$(".drag").draggable({

            revert: function (event, ui) {

                //droppable 객체가 아닌곳에 드래그 됫을때

                if (event == false) {

                    isRevert = false;

                    return true;

                    //droppable 객체에 들어갓을때

                } else {

                    isRevert = true;

                }

            },

        });
        $("#drop").droppable(
            {
                drop: function (event, ui) {
                    //alert($(event.source).html());
                    //alert($(e).html());
                    //alert($('#' + ui).html());
                    ==>Drop된 객체에(span)태그의 글자를 읽어오고
                  내가 원하는 글자가 아니면 드래그된 <span을 원위치로 돌리고 싶습니다.
                 어떻게 해야 할까요? 감사합니다.
                   
                    event.preventDefault();
                }
            });
    });


        <div >
            <span class="drag" id="a1" style="margin:5px 5px 5px 5px">aaaa</span>
            <span class="drag"  id="a2" style="margin:5px 5px 5px 5px">bbbb</span>
            <span class="drag"  id="a3" style="margin:5px 5px 5px 5px">cccc</span>
            <span class="drag"  id="a4" style="margin:5px 5px 5px 5px">dddd</span>
            <span class="drag"  id="a5" style="margin:5px 5px 5px 5px">eeee</span>
        </div>

        <br />
        <br />
        <div id="drop" style="height:500px; width:500px ; border:1px solid; vertical-align: middle">
        Drop On Me
        </div>