string tempStr = "http://www.rczjp.cn/A/B/C/index.aspx?cid=11&sid=22";
            string sid = Regex.Match(tempStr, @"(?<=http://.+?\?.*?)sid=\d+").Value;//sid=22

------解决方案--------------------
str=url.Substring(LastIndexOf("&")+1);

------解决方案--------------------
C# code


var uri = new System.Uri("http://www.rczjp.cn/A/B/C/index.aspx?cid=11&sid=22");
var query = System.Web.HttpUtility.ParseQueryString(uri.Query.SubString(1));

------解决方案--------------------
js
JScript code

正则分析法。
function GetQueryString(name) 
{
   var reg = new RegExp("(^|&)" + name + "=([^&]*)(&|$)","i");
   var r = window.location.search.substr(1).match(reg);
   if (r!=null) return unescape(r[2]); return null;
}
alert(GetQueryString("参数名1"));
alert(GetQueryString("参数名2"));
alert(GetQueryString("参数名3"));