text.replace(new RegExp('(<a[^>].*>.*)('+keyword+')(.*</a>)'),'$1<b>$2</b>$3')

------解决方案--------------------

<script type="text/javascript">
           var text = "<a href='http://www.baidu.com'>http://www.baidu.com</a>";
           var keyword = "www";
           var pattern = new RegExp('^(<[^>]+>[^<]*)(' + keyword + ')([^<]*<[^>]*>)$');
           text = text.replace(pattern, "$1<b>$2</b>$3");
           alert(text);
       </script>

------解决方案--------------------
JScript code
    var text = "<a href='http://www.baidu.com'>http://www.baidu.com</a>afefeeaaa<A href='http://www.code-design.cn'>http://www.code-design.cn</a>";
    var keyword = "www";
    var rx = /(<a[^>]+>)([^>]+)(<\/a>)/ig
    text = text.replace(rx, function ($0, $1, $2, $3) { return $1 + $2.replace(keyword, '<b>' + keyword + '</b>') + $3; });
    alert(text)