$(document).ready(function(){
   $(".over").hover(function(){
	   var re = new RegExp("(.+)\\.(gif|png|jpg)", "g");
       var hoverImg = $(this).attr("src").replace(re, "$1_over.$2");
       $(this).attr("src", hoverImg);
     }, function(){
		var re = new RegExp("(.+)_over\\.(gif|png|jpg)", "g");
       var normalImg = $(this).attr("src").replace(re, "$1.$2");
       $(this).attr("src", normalImg);
     }
   );
});