下面这个是判断输入是否为6位数字和字母的组合

NSString * str = _tf.text;
    NSError * error = NULL;
    NSRegularExpression * exp = [NSRegularExpression regularExpressionWithPattern:@"[0-9a-zA-Z]{6,6}" options:NSRegularExpressionCaseInsensitive error:&error];
    if (error) {
        NSLog(@"%@",error);
    }else {
        NSTextCheckingResult * result = [exp firstMatchInString:str options:0 range:NSMakeRange(0,str.length)];
        if (result) {
            NSLog(@"%@",[str substringWithRange:result.range]);
        }else {
            NSLog(@"输入失败");
        }
    }

常见输入判断对应的正则表达式有: