That’s precisely what nimble_parsec is for. ^.^

However I’d probably just be lazy and swap the . and , with : then just run it through Timex.parse. The Float.parse and elem calls on that are definitely a bit flimsy. but they could be cleaned up. The zip is an interesting method of setting up the additive part though.

# lib/parse_interval.ex
defmodule ParseInterval do
  @doc """
  Parse a given string as either a time interval or a fractional number of hours and return the equivalent number of
  seconds.

  ## Examples

      iex> ParseInterval.parse("02:15")
      8100

      iex> ParseInterval.parse("0,25")
      900

      iex> ParseInterval.parse("8.00")
      28800

      iex> ParseInterval.parse("8.13:05")
      {:error, :invalid_format}

  """
  def parse(str) do
    with :error <- try_parse_as_time(str),
         :error <- try_parse_as_number(str) do
      {:error, :invalid_format}
    end
  end

  ###

  defp try_parse_as_time(str) do
    case String.split(str, ":") do
      [hours, minutes] -> as_time(hours, minutes)
      _ -> :error
    end
  end

  defp as_time(hours_str, minutes_str) do
    with {:ok, hours} <- parse_integer(hours_str),
         {:ok, minutes} <- parse_integer(minutes_str) do
      hours * 3600 + minutes * 60
    end
  end

  defp parse_integer(str) do
    case Integer.parse(str) do
      {num, ""} -> {:ok, num}
      _ -> :error
    end
  end

  defp try_parse_as_number(str) do
    case str |> String.replace(",", ".") |> Float.parse() do
      {num, ""} -> trunc(num * 3600)
      _ -> :error
    end
  end
end

# test/parse_interval_test.exs
defmodule ParseIntervalTest do
  use ExUnit.Case
  doctest ParseInterval
end

$ mix test
....

Finished in 0.03 seconds
4 doctests, 0 failures

Thanks @alco! Wasn’t aware of doctest yet, so even learned that one along the way