multi_replace.rb

def multi_replace(string, *rules)
  scanning_pattern = Regexp.union(rules.map{|e| e.first} | [/./])
  string.scan(scanning_pattern).map do |match|
    rules.select{|e| match =~ e.first}.flatten.last || match
  end.join
end
 
multi_replace("the first problem is not egal to second problem", [/first problem/, "older problem"], [/second problem/, "first problem"])