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

You can do this with pattern matching in the function args.

defmodule TextProcessor do
  def process_string(string) do
    string
    |> String.split("\n")
    |> Enum.each(&process_line/1)
  end

  defp process_line("LYRICS:" <> rest) do 
    # Process lines starting with "LYRICS:"
    IO.puts("Processing lyrics line: #{rest}")
  end

  defp process_line("CHORDS:" <> rest) do 
    # Process lines starting with "CHORDS:"
    IO.puts("Processing chords line: #{rest}")
  end

  defp process_line(line) do
    # Default processing for other lines
    IO.puts("Processing line: #{line}")
  end
end

TextProcessor.process_string("""
LYRICS: foo 
CHORDS: bar
foobar\
""")

Processing lyrics line:  foo 
Processing chords line:  bar
Processing line: foobar

There are constraints around guards, which prevent many higher level/more complex apis from being usable:

Not all expressions are allowed in guard clauses, but only a handful of them. This is a deliberate choice. This way, Elixir (and Erlang) can make sure that nothing bad happens while executing guards and no mutations happen anywhere. It also allows the compiler to optimize the code related to guards efficiently.

https://hexdocs.pm/elixir/patterns-and-guards.html