Part 2 threw me for a loop because the inputs were all separated by lines. But overall pretty fun problem
defmodule AOC.Y2024.Day3 do
@moduledoc false
use AOC.Solution
@mul_regex ~r/mul\((\d+),(\d+)\)/
@do_regex ~r/do\(\)/
@dont_regex ~r/don\'t\(\)/
@impl true
def load_data() do
Data.load_day(2024, 3)
|> Enum.join()
end
@impl true
def part_one(data) do
Regex.scan(@mul_regex, data, capture: :all_but_first)
|> Enum.map(fn v -> Enum.map(v, &String.to_integer(&1)) end)
|> General.map_sum(fn [a, b] -> a * b end)
end
@impl true
def part_two(data) do
data
|> get_do_instructions()
|> Enum.concat(get_dont_instructions(data))
|> Enum.concat(get_mul_instructions(data))
|> Enum.sort(fn a, b -> elem(a, 0) < elem(b, 0) end)
|> Enum.reduce({0, :do}, fn
{_, :do}, {acc, _} -> {acc, :do}
{_, :dont}, {acc, _} -> {acc, :dont}
{_, :mul, a, b}, {acc, :do} -> {acc + a * b, :do}
{_, :mul, _, _}, {acc, :dont} -> {acc, :dont}
end)
|> elem(0)
end
defp get_mul_instructions(data),
do:
Regex.scan(@mul_regex, data, return: :index)
|> Enum.map(fn [{mul_idx, _}, {f, fl}, {s, sl}] ->
{mul_idx, :mul, data |> String.slice(f, fl) |> String.to_integer(),
data |> String.slice(s, sl) |> String.to_integer()}
end)
defp get_do_instructions(data),
do:
Regex.scan(@do_regex, data, capture: :first, return: :index)
|> Enum.flat_map(& &1)
|> Enum.map(fn {i, _} -> {i, :do} end)
defp get_dont_instructions(data),
do:
Regex.scan(@dont_regex, data, capture: :first, return: :index)
|> Enum.flat_map(& &1)
|> Enum.map(fn {i, _} -> {i, :dont} end)
end
This solution is RegEx free 
