See the below given example ..that check for the string length and returns alert if the string length is greater than required one in UITextField.

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    if (range.location == 0 && string.length == 0) {
        return YES;
    }

    // Build up the resulting string?
    NSMutableString *fullString = [[NSMutableString alloc] init];

    [fullString appendString:[textField.text substringWithRange:NSMakeRange(0, range.location)]];
    [fullString appendString:string];

    // Set up number formatter?
    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
    NSNumber *replaceNumber = [formatter numberFromString:fullString];

    [fullString release];
    [formatter release];

    return !(replaceNumber == nil || [replaceNumber intValue] > 40);
}