<div\s+class=""abc""\s+id=""efg"">(((?<o>)<div[^>]*>|(?<-o>)</div>|(?:(?!</?div)[\s\S]))*)(?(o)(?!))</div>

------解决方案--------------------

没有那么复杂


  String re = @"<div class=""abc"" id=""efg"">([\s|\S|.]+)</div>";
       String test = @"<div class=""abc"" id=""efg""> <div>content 1 </div> <div>content 2 </div> <ul> <li>li 1 </li> </ul> <a href=""#"">here </a> </div> ";
       Regex reg = new Regex(re, RegexOptions.Multiline & RegexOptions.IgnoreCase);
       Match m = reg.Match(test);
       Response.Write(m.Groups[1]);
       Response.End();

测试通过。