<script   language= "javascript ">
function   IsUrl(str)
{
var   regUrl   =   /(http\:\/\/)?([\w.]+)(\/[\w-   \.\/\?%&=]*)?/gi;
var   result   =   str.match(regUrl);
if(result!=null)   {alert( "Yes ")}  
else   {alert( "No! ")}
}
</script>

<FORM   name= "bao ">
<INPUT   TYPE= "text "   NAME= "txt ">
<INPUT   TYPE=BUTTON   value= "TestURL "   onClick= "IsUrl(document.bao.txt.value) ">
</FORM>
(http\:\/\/)?代表匹配http://0或1次，
([\w.]+)匹配www.sian.com
\/[\w-   \.\/\?%&=]*)?匹配后面子串0或1次