C#

public partial class Day03 : Solver
{
  [GeneratedRegex(@"mul[(](\d{1,3}),(\d{1,3})[)]")]
  private partial Regex mulRegex();

  [GeneratedRegex(@"(do)[(][)]|(don't)[(][)]|(mul)[(](\d{1,3}),(\d{1,3})[)]")]
  private partial Regex fullRegex();

  private string input;

  public void Presolve(string input)
  {
    this.input = input.Trim();
  }

  public string SolveFirst() => mulRegex().Matches(input)
      .Select(match => int.Parse(match.Groups[1].Value) * int.Parse(match.Groups[2].Value))
      .Sum().ToString();

  public string SolveSecond()
  {
    bool enabled = true;
    int sum = 0;
    foreach (Match match in fullRegex().Matches(input)) {
      if (match.Groups[1].Length > 0) {
        enabled = true;
      } else if (match.Groups[2].Length > 0) {
        enabled = false;
      } else if (enabled) {
        sum += int.Parse(match.Groups[4].Value) * int.Parse(match.Groups[5].Value);
      }
    }
    return sum.ToString();
  }
}

I did part 2 live with the python interactive shell. I deleted all the stuff where I was just exploring ideas.

part 1:

import re

def multiply_and_add(data: "str") -> int:
    digit_matches = re.findall(r"mul\(\d{0,3},\d{0,3}\)", data)
    result = 0
    for _ in digit_matches:
        first = _.split("(")[1].split(")")[0].split(",")[0]
        second = _.split("(")[1].split(")")[0].split(",")[1]
        result += int(first) * int(second)

    return result

with open("input") as file:
    data = file.read()


answer = multiply_and_add(data)
print(answer)

part 2:

Python 3.11.2 (main, Aug 26 2024, 07:20:54) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import solution2
<re.Match object; span=(647, 651), match='do()'>
>>> from solution2 import *
>>> split_on_dont = data.split("don't()")
>>> valid = []
>>> valid.append(split_on_dont[0])
>>> for substring in split_on_dont[1:]:
...     subsubstrings = substring.split("do()", maxsplit=1)
...     for subsubstring in subsubstrings[1:]:
...             valid.append(subsubstring)
...
>>> answer = 0
>>> for _ in valid:
...     answer += multiply_and_add(_)
...
>>> answer
103811193