iphone - What's the best way to validate currency input in UITextField?

Question

My application allows the user to enter a numeric value (currency) in a UITextField control, but the keyboard layout that I wish was available is unfortunately not one of the built-in options, so I had to choose the "Numbers & Punctuation" option in Interface Builder. Here's the corresponding dialog window in IB:

So when my application asks the user for the input, it is displaying the following:

Which is perfectly fine, but look at all of the extra keys available to the user! One could easily enter "12;56!" in the text field, and I assume I have to validate that somehow.

So my question is: how do I validate currency values entered into a UITextField?

Answer 1

I have the urge to answer because this was the first entry I saw when I googled and the highest ranked answer wouldn't allow me to enter a currency.
I'm german, and in germany (and in many other countries) we use , as the decimalseparator.

I just wrote a similar method and this is what I have right now.

- (BOOL) textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    static NSString *numbers = @"0123456789";
    static NSString *numbersPeriod = @"01234567890.";
    static NSString *numbersComma = @"0123456789,";

    //NSLog(@"%d %d %@", range.location, range.length, string);
    if (range.length > 0 && [string length] == 0) {
        // enable delete
        return YES;
    }

    NSString *symbol = [[NSLocale currentLocale] objectForKey:NSLocaleDecimalSeparator];
    if (range.location == 0 && [string isEqualToString:symbol]) {
        // decimalseparator should not be first
        return NO;
    }
    NSCharacterSet *characterSet;
    NSRange separatorRange = [textField.text rangeOfString:symbol];
    if (separatorRange.location == NSNotFound) {
        if ([symbol isEqualToString:@"."]) {
            characterSet = [[NSCharacterSet characterSetWithCharactersInString:numbersPeriod] invertedSet];
        }
        else {
            characterSet = [[NSCharacterSet characterSetWithCharactersInString:numbersComma] invertedSet];              
        }
    }
    else {
        // allow 2 characters after the decimal separator
        if (range.location > (separatorRange.location + 2)) {
            return NO;
        }
        characterSet = [[NSCharacterSet characterSetWithCharactersInString:numbers] invertedSet];               
    }
    return ([[string stringByTrimmingCharactersInSet:characterSet] length] > 0);
}