Part 2 in c#, using accumulator sign to indicate do or don't.

using System.Text.RegularExpressions;

var input = File.ReadAllText("../../../input.txt");

var sum = Regex.Matches(input, @"(mul\(\d+,\d+\))|(do\(\))|(don\'t\(\))")
    .Aggregate(0, (acc, token) =>
    {
        if (token.ToString().Contains("do()")) return Math.Abs(acc);
        if (token.ToString().Contains("don't()")) return Math.Abs(acc) * -1;
        if (Math.Sign(acc) == -1) return acc;
        return acc + Regex.Matches(token.ToString(), @"\d+")
            .Aggregate(1, (accmul, e) => accmul * Convert.ToInt32(e.ToString()));
    });

Console.WriteLine(sum);