<script>
$('p:first').data('info', 'value'); // populates $'s data object to have something to work with
 
$.extend(
jQuery.expr[":"], {
block: function(elem) {
return $(elem).css("display") === "block";
},
 
hasData : function(elem) {
return !$.isEmptyObject( $(elem).data() );
}
}
);
 
$("p:hasData").text("has data"); // grabs paras that have data attached
$("p:block").text("are block level"); // grabs only paragraphs that have a display of "block"
</script>