eh_it_works.rb

sentence = "For now you ask why."

hash = {
  FOR: "4",
  YOU: "U",
  WHY: "Y",
}

# Start with an upcased copy of the original.
slang = sentence.upcase

hash.each do |word, alt|
  # Use gsub! (note the bang) to modify the string in-place.
  slang.gsub!(/\b#{word}\b/, alt)
end

puts slang

sexy.rb

sentence = "For now you ask why."

hash = {
  "FOR" => "4",
  "YOU" => "U",
  "WHY" => "Y",
}
hash.default_proc = -> k, v { v }

puts sentence.upcase.gsub /\w+/, hash