Catch an exception for invalid user input in swift

I am trying this code that is a calculator. How can I handle input from the user that is not valid?

//ANSWER: Bridging header to Objective-C// https://github.com/kongtomorrow/TryCatchFinally-Swift

Here is the same question but in objc but I want to do this in swift. Catching NSInvalidArgumentException from NSExpression

All I want to show is a message if it doesn't work, but now I am getting an exception when the user doesn't input the correct format.

import Foundation

var equation:NSString = "60****2"  // This gives a NSInvalidArgumentException', 
let expr = NSExpression(format: equation) // reason: 'Unable to parse the format string
if let result = expr.expressionValueWithObject(nil, context: nil) as? NSNumber {
    let x = result.doubleValue
    println(x)
} else {
    println("failed")
}

This is still an issue in Swift 2. As noted, the best solution is to use a bridging header and catch the NSException in Objective C.

https://medium.com/swift-programming/adding-try-catch-to-swift-71ab27bcb5b8 describes a good solution, but the exact code doesn't compile in Swift 2 because try and catch are now reserved keywords. You'll need to change the method signature to workaround this. Here's an example:

// https://medium.com/swift-programming/adding-try-catch-to-swift-71ab27bcb5b8

@interface TryCatch : NSObject

+ (void)tryBlock:(void (^)())try catchBlock:(void (^)(NSException *))catch finallyBlock:(void (^)())finally;

@end

@implementation TryCatch

+ (void)tryBlock:(void (^)())try catchBlock:(void (^)(NSException *))catch finallyBlock:(void (^)())finally {
    @try {
        try ? try() : nil;
    }
    @catch (NSException *e) {
        catch ? catch(e) : nil;
    }
    @finally {
        finally ? finally() : nil;
    }
}

@end