Merge: regex: bug fix, less warnings and rename

Fix bug with sub-expressions when using a `search_from`, fix a few compiler warnings and rename `optimize_is_in` to `optimize_has` as it is closer to the public API.

Pull-Request: #1679
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>