string s = "201206030";
string pattern = @"^(\d{4})0*?(\d{2})0*?(\d{2})$";
string result = System.Text.RegularExpressions.Regex.Replace(s, pattern, "$1$2$3");

------解决方案--------------------
有时最简单的也是最有效的一种方式

string s = "201206030";
string ss = s.Substring(0,6)+s.Substring(7,2);
Resoponse.Write(ss);