protected function _splitFieldname($field) {
		$regex = '/^([a-z0-9_-]+)\.([a-z 0-9_-]+|\*)$/iS';

		if (strpos($field, '.') !== false && preg_match($regex, $field, $matches)) {
			return [$matches[1], $matches[2]];
		}
		return [null, $field];
	}