Bad UTF8 in Elixir

bad_utf8.exs

defmodule Check do

  def is_utf8?(""), do: true
  def is_utf8?(<< char :: size(8), rest :: binary >>) when char <= 0x7f,  do: is_utf8?(rest)

  def is_utf8?(<< f :: size(8), s :: size(8), rest :: binary >>)
      when f >= 0xc0 and f <= 0xdf and # 110* ****
           s >= 0x80 and s <= 0xbf do  # 10** ****
    is_utf8?(rest)
  end

  def is_utf8?(<< f :: size(8), s :: size(8), t :: size(8), rest :: binary >>)
      when f >= 0xe0 and f <= 0xef and # 1110 ****
           s >= 0x80 and s <= 0xbf and # 10** ****
           t >= 0x80 and t <= 0xbf do  # 10** ****
    is_utf8?(rest)
  end

  def is_utf8?(_rest), do: false
end