func urldecode(str) {
    str.gsub!('+', ' ');
    str.gsub!(/\%([A-Fa-f0-9]{2})/, {|a| 'C'.pack(a.hex)});
    return str;
}

say urldecode('http%3A%2F%2Ffoo+bar%2F');  # => "http://foo bar/"