nimble_parsec example with RFC822 datetime parser (https://www.ietf.org/rfc/rfc822.txt)

rfc822.ex

defmodule RFC822 do
  import NimbleParsec

  day =
    choice([
      string("Mon"),
      string("Tue"),
      string("Wed"),
      string("Thu"),
      string("Fri"),
      string("Sat"),
      string("Sun")
    ])
    |> string(",")
    |> string(" ")

  date =
    integer(min: 1, max: 2)
    |> ignore(string(" "))
    |> choice([
      string("Jan"),
      string("Feb"),
      string("Mar"),
      string("Apr"),
      string("May"),
      string("Jun"),
      string("Jul"),
      string("Aug"),
      string("Sep"),
      string("Oct"),
      string("Nov"),
      string("Dec")
    ])
    |> ignore(string(" "))
    |> integer(2)

  time =
    integer(2)
    |> ignore(string(":"))
    |> integer(2)
    |> ignore(string(":"))
    |> optional(integer(2))

  timezone =
    choice([
      string("EST"),
      string("EDT"),
    ])

  defparsec :rfc822,
    optional(ignore(day))
    |> concat(date)
    |> ignore(string(" "))
    |> concat(time)
    |> ignore(string(" "))
    |> concat(timezone)

  def parse(binary) do
    case rfc822(binary) do
      {:ok, result, _, _, _, _} ->
        # FIXME: irreponsible pattern matching.
        [day, month, year, hour, minute, second, zone] = result

        {:ok, naive_datetime} =
          NaiveDateTime.new(year(year), month(month), day, hour, minute, second)

        {
          :ok,
          naive_datetime
          |> NaiveDateTime.add(zone_to_offset(zone), :second)
          |> DateTime.from_naive!("Etc/UTC")
        }

      {:error, _, rest, _, _, _} ->
        {:error, :invalid}
    end
  end

  defp year(year), do: 2000 + year

  defp month("Jan"), do: 1
  defp month("Feb"), do: 2
  defp month("Mar"), do: 3
  defp month("Apr"), do: 4
  defp month("May"), do: 5
  defp month("Jun"), do: 6
  defp month("Jul"), do: 7
  defp month("Aug"), do: 8
  defp month("Sep"), do: 9
  defp month("Oct"), do: 10
  defp month("Nov"), do: 11
  defp month("Dec"), do: 12

  defp zone_to_offset("EDT"), do: -4 * 60 * 60
  defp zone_to_offset("EST"), do: -5 * 60 * 60
  defp zone_to_offset("CST"), do: -6 * 60 * 60
  # More zones to come
end

defmodule RFC822Test do
  use ExUnit.Case

  test "parses the string" do
    string = "Wed, 02 Oct 02 08:00:00 EST"
    assert {:ok, datetime} = RFC822.parse(string)
    assert datetime.year == 2002
    assert datetime.month == 10
    assert datetime.day == 2
    assert datetime.hour == 3
    assert datetime.minute == 0
    assert datetime.second == 0

    string = "02 Oct 19 08:00:00 EST"
    assert {:ok, datetime} = RFC822.parse(string)
    assert datetime.year == 2019
    assert datetime.month == 10
    assert datetime.day == 2
    assert datetime.hour == 3
    assert datetime.minute == 0
    assert datetime.second == 0

    string = "Wednesday, 02 Oct 19 08:00:00 EST"
    assert {:error, invalid} = RFC822.parse(string)
  end
end