jQuery.noConflict();

String.prototype.contains = function(it) { return this.indexOf(it) != -1; };

jQuery(document).ready(function(){
    jQuery('.nav_button').each(function(){
        var t=jQuery(this);
        var src1= t.attr('src'); // initial src
        if(!src1.contains('_over')){
            var newSrc = src1.substring(src1.lastIndexOf('/'), src1.lastIndexOf('.')); // let's get file name without extension
            t.hover(function(){
                jQuery(this).attr('src', 'images/'+newSrc+ '_over.' + /[^.]+$/.exec(src1)); //last part is for extension
            }, function(){
                jQuery(this).attr('src', 'images/'+newSrc + '.' + /[^.]+$/.exec(src1)); //removing '_over' from the name
            });
        }
    });
});