def _exclude_regex(self, which: str) -> str:
        """Return a regex string for the given exclusion list."""
        if which not in self._exclude_re:
            excl_list = getattr(self.config, which + "_list")
            self._exclude_re[which] = join_regex(excl_list)
        return self._exclude_re[which]
Return a regex string for the given exclusion list.
coverage/control.py