Replace the first word of a string

How about something like

words = {}
words['word'] = 'newword'
words['otherword'] = 'newotherword'

outmessage = message
splits = message.split(' ')
if (words[splits[0]])
  splits[0] = words[splits[0]]
  outmessage = splits.join(' ')
end

obviously it can be cleaned up a fair bit, but the hash will save having a god awful if or switch statement

Simon