Js检测判断URL网址输入是否正确

<meta http-equiv="Content-Type" content="text/html;charset=UTF-8"> <title>检查是否为url</title> <form name="form1" onsubmit="return check()"> <input type="text" name="txt"><input type="submit" value="检验"> </form>
[ctrl+a 全选 注:如需引入外部js需刷新才能执行]