text.replace(new RegExp('(<a[^>].*>.*)('+keyword+')(.*</a>)'),'$1<b>$2</b>$3')
var text = "<a href='http://www.baidu.com'>http://www.baidu.com</a>";
var keyword = "www";
var regexp = new RegExp((
'(<a.*?>\\s*)'
+ '(https?://\\s*)' //开头的http://或https://
+ '(\\w+)' //主机名
+ '(.\\w+)+' //域名
+ '(\\s*</a>)'
), 'gi');
console.log(text.replace(regexp, '$1$2<b>$3</b>$4'));
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)
