lookup_collatename() recognize? #5393. (If preferred, we could make an additional distinction: the "locale" test failures are about the "can" in the issue question, the failures of the other grammar-specific tests are about the "should", and the failure of the lookup_collatename() test is about both.)