input元素required属性无效

 1 $(document).ready(function(){
 2     $("form").submit(function(e) {
 3         var ref = $(this).find("[required=required]");
 4         alert(‘ref‘);
 5         $(ref).each(function(){
 6             if ( $(this).val() == ‘‘ )
 7             {
 8                 alert("Required field should not be blank.");
 9                 $(this).focus();
10                 e.preventDefault();
11                 return false;
12             }
13         });  return true;
14     });
15 });