银行卡卡号4位一空格

//检测是否为纯数字
- (BOOL)isPureInt:(NSString *)string{
    NSScanner* scan = [NSScanner scannerWithString:string];
    int val;
    return [scan scanInt:&val] && [scan isAtEnd];
}
//在UITextField的代理方法中
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
        NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string]; //得到输入框的内容
        //检测是否为纯数字
        if ([self isPureInt:string]) {
            //添加空格，每4位之后，4组之后不加空格，格式为xxxx xxxx xxxx xxxx xxxxxxxxxxxxxx
            if (textField.text.length % 5 == 4 && textField.text.length < 22) {
                textField.text = [NSString stringWithFormat:@"%@ ", textField.text];
            }
            //只要30位数字
            if ([toBeString length] >= 19+4+11)
            {
                toBeString = [toBeString substringToIndex:19+4+11];
                textField.text = toBeString;
                [textField resignFirstResponder];
                return NO;
            }
        }
        else if ([string isEqualToString:@""]) { // 删除字符
            if ((textField.text.length - 2) % 5 == 4 && textField.text.length < 22) {
                textField.text = [textField.text substringToIndex:textField.text.length - 1];
            }
            return YES;
        }
        else{
            return NO;
        }
        return YES;
}