JavaScript正则表达式实现关键字高亮加粗

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>正则替换</title>
</head>
<body>
<script type="text/javascript">
var str = "使用正则表达式把关键字替换加粗，文字加粗";
var word = "加粗";
str = str.replace(new RegExp("(" + word + ")","ig"), "<strong>" + word + "</strong>");
document.write(str);
</script>
</body>
</html>