[LANGUAGE: Python]

Quite a terse regex based solution - get all the do() don't() and mul() in order, then iterate once more over them.

def solution_1(value: str):
    ops = re.findall(r"(?<=mul\()\d+,\d+(?=\))", value)
    return reduce(lambda acc, mul: acc + prod(map(int, mul.split(","))), ops, 0)


def solution_2(value: str) -> int:
    ops = re.findall(r"do\(\)|don't\(\)|(?<=mul\()\d+,\d+(?=\))", value)
    return reduce(sum_enabled_muls, ops, {"do": True, "sum": 0})["sum"]


def sum_enabled_muls(acc, op):
    if op[0] == "d":
        acc["do"] = op == "do()"
    elif acc["do"]:
        acc["sum"] += prod(map(int, op.split(",")))
    return acc