php 正则表达式匹配非链接文字

//$str='<a href=”aaaaa”>aaa百度aaaaa</a><div>test</div><p>crumbs</p></a>百度知道<a href=”bbbb”>bbbbbbb</a><p>crumbs</p><p>crumbs</p>百度<p>crumbs</p><p>crumbs</p>贴吧<p>crumbs</p><p>crumbs</p><a href=”ccccc”>ccccccc</a>’;

$str='<a href=”aaaaa”>aaa百度aaaaa</a><div>test</div><p>crumbs</p></a>百度知道’;

//$str = preg_replace(‘/(?<=<\/a>)(.*?)(百度)(.*?)(?=(?<!(<a))<a)/i’,’\1<a href=\”baidu.com\”>\2</a>\3`’,$str);
$str = preg_replace(‘/(?<=<\/a>)(.*?)(百度)(.*?)(?=(?<!(<a)))/i’,’\1<a href=\”baidu.com\”>\2</a>\3`’,$str);
echo $str;