Answer by Brian L for What's the best way to validate currency input in UITextField?

After finding a quick solution on stack overflow to handle US currency, I rewrote the function to safely handle international currencies as well. This solution will dynamically validate user input from a UITextField, correcting as they type.

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {      

   NSNumberFormatter *nf = [[NSNumberFormatter alloc] init];

   [numberFormatter setFormatterBehavior:NSNumberFormatterBehavior10_4];
   [numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];
   [numberFormatter setMaximumFractionDigits:2];
   [numberFormatter setMinimumFractionDigits:0];

    // Grab the contents of the text field
    NSString *text = [textField text];  

    // the appropriate decimalSeperator and currencySymbol for the current locale
    // can be found with help of the
    // NSNumberFormatter and NSLocale classes.
    NSString *decimalSeperator = numberFormatter.decimalSeparator;  
    NSString *currencySymbol = numberFormatter.currencySymbol;

    NSString *replacementText = [text stringByReplacingCharactersInRange:range withString:string];
    NSMutableString *newReplacement = [[ NSMutableString alloc ] initWithString:replacementText];

    // whenever a decimalSeperator or currencySymobol is entered, we'll just update the textField.
    // whenever other chars are entered, we'll calculate the new number and update the textField accordingly.
    // If the number can't be computed, we ignore the new input.
    NSRange decimalRange = [text rangeOfString:decimalSeperator];
    if ([string isEqualToString:decimalSeperator] == YES && 
        [text rangeOfString:decimalSeperator].length == 0) {
        [textField setText:newReplacement];
    } else if([string isEqualToString:currencySymbol] == YES&& 
              [text rangeOfString:currencySymbol].length == 0) {
        [textField setText:newReplacement];        
    } else if([newReplacement isEqualToString:currencySymbol] == YES) {
        return YES;
    }else {

        NSString *currencyGroupingSeparator = numberFormatter.currencyGroupingSeparator;

        [newReplacement replaceOccurrencesOfString:currencyGroupingSeparator withString:@"" options:NSBackwardsSearch range:NSMakeRange(0, [newReplacement length])];        
        [numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];

        NSNumber *number = [numberFormatter numberFromString:newReplacement];

        if([newReplacement length] == 1) {
            [numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
            number = [numberFormatter numberFromString:newReplacement];
        }

        if (number == nil) { 
            [newReplacement release];
            return NO;
        }
        [numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];    
        text = [numberFormatter stringFromNumber:number];
        [textField setText:text];       
    }

    [newReplacement release];

    return NO; // we return NO because we have manually edited the textField contents.
}