More regexes.
I’m not sure why do produces ["", "", "do()"] and don't produces ["", "", "", "don't()"]. The number of empty strings is consistent, but my regex-fu is not enough to figure out why / prevent it.
def part1(input) do
Regex.scan(~r/mul\((\d+),(\d+)\)/, input, capture: :all_but_first)
|> Enum.map(fn [a, b] -> String.to_integer(a) * String.to_integer(b) end)
|> Enum.sum()
end
def part2(input) do
Regex.scan(~r/mul\((\d+),(\d+)\)|(do\(\))|(don't\(\))/, input, capture: :all_but_first)
|> Enum.reduce({0, :process}, fn
["", "", "do()"], {sum, _} -> {sum, :process}
["", "", "", "don't()"], {sum, _} -> {sum, :skip}
_, {sum, :skip} -> {sum, :skip}
[a, b], {sum, :process} -> {sum + String.to_integer(a) * String.to_integer(b), :process}
end)
|> elem(0)
end
https://git.adamu.jp/adam/AdventOfCode/src/branch/main/2024/day3.exs
This solution is RegEx free 
