local dict = {
  a = "あ",
  i = "い",
  u = "う",
  e = "え",
  o = "お"
}

local function roman_to_japanese(text)
  local converted = ""
  while text ~= "" do
    local ok = false
    for i = 1, 3 do
      local subbed = text:sub(1, i)
      if dict[subbed] then
        converted = converted..dict[subbed]
        text = text:sub(i + 1, -1)
        ok = true
        break
      end
    end
    if not ok then
      error("no!")
    end
  end
  return converted
end

print(roman_to_japanese("aiueoaieueo"))