I am pretty late here but I’d advise against regexes unless they are very straightforward. Haven’t looked into yours in details (and length is not always an indicator of a complex regex) but I’d probably reach for nimble_parsec and give it a go for a day or two.

That being said, being productive in that particular library is a skill in and of itself so if you are pressed for time you should probably keep your regex solution but also add edge case unit tests.

Not in guards/function heads. You’ll need to use a single function with something like a cond inside.

If anyone is interested I think I kind of got to where I wanted like this:

defmodule Textprocessor do
  @regex_find_chords ~r/\b(?:[BE]b?|[ACDFG]#?)(?:sus|m|maj|min|[-1-9\/m])?(?:sus|m|maj|min|[-1-9\/m])?\b#?/
  @empty_line ~r/^\s*$/
  @instruction ~r/^\[[\w+ ]+\]\s*$/

  @moduledoc """
  Documentation for `Textprocessor`.
  """

  @doc """
  Process song

  ## Examples
    # iex> Textprocessor.process_file("./lyrics.text")
    # :ok
  """

  def process_file(filename) do
    File.stream!(filename)
    |> Enum.map(&String.trim/1)
    |> Enum.each(&process_line/1)
  end

  defp process_line(line) do
    cond do
      String.match?(line, @regex_find_chords) ->
        IO.puts("CHORDS: #{line}")

      String.match?(line, @instruction) ->
        IO.puts("INSTRUCTION: #{line}")

      String.match?(line, @empty_line) ->
        IO.puts("EMPTY: #{line}")

      true ->
        IO.puts("LYRICS: #{line}")
        :ok
    end
  end
end

Not sure this is optimal but I would be interested in any criticism