XOR Strings

package test.padencryption;
import java.util.regex.*;
import java.util.*;

public class StringEncrypt
{
    String s1, s2, result;
    static boolean isValid(String test)
    {
        if(test.matches("([0-9A-F])+")) //Regex expression for hexadecimal number
                return true;
        else
                return false;
    }

    public String encrypt(String data, String pad)
    {
        int x1 = Integer.parseInt(data, 16); //Parse string into hex integer.
        int x2 = Integer.parseInt(pad, 16);
        int x3 = x1 ^ x2; //XORing
        String res = String.format("%06x", x3); //Format string. Pad with 0s if required, min width 6.
        res = res.replace('a', 'A');
        res = res.replace('b', 'B');
        res = res.replace('c', 'C');
            res = res.replace('d', 'D');
        res = res.replace('e', 'E');
        res = res.replace('f', 'F');
        return res;
    }

    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        StringEncrypt obj = new StringEncrypt();
        System.out.println("Enter the first string : ");
        obj.s1 = input.next();
        System.out.println("Enter the second string: ");
        obj.s2 = input.next();
                //Check for validity. Strings must be equal length.
        if((isValid(obj.s1)) && (isValid(obj.s2)) && (obj.s1.length() == obj.s2.length()))
        {
            obj.result = obj.encrypt(obj.s1, obj.s2);
            System.out.println("The result is = " + obj.result);
        }
        else
            System.out.println("Invalid entries");
    }
}