private function preg_error_message( $error ) {
		static $error_names = null;

		if ( null === $error_names ) {
			$definitions    = get_defined_constants( true );
			$pcre_constants = array_key_exists( 'pcre', $definitions )
				? $definitions['pcre']
				: array();
			$error_names    = array_flip( $pcre_constants );
		}

		return isset( $error_names[ $error ] )
			? $error_names[ $error ]
			: '<unknown error>';
	}