Ruby encoding is fun!

gistfile1.rb

[29] pry(main)> foo = 'a&b*c d'
=> "a&b*c d"
[30] pry(main)> URI::escape(foo, /[^a-zA-Z0-9\-\.\_\~]/)
=> "a%26b%2Ac%20d"
[31] pry(main)> uri_val = URI.escape(foo, Regexp.new("[^#{URI::PATTERN::UNRESERVED}]"))
=> "a%26b*c%20d"
[32] pry(main)> CGI.escape(foo)                                      
=> "a%26b%2Ac+d"
[33] pry(main)> URI.escape(foo)                                      
=> "a&b*c%20d"

You should try [].

>> foo = '[]a&b*c d'
=> "[]a&b*c d"
>> URI::escape(foo, /[^a-zA-Z0-9\-\.\_\~]/)
=> "%5B%5Da%26b%2Ac%20d"
>> URI.escape(foo, Regexp.new("[^#{URI::PATTERN::UNRESERVED}]"))
=> "%5B%5Da%26b*c%20d"
>> CGI.escape(foo)
=> "%5B%5Da%26b%2Ac+d"
>> URI.escape(foo)
=> "[]a&b*c%20d"

See? It's so simple! :-/