def check(self, diff):
        """Check that the name of the subpackage within contrib is valid

        The package name must match ``user_[a-zA-Z0-9_]+``.
        """
        relative_path = relative_to_contrib(diff, self.project)
        subpackage_name = relative_path.parts[0]
        assert re_test(SUBPACKAGE_NAME_REGEX, subpackage_name)