How to know the number is a double

I coded something in Java looks what was imputed as string and then to convert that string into a double or an int IF there is a decimal (period) found. In my code it always jumps out of the loop and prints "no numbers found". Why is that? What is wrong.

My code:

    //check if its actually numbers
    if(leftSide.matches("[0-9]+") && rightSide.matches("[0-9]+")){
        System.out.println("numbers found");
        if(leftSide.contains(".") || rightSide.contains(".")){
            System.out.println("convert to doubles");
        }
        else{
            System.out.println("convert to ints");
        }
    }
    else{
        System.out.println("no numbers found");
    }