Jquery validation by class

Hello experts.
I need help to validate a form in a modal window by classname using jquery validation plugin.
My form is  id="formcategories"
My required textfields : class="categoryclass"

If one of the textfields doesn't pass the validation - add to this class error (like i would handle this with id validation)
Attached my code

var editcategory = function(id){
    var p = $("#popuploader");
    p.dialog({
        autoOpen: false,
        width: 300,
        height: 300
    });
    p.dialog({
       buttons: {
            "Save": function () {
                var targetUrl = "index.cfm";
                var b = $("#formcategories");
                var a = b.valid();
                if (a) {
                    var f = b.serialize();
                    $.post("actions/actupdate.cfm", f, function (g) {
                        if (g) {
                            
							window.location.href = targetUrl;
                           
                        }
                    })
                }
            
            },
            "Cancel": function () {
                $(this).dialog("close");
                $(this).dialog("destroy");
            }
        }
    });
	p.dialog("open");
	p.load("forms/frmtext.cfm?categoryid=" + id, function (c, e, d) {
            if (e == "success") {
			 $("#formcategories").validate({
				 
				 }
			   )
	
            } else {
                alert("errorload")
            }
        });
};

Select allOpen in new window

Any help?