PEMDAS/Main.java

import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.Scanner;
import java.util.ArrayList;
public class Main{
    public static ArrayList<String> pemdas = new ArrayList<String>();
    public static boolean pClose(String input){
        int tracker = 0;
        for(int i =0; i < input.length(); i++){
            if(input.charAt(i) == '(') tracker++;
            else if (input.charAt(i) == ')') tracker--;
            if(tracker < 0) return false;
        }
        return tracker == 0; 
    }

    public static Double calc(String input, int num){ // p =0, e = 1, md = 2, as=3
        //System.out.println(input);
        Double result = null;
        String next;
        Pattern filter = Pattern.compile(pemdas.get(num));
        Matcher calcMatch = filter.matcher(input);
        if(!calcMatch.find()){
            if(num < 3) return calc(input, num+1);
            return null;
        }
        if(num == 0){
            Double pSol = calc(calcMatch.group(2), 1);
            if(pSol !=null){ result = pSol;
                next = input.substring(0, calcMatch.start()) + result.toString() + input.substring(calcMatch.end());
            }
            else{
                next = input.substring(0, calcMatch.start()) + input.substring(calcMatch.end());
            }
        }
        else{
            switch(calcMatch.group(2)){
                case "+":
                result = Double.parseDouble(calcMatch.group(1)) + Double.parseDouble(calcMatch.group(3));
                break;
                case "-":
                result = Double.parseDouble(calcMatch.group(1)) - Double.parseDouble(calcMatch.group(3));
                break;
                case "/":
                result = Double.parseDouble(calcMatch.group(1)) / Double.parseDouble(calcMatch.group(3));
                break;
                case "*":
                result = Double.parseDouble(calcMatch.group(1)) * Double.parseDouble(calcMatch.group(3));
                break;
                case "^":
                result = Math.pow(Double.parseDouble(calcMatch.group(1)), Double.parseDouble(calcMatch.group(3)));
                break;
                
            }
            next = input.substring(0, calcMatch.start()) + result.toString() + input.substring(calcMatch.end());
        }
        //for(int i = num; i <= 3; i++){
        //if(calc(next,  i) != null) {return calc(next, i);}
        //}
        if(filter.matcher(next).find()) return calc(next, num);
        if(num < 3){
            Double nextResult = calc(next, num+1);
            if(nextResult !=null) return nextResult;
        }
        return result;
    }

    public static void main(String args[]){
        System.out.println("PEMDAS calculator! Just write an expression to solve how you normally would, like \"(2+3)*3\"! Supports + - * / ^ % as its operators. \"stop\" will stop the program.");
        Scanner scanner = new Scanner(System.in);
        String input;
        pemdas.add("(\\()([^\\(\\)]*)(\\))");//detects anything in parenthesis
        pemdas.add("(-?\\d+\\.?\\d*)\\D*?(\\^)\\D*?(-?\\d+\\.?\\d*)"); //exponents
        pemdas.add("(-?\\d+\\.?\\d*)\\D*?(\\*|\\/|%)\\D*?(-?\\d+\\.?\\d*)"); //multiplication, division, 
        pemdas.add("(-?\\d+\\.?\\d*)\\D*?(\\+|-)\\D*?(-?\\d+\\.?\\d*)"); // addition, subraction
        while(true){
            input = scanner.nextLine();
            if(input.toLowerCase().equals("stop")) break; 
            if(!pClose(input)) System.out.println("Error: There's a problem with your parenthesis, so the result may be different from what you expect.");
            System.out.println(input + " is " + calc(input, 0));
        }
    }
}