Helper help - inserting delimiters

My ruby skills are a bit underdeveloped, but I managed to whip up a
Helper that inserts a delimiter at a fixed position in a number.
Any wizards out there that can give tips on how to scale this to
accepting calls for putting a delimiter at more than 1 position?

  def insert_delimiter_at(number, pos, delimiter="-")
    begin
      number = number.to_s
      parts = []
      parts[0] = number.to(pos-1)
      parts[1] = number.from(pos)
      number = parts.join delimiter
    rescue
      number
    end
  end