function checkUrl(urlString){
if(urlString!=""){
var reg=/(http|ftp|https):\/\/[\w\-_]+(\.[\w\-_]+)+([\w\-\.,@?^=%&:/~\+#]*[\w\-\@?^=%&/~\+#])?/;
if(!reg.test(urlString)){
alert("不是正确的网址吧，请注意检查一下");
                        }
                    }
}

方法二：

function IsURL(str_url){
 var strRegex = "^((https|http|ftp|rtsp|mms)?://)"
 + "?(([0-9a-z_!~*'().&=+$%-]+: )?[0-9a-z_!~*'().&=+$%-]+@)?" //ftp的user@ 
  + "(([0-9]{1,3}\.){3}[0-9]{1,3}" // IP形式的URL- 199.194.52.184 
  + "|" // 允许IP和DOMAIN（域名）
  + "([0-9a-z_!~*'()-]+\.)*" // 域名- www. 
  + "([0-9a-z][0-9a-z-]{0,61})?[0-9a-z]\." // 二级域名 
  + "[a-z]{2,6})" // first level domain- .com or .museum 
  + "(:[0-9]{1,4})?" // 端口- :80 
  + "((/?)|" // a slash isn't required if there is no file name 
  + "(/[0-9a-z_!~*'().;?:@&=+$,%#-]+)+/?)$"; 
  var re=new RegExp(strRegex); 
 //re.test()
  if (re.test(str_url)){
  return (true); 
  }else{ 
  return (false); 
  }
 }
var testUrl;
testUrl="http://harveyzeng.iteye.com/blog/1776991";
//var testUrl="http://www.jb51.net/article/1.htm";
alert(IsURL(testUrl));