- (void)reTextFieldDidChange:(UITextField *)textField{//输入文本时调用

    //将输入的小写x转换成大写

    textField.text = [textField.text stringByReplacingOccurrencesOfString:@"x" withString:@"X"];

    //解决联想输入的问题

    NSCharacterSet *cs = [[NSCharacterSet characterSetWithCharactersInString:@"Xx0123456789"] invertedSet];//里面放允许输入的字符

   textField.text = [[textField.text componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:];

    if (textField.text.length > 18) {

        textField.text = [textField.text substringToIndex:18];

    }

}

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

    //设置只能输入字母数字  和 文本框最大长度

    NSCharacterSet *cs = [[NSCharacterSet characterSetWithCharactersInString:@"Xx0123456789"] invertedSet];

    NSString *filtered = [[string componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:@""];

    return [string isEqualToString:filtered] && (textField.text.length - range.length + string.length) <= 18;

}