Hi, I need some help on this, I saw a piece of code of javascript and i was confused on this,can you help me please or enlighten my mind,what does this javascript mean or how this behave.Thank you in advance.

this.className=this.className.replace(new RegExp(" over\\b"), "");



Code:
<!--[if lt IE 7]>
<script type="text/javascript">
sfHover = function() {
	var sfEls = document.getElementById("nav").getElementsByTagName("LI");
	for (var i=0; i<sfEls.length; i++) {
		sfEls[i].onmouseover=function() {
			this.className+=" over";
		}
		sfEls[i].onmouseout=function() {
			this.className=this.className.replace(new RegExp(" over\\b"), "");
		}
	}
}
if (window.attachEvent) window.attachEvent("onload", sfHover);
</script>
<![endif]-->