请教C#下一个多行正则的问题

[hometohome]

多行文本如下：
<br />
<b>Notice</b>: Undefined index: HTTP_USER_AGENT in <b>/include.php</b> on line <b>42</b><br />
<br />
<b>Notice</b>: Undefined index: HTTP_USER_AGENT in <b>/include.php</b> on line <b>43</b><br />
<?xml version="1.0"?><rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
。。。省略若干代码。。。

请问我如何删掉<?xml version="1.0"?>以上的部分？

[必结贴送分，谢谢了]

[hometohome]

补充：<?xml version="1.0"?>以上的部分是不确定有什么的，也可能什么都没有

[HDNGO]

([\s\S]*)<\?xml version=""1.0""\?>([\s\S]*)

[HDNGO]

string sDetail = "<b>Notice </b>:  Undefined index:  HTTP_USER_AGENT in <b>/include.php </b> on line <b>42 </b>\r\n<b>Notice </b>:  Undefined index:  HTTP_USER_AGENT in <b>/include.php </b> on line <b>43 </b>\r\n<?xml version=\"1.0\"?> <rss version=\"2.0\" xmlns:atom=\"http://www.w3.org/2005/Atom\">";
Regex r;
Match m;
r = new Regex(@"([\s\S]*)<\?xml version=""1.0""\?>([\s\S]*)", RegexOptions.Multiline);
m = r.Match(sDetail);
sDetail = sDetail.Replace(m.Groups[1].ToString(), "");
Console.Write(sDetail);
Console.Read();

[蓝海D鱼]

这个才可以

    string sDetail = "<b>Notice </b>:  Undefined index:  HTTP_USER_AGENT in <b>/include.php </b> on line <b>42 </b>\r\n<b>Notice </b>:  Undefined index:  HTTP_USER_AGENT in <b>/include.php </b> on line <b>43 </b>\r\n<?xml version=\"1.0\"?> <rss version=\"2.0\" xmlns:atom=\"http://www.w3.org/2005/Atom\">";
            Regex r;
            Match m;
            r = new Regex(@"([\s\S]*)(?=<\?xml version=""1.0""\?>)", RegexOptions.Multiline);
            m = r.Match(sDetail);
            sDetail = sDetail.Replace(m.Groups[1].ToString(), "");
            Console.Write(sDetail);
            Console.Read();

[蓝海D鱼]

都可以

[-过客-]

为什么都要先匹配一次再替换呢？直接替换就是了
另外这种需求一定要加“^”，因为.NET的正则引擎对“^”做了优化，从开始位置匹配，无论第一次匹配成功还是不成功，都不会继续尝试匹配，可以有效的提升效率

string result = Regex.Replace(yourStr, @"^(?:(?!<\/xml)[\s\S])*(?=<\?xml\s+version=""1.0""\?>)", "", RegexOptions.IgnoreCase);