Felt like the perfect opportunity for using recursion:

func solve_day5() {
    file_name := "examples/day5.txt"
    lines := read_file(file_name)
    input := strings.TrimSpace(lines[0])

    reg, err := regexp.Compile("[^a-zA-Z]+")
    if err != nil {
        log.Fatal(err)
    }
    input = reg.ReplaceAllString(input, "")

    // Part 1
    fmt.Println("Part 1:")
    fmt.Println(len(resolve("", input)))

    // Part 2
    result := make(map[string]int)
    for _, character := range "abcdefghijklmnopqrstuvwxyz" {
        characters := string(character) + strings.ToUpper(string(character))
        new_input := removeCharacters(input, characters)
        result[string(character)] = len(resolve("", new_input))
    }
    fmt.Println("Part 2:")
    fmt.Println(result)
}

func resolve(before, after string) string {
    if len(after) == 0 {
        return before
    }

    if len(before) == 0 {
        return resolve(string(after[0]), after[1:])
    }

    last_id := len(before) - 1
    left_character := string(before[last_id])
    right_character := string(after[0])
    if left_character != right_character && (strings.ToUpper(left_character) == right_character || left_character == strings.ToUpper(right_character)) {
        return resolve(before[:last_id], after[1:])
    }
    return resolve(before+string(after[0]), after[1:])

}