Catching NSInvalidArgumentException from NSExpression

In my code I am evaluating strings as mathematical expressions for example: ``` NSString *formula=@"9*7"; NSExpression *expr =[NSExpression expressionWithFormat:formula]; NSLog(@"%@", [[expr expressionValueWithObject:nil context:nil]intValue]); ``` The above works fine but I will be handling dynamic input from users so I need to be able to catch the exception when the user enters faulty data, thus I need to be able to catch the exception in situations like the following: ``` NSString *formula=@"9*"; //note the deliberately invalid expression NSExpression *expr =[NSExpression expressionWithFormat:formula]; @try { [[expr expressionValueWithObject:nil context:nil]intValue]; } @catch (NSException *exception) { NSLog(@"Exception"); } @finally { NSLog(@"Finally"); } ``` However when I run this code I get: Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse the format string "9* == 1"' Is there some way to catch this exception? Or alternatively is there some way to test if an expression is valid before I pass it off? Thanks!