Day3Part1.java

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Day3Part1 {
    public static void main(String[] args) throws FileNotFoundException {
        Pattern pattern = Pattern.compile("mul\\((\\d{1,3}),(\\d{1,3})\\)");

        Scanner input = new Scanner(new File("day3-input.txt"));

        int answer = 0;

        while (input.hasNextLine()) {
            String line = input.nextLine();
            Matcher matcher = pattern.matcher(line);

            while (matcher.find()) {
                String op = matcher.group(0);
                String num1 = matcher.group(1);
                String num2 = matcher.group(2);

                int mulResult = Integer.parseInt(num1) * Integer.parseInt(num2);
                System.out.println(op + " = " + mulResult);

                answer += mulResult;
            }
        }

        System.out.println(answer);
    }
}