isbn.ex

defmodule Isbn do
  require Record
  Record.defrecord(:isbn, raw: "", gs1: "", group: "", publisher: "", title: "", check_digit: "")
end

parser.ex

defmodule Isbn.Parser do
  import Isbn

  def parse(isbn) do
    record =
      isbn
      |> String.graphemes
      |> Enum.map(&lexeme/1)
      |> join
      |> parse_structure
  end

  defp lexeme(l) do
    if String.match?(l, ~r/[\d -X]/) do
      l
    else
      raise ArgumentError, "ISBN contains an invalid character: " <> l <> ". Valid characters are: 0-9, X, (space), -."
    end
  end

  defp join(lexemes) do
    Enum.reduce(lexemes, "", fn(my_lx, isbn) -> isbn <> my_lx end)
  end

  defp parse_structure(isbn) do
    isbn
    |> String.split([" ", "-"], trim: true)
    |> record_from(isbn)
  end

  defp record_from(parts, raw) when length(parts) == 4 do
    attrs = [:group, :publisher, :title, :check_digit]
    |> Enum.zip(parts)
    |> Kernel.++([raw: raw])
    |> isbn
  end
  defp record_from([gs1 | parts], raw) when length(parts) == 4 do
    record_from(parts, raw)
    |> isbn(gs1: gs1)
  end
end

parser_test.exs

defmodule IsbnParserTest do
  use ExUnit.Case
  require Record
  import Isbn

  test "parsing stores various parts of the isbn separately" do
    record = Isbn.Parser.parse("3-598-21508-8")
    assert(isbn(record, :group)) == "3"
    assert(isbn(record, :publisher)) == "598"
    assert(isbn(record, :title)) == "21508"
    assert(isbn(record, :check_digit)) == "8"
    assert(isbn(record, :raw)) == "3-598-21508-8"
  end
end