defmodule Username do
  def sanitize([]), do: []

  def sanitize([first | other]) do
    sanitized =
      case first do
        ?ä -> ~c"ae"
        ?ö -> ~c"oe"
        ?ü -> ~c"ue"
        ?ß -> ~c"ss"
        ok when ok in ?a..?z or ok == ?_ -> [ok]
        _ -> []
      end

    sanitized ++ sanitize(other)
  end
end