Its really all in the examples, stupid me just looked in hexdocs.
I have to say this is more fun than writing push-down-automata in Modula-3.

example how to do parentheses and also convert a production rule to a parser:

  # factor := ( expr ) | nat

  factor =
    empty()
    |> choice(
      [
        ignore(ascii_char([?(]))
        |> concat(parsec(:expr))
        |> ignore(ascii_char([?)])),
        nat
      ]
    )

example how to be a little sensitive.

  defp match_and_emit_tag(_rest, [tag, [tag, text]], context, _line, _offset), do: ...

  defp match_and_emit_tag(_rest, [opening, [closing | _]], _context, _line, _offset),
    do: {:error, "closing tag #{inspect(closing)} did not match opening tag #{inspect(opening)}"}

thats really cool. If i remember correctly this is a real pain in theory.