string source = @"href=""/Product/List-0039,0084.shtml""";
            Regex reg = new Regex(@"(?is)(?<=href="")/[^""]+(?="")");
            source = reg.Replace(source, @"http://abc.sina.com.cn$0");
            MessageBox.Show(source);

------解决方案--------------------

正则没问题，你我自己使用的问题
你代码里的双引号转义了么？

Regex reg = new Regex(@"(?i)href=(['"]?)(?=(?!https?://)[^'"\s]+\.(?:shtml|css|js|aspx)\1)");
==============
 Regex reg = new Regex(@"(?i)href=(['""]?)(?=(?!https?://)[^'""\s]+\.(?:shtml|css|js|aspx)\1)");



 source = reg.Replace(source, @"http://abc.sina.com.cn$0");  
========
 source = reg.Replace(source, @"$0http://abc.sina.com.cn");  

最终正确代码：

C# code


  Regex reg = new Regex(@"(?i)href=(['""]?)(?=(?!https?://)[^'""\s]+\.(?:shtml|css|js|aspx)\1)");
  source = reg.Replace(source, @"$0http://abc.sina.com.cn");