Problems with a regex

I have this equation 2x ^ 2 + 7x-1 which by means of a regex removes what are the following characters: [x], [x ^ n]. to be able to solve them by means of of the general formula.
This is my next code

public void resolver(String msg) throws IOException {
        if(!msg.matches("^(?:([+-]?\d+|)x\^[0-9])?(?:([+-]?\d+)x)?([+-]?\d+)?$")){
            msg.trim();
            System.out.println("No es una ecuación cuadrática");
            System.exit(1);
        } else {
            //  System.out.println(msg);
        }


        String[] lista = new String[3];
        Pattern p = Pattern.compile("^(?:([+-]?[0-9]+?|)x\^2)?(?:([+-]?\d+)x)?([+-]?\d+)?$");
        Matcher m = p.matcher(msg);
        while (m.find()){


            lista[0] = m.group(1);
            lista[1] = m.group(2);
            lista[2] = m.group(3);
        }
        if(lista[0].isEmpty()){
            lista[0] = "1";
        }
        String a =  lista[0];
        String b =  lista[1];
        String c =  lista[2];


        Double a1 = Double.valueOf(a);
        Double b1 = Double.valueOf(b);
        Double c1 = Double.valueOf(c);

       System.out.println(a1+" "+b1+""+" "+c1);

     //  Double p1 = ((b1*b1)-(4*(a1*c1)));
       Double p2 = Math.sqrt((b1*b1)-(4*(a1*c1)));
       // System.out.println(p1);

       if(p2 <= 0){
           System.out.println("Raiz Imaginaria. No se puede Resolver");
       } else {

           x1 = (-b1  + p2) / (2*a1);
           x2 = (-b1  - p2) / (2*a1);
           System.out.println("X1 = " + " "+ x1);
           System.out.println("X2 = " + " "+ x2);
           String mensaje2 = "\n"+"X1 = "+x1+"\n"+"X2"+" "+x2;

The problem is that when someone puts a negative sign on the operation for example -x ^ 2 + 7x-1, the program takes it as if it were not a regular expression given to the validations.

Edit : Sorry, it does not appear as such an error, but in the regular expression does not recognize the equation if there is no number after the sign "-" and before "x ^ 2"