Program does not recognize empty expressions in brackets

[bryjen]

Program does not recognize empty expressions in brackets

 - ()                //returns valid
 - () + 5            //returns valid
 - (-4 + ()) + 5     //returns valid

Make it so that the program can recognize empty brackets. Also make it so that the expressions in parentheses can be evaluated as expressions also (maybe recursively)

 - (-4 +4-) + 5    //returns valid

The expression is seen as valid since the program only checks that the end of the expression ends in a non-operator, not in the expressions in parentheses (since it cannot distinguish which parenthesis ends if it is nested)