Modified Version price.ex

price.ex

defmodule X do
  
  def format(number) when is_integer(number), do:  number |> to_string |> format
  
  def format(amount) when is_binary(amount) do   
    formatted_amount =
    amount 
    |> String.replace(~r/\D/, "")
    |> String.replace(~r/^0*(\d+)(\d{2})$/, "\\1,\\2")
    |> pad
    |> String.split( ",", parts: 2)
    |> concat_format
    "€ " <>  formatted_amount
  end

  def amount_to_cents(amount) do
    case Integer.parse(amount) do
      {number,""} -> number * 100
      {_,_} ->
        {number, _} = amount |> String.replace(~r/\D/, "") |> Integer.parse
        number
    end
  end
  
  defp pad(""), do: "0,00"
  defp pad(centified) when byte_size(centified) == 1, do: "0,0" <> centified
  defp pad(centified) when byte_size(centified) == 2, do: "0," <> centified
  defp pad(centified), do: centified
  
  defp concat_format([x , "00"]), do: concat_format([x, "-"])  
  defp concat_format([x, y]), do: set_separators(x) <> "," <> y
  
  defp set_separators(europart) do
    f = fn
	(r, []) -> [[r]] 
	(r, [h|acc]) when length(h) < 3 -> [[r|h]|acc]
	(r, [h|acc]) -> [[r],h|acc] 
    end
    List.foldr(europart |> to_char_list, [], f) |> :string.join('.') |> to_string
  end
  
  def test do
    keylist = [
    {"€ 0,-", ""}, 
    {"€ 0,-", "0000"}, 
    {"€ 1.234.567,89", "123456,789"}, 
    {"€ 1.345,90", "001we345fs90"},
    {"€ 0,01", "1"},
    {"€ 0,12", "12"},
    {"€ 1,23", "12.3"},
    {"€ 12,34", "12.34"},
    {"€ 123,45", "123,45"},
    {"€ 1.234,56", "1234,56"},
    {"€ 12.345,67", "12345.67"},
    {"€ 123.456,78", "123456,78"},
    {"€ 1.234.567,89", "1234567.89"},
    {"€ 12.345.678,90", "12345678,90"},
    {"€ 1,-", "1.00"},
    {"€ 1,-", "1,00"}
    ]
    :lists.foreach(
      fn ({f, i}) -> 
        calc = format(i)
        result = f == calc
        IO.puts "#{i} -> #{f} == (#{calc}) : #{result} "
      end,
      keylist
    )    
  end

end

Additional test cases:-

iex(1)> X.format("1")
"€ 0,01"
iex(2)> X.format("")
"€ 0,00"
iex(3)> X.format(101)
"€ 1,01"
iex(4)> X.format(1)
"€ 0,01"
iex(5)> X.format(0)
"€ 0,-"

iex(1)> X.format("1,00")
"€ 1,-"
iex(2)> X.format("1, 00")
"€ 1,-"
iex(3)> X.format("12345678,90")
"€ 12.345.678,90"