abdddc<div class="content" id="content5243635">还有<a href ="http://www.163.com" target="_blank">这样</a>

<div class="ad">abddddsss

<div class="content" id="content\d*">((.|\n)*?)<div class="ad">

            string str = @"abdddc<div class=""content"" id=""content5243635"">还有<a href =""http://www.163.com"" target=""_blank"">这样</a>
没有<a href =""http://www.163.com"" target=""_blank"">那样</a>很好

<div class=""ad"">abddddsss";
            Regex reg = new Regex(@"(?is)<div[^>]*?id=""content5243635""[^>]*?>(?:\s*([^<>]*)(?:<[^>]+>([^<>]+)</[^>]+>)?)*(?=\s*<div[^>]*?class=""ad"")");
            Match m = reg.Match(str);
            int length = m.Groups[1].Captures.Count;
            int minlength = m.Groups[2].Captures.Count;
            string result = string.Empty;
            for (int i = 0; i < length; i++)
            {
                result += m.Groups[1].Captures[i].Value;
                result += i>=minlength ? "" : m.Groups[2].Captures[i].Value;
            }
            Console.WriteLine(result);
/*
还有这样没有那样很好
*/