Format big numbers with Elixir

format.ex

defmodule Number do
  def format(input, separator \\ ",") do
    prefix_size = rem(byte_size(input), 3)
    {prefix, rest} = String.split_at(input, prefix_size)
    segments = for <<chunk::binary-size(3) <- rest>>, do: chunk

    Enum.join([prefix | segments], separator)
  end
end

# "2342346546" => "2.342.346.546"