<input type="text" id="itext" /><input type="button" value="测试" onclick="test()" />
<script language="JavaScript" type="text/javascript" >  
function test()
{ 
    var reg = /^\w+$/;
    var val = document.getElementById("itext").value;
    if(val!="")
    {
        alert(reg.test(val));
    }
}
</script>

------解决方案--------------------
function checkUserName(){
	try{
		usernameObj = document.loginform.username;
		username_re=/(\d|[a-z]|_)+/;
		if(! username_re.test(usernameObj.value)){
			alert("用户名格式不正确!");
			usernameObj.focus();
			return false;	
		}
	}catch(exception){
		alert("Make sure the object id is correct!");
		return false;
	}
	return true;
}

------解决方案--------------------
<input type="text" id="itext" /><input type="button" value="测试" onclick="test()" />
<script language="JavaScript" type="text/javascript" > 
function test()
{  
   var reg = /^\w+$/;
   var val = document.getElementById("itext").value;
   if(val!="")
   {
       alert(reg.test(val));
   }
}
</script>