Use:

• String#scan to get all occurences of the smiley
• Array#sample to make a random selection of which to remove
• String#gsub with a block parameter to iterate through matches and select positions to substitute with empty string

Code:

class String
  def magic()
    len = scan(/:\)/).length
    pos_to_remove = (0 ... len).to_a.sample(len / 2)
    gsub(/:\)/).with_index { |_, i|
      if pos_to_remove.include?(i) then "" else ":)" end
    }
  end
end

The key is to match two :) at the same time but only capture one of them in the group.

• regex

(:\).*?):\)\s*

or

:\)\s*(.*?:\))

• substitution

\1

See the test case here

"Hi! How :) are :) you? I'm :) fine.:)".gsub(/(:\).*?):\)\s*/, '\1')
# => Hi! How :) are you? I'm :) fine.