Decoding URL encoding (percent encoding)

I want to decode URL encoding, is there any built-in tool for doing this or could anyone provide me with a sed code that will do this? I did search a bit through unix.stackexchange.com and on the internet but I couldn't find any command line tool for decoding url encoding. What I want to do is simply in place edit a txt file so that: - %21 becomes ! - %23 becomes # - %24 becomes $ - %26 becomes & - %27 becomes ' - %28 becomes ( - %29 becomes ) And so on.