val finish = fn s:b2s(map(fn x:number(x, 16), rest(split("%", s))))
val decode = fn s:replace(s, re/(%[0-9A-Fa-f]{2})+/, finish)

writeln decode("http%3A%2F%2Ffoo%20bar%2F")
writeln decode("google.com/search?q=%60Abdu%27l-Bah%C3%A1")