function tr(a, b, s)
  r = ''
  for i = 1, s:len() do
    j = a:find(s:sub(i, i))
    r = r .. (j and b:sub(j, j) or s:sub(i, i))
  end
  return r
end

print(tr('qwertyuiop', 'QWERTYUIOP', 'typewriter'))