How do I find out if a number is an integer in this expression?

What I'm trying to do is determine if a certain number is an integer in an expression like this. (10 * 2) + 5. If the expression was (10 * 2) + 5.24 then I want the program to say that there's a non-integer in that expression. What I have right now is this:

public static boolean isInt(String expr) {
    for (int i = 0; i<expr.length(); i++){
        if (expr. charAt(i) != (int)i){
        return false;
        }
    }
    return true;
}

The problem is there is alot more characters besides just numbers. So I want it to just check the numbers and ignore all the other symbols to determine if every number in the string is an integer.