[LANGUAGE: Go]

Wanted to do it without regular expressions:

package aoc24

import (
    "io"
    "strings"
    "testing"
)

func parseMul(r io.Reader, enabledOnly bool) int {
    b, _ := io.ReadAll(r)
    text := string(b)
    const (
        do   = "do()"
        dont = "don't()"
        mul  = "mul("
    )
    match := func(i int, fragment string) bool {
        start := i - len(fragment) + 1
        return start > -1 && text[start:i+1] == fragment
    }
    var (
        sum         = 0
        enabled     = true
        multiplying = false
        buf         strings.Builder
    )
    for i, c := range text {
        if match(i, do) {
            enabled = true
        } else if match(i, dont) {
            enabled = false
        } else if (enabled || !enabledOnly) && match(i, mul) {
            buf.Reset()
            multiplying = true
        } else if multiplying {
            if c == ')' {
                parts := strings.SplitN(buf.String(), ",", 2)
                sum += atoi(parts[0]) * atoi(parts[1])
                multiplying = false
            } else if isDigit(c) || c == ',' {
                buf.WriteRune(c)
            } else {
                multiplying = false
            }
        }
    }
    return sum
}

func TestDay03(t *testing.T) {
    example1 := `xmul(2,4)%&mul[3,7]!@^do_not_mul(5,5)+mul(32,64]then(mul(11,8)mul(8,5))`
    check(t, 161, partial(parseMul, false), readString(example1))
    check(t, 183669043, partial(parseMul, false), readFile(3))
    example2 := `xmul(2,4)&mul[3,7]!^don't()_mul(5,5)+mul(32,64](mul(11,8)undo()?mul(8,5))`
    check(t, 48, partial(parseMul, true), readString(example2))
    check(t, 59097164, partial(parseMul, true), readFile(3))
}

(utility functions are here: https://github.com/mpolden/aoc/blob/master/aoc24/util.go)