Elixir

defmodule AdventOfCode.Solution.Year2024.Day03 do
  def part1(input) do
    Regex.scan(~r/mul\((?<l>\d+),(?<r>\d+)\)/, input, capture: ["l", "r"])
    |> Stream.map(fn l -> Enum.reduce(l, 1, &(&2 * String.to_integer(&1))) end)
    |> Enum.sum()
  end

  def part2(input) do
    input |> String.replace(~r/don't\(\).*(do\(\)|$)/Us, "") |> part1
  end
end

Rust feat. pest

No Zalgo here! I wasted a huge amount of time by not noticing that the second part's example input was different - my code worked fine but my test failed 🤦‍♂️

pest.rs is lovely, although part two made my PEG a bit ugly.

part1    =  { SOI ~ (mul_expr | junk)+ ~ EOI }
part2    =  { (enabled | disabled)+ ~ EOI }
mul_expr =  { "mul(" ~ number ~ "," ~ number ~ ")" }
number   =  { ASCII_DIGIT{1,3} }
junk     = _{ ASCII }
on       = _{ "do()" }
off      = _{ "don't()" }
enabled  = _{ (SOI | on) ~ (!(off) ~ (mul_expr | junk))+ }
disabled = _{ off ~ (!(on) ~ junk)+ }

use std::fs;

use color_eyre::eyre;
use pest::Parser;
use pest_derive::Parser;

#[derive(Parser)]
#[grammar = "memory.pest"]
pub struct MemoryParser;

fn parse(input: &str, rule: Rule) -> eyre::Result<usize> {
    let sum = MemoryParser::parse(rule, input)?
        .next()
        .expect("input must be ASCII")
        .into_inner()
        .filter(|pair| pair.as_rule() == Rule::mul_expr)
        .map(|pair| {
            pair.into_inner()
                .map(|num| num.as_str().parse::<usize>().unwrap())
                .product::<usize>()
        })
        .sum();
    Ok(sum)
}

fn part1(filepath: &str) -> eyre::Result<usize> {
    let input = fs::read_to_string(filepath)?;
    parse(&input, Rule::part1)
}

fn part2(filepath: &str) -> eyre::Result<usize> {
    let input = fs::read_to_string(filepath)?;
    parse(&input, Rule::part2)
}

fn main() -> eyre::Result<()> {
    color_eyre::install()?;

    let part1 = part1("d03/input.txt")?;
    let part2 = part2("d03/input.txt")?;
    println!("Part 1: {part1}\nPart 2: {part2}");
    Ok(())
}

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();
  }
}