NumberFormatException when running a calculator program with regular expression

Question

I am trying to program a simple calculator.

public static String cal(){
        String a="-60-1+40";
        if(a.matches("-?[1-9][0-9]?([\\+-][1-9][0-9]?)+")){
            System.out.println(a);
            String operators[]=a.split("[0-9]+");
            String operands[]=a.split("[+-]");
            int agregate = Integer.parseInt(operands[0]);
            for(int i=1;i<operands.length;i++){
                if(operators[i].equals("+"))
                    agregate += Integer.parseInt(operands[i]);
                else 
                    agregate -= Integer.parseInt(operands[i]);
            }
             return Integer.toString(agregate);
        }else throw new invalidExpressionException(a+" is a Invalid expression");
    }

I have this function but when I try to run it I get a NumberFormatException if the first character of the string is a "-". All other cases seems to be right and only that one is failing. I've been trying to fix for a while but I don't find how.

run:
Exception in thread "main" java.lang.NumberFormatException: For input string: ""
-60-1+40
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
    at java.lang.Integer.parseInt(Integer.java:592)
    at java.lang.Integer.parseInt(Integer.java:615)
    at calculator.Calculator.cal(Calculator.java:20)
    at calculator.Calculator.main(Calculator.java:9)

Answer 1

Following @Ben's comment, a workaround would be to check if the first character is a '-', and in that case preppend a 0 to the string before processing it (-n can be obtained as sustracting the number n from 0):

-1+2-3, should become 0-1+2-3

if(a.startsWith("-")){
    a = "0" + a;
}