Using nimble_parsec I tend to prefer using a helper module for combinators rather than inline. I also tend to create semantic functions because sooner or later debugging a nimble_parsec combinator - and providing reasonable messages back to a user on parse errors - becomes a challenge.

Example parser

This is a pattern I usually follow, here just parsing your first example.

defmodule Parse.Helpers do
  import NimbleParsec

  def version() do
    head()
    |> concat(hex_integer(4))
    |> label("version")
    |> tag(:version)
  end

  def hex_integer(digits) do
    ascii_string([?a..?f, ?A..?F, ?0..?9], digits)
    |> reduce(:hex_to_integer)
  end

  def count() do
    hex_integer(4)
    |> label("count")
    |> unwrap_and_tag(:count)
  end

  def head() do
    ascii_string([?A..?Z, ?0..?9], 2)
    |> label("head")
  end

  def tail() do
    string("$")
    |> label("tail")
    |> unwrap_and_tag(:tail)
  end

  def hex_to_integer([string]) do
    String.to_integer(string, 16)
  end

end

defmodule Parser do
  import NimbleParsec
  import Parse.Helpers

  defparsec :parse,
    version()
    |> concat(count())
    |> concat(tail())

end

Example usage

iex> Parser.parse "AB98761234$"
{:ok, [version: ["AB", 39030], count: 4660, tail: "$"], "", %{}, {1, 0}, 11}

iex> Parser.parse "AB98761234" 
{:error, "expected version, followed by count, followed by tail", "AB98761234",
 %{}, {1, 0}, 0}