- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    
    if (textField == self.invoiceNumField||textField==self.bankNumField) {
        NSString *text = [textField text];
        NSCharacterSet *characterSet = [NSCharacterSet characterSetWithCharactersInString:@"0123456789\b"];
        string = [string stringByReplacingOccurrencesOfString:@" " withString:@""];
        if ([string rangeOfCharacterFromSet:[characterSet invertedSet]].location != NSNotFound) {
            return NO;
        }
        
        text = [text stringByReplacingCharactersInRange:range withString:string];
        text = [text stringByReplacingOccurrencesOfString:@" " withString:@""];
        
        NSString *newString = @"";
        while (text.length > 0) {
            NSString *subString = [text substringToIndex:MIN(text.length, 4)];
            newString = [newString stringByAppendingString:subString];
            if (subString.length == 4) {
                newString = [newString stringByAppendingString:@" "];
            }
            text = [text substringFromIndex:MIN(text.length, 4)];
        }
        
        newString = [newString stringByTrimmingCharactersInSet:[characterSet invertedSet]];
        
//        if (newString.length >= 20) {
//            return NO;
//        }
        
        [textField setText:newString];
        return NO;
    }
    
    return NO;

}