Multi-replace problem

problem.txt

QUESTION 6: Write a function multi_gsub that performs multiple, simultaneous
search-and-replace operations on a string.

Example:

"Lorem Ipsum #9191".multi_gsub([[/[a-z]/i, '#'], [/#/, 'P']])
=> "##### ##### P9191"

solution.rb

class String

  def mgsub(opts)
    u = Regexp.union(opts.keys)

    gsub(u) do |m|
      opts.find { |re, s| m =~ re }.last
    end
  end

end

p "Lorem Ipsum #9191".mgsub(/[a-z]/i => '#', /#/ => 'P')