php preg仅匹配标签innerhtml

Question

如果标签有一个url而不更改href atrribute值，如何只更改标签的innerHTML？ 我正在使用

$str=preg_replace('/((((?:http|https|ftp):\/\/)|(www\.))(?:[A-Z0-9][A-Z0-9_-]*(?:\.[A-Z0-9][A-Z0-9_-]*)+):?(\d+)?\/?[^\s\"\']+)/i','<a href="$1" rel="nofollow" target="_blank">$1</a>',$str);

我想再次强调<a>标记的innerhtml和一些关键字 例如：$str=www.google.com, google is a search engine.，搜索关键字为google 现在输出看起来应该像html

<a href='www.google.com' >www.<b>google</b>.com</a>, <b>google</b> is a search engine.

I used code for that is
$keyword='google';//in this example

$keywords = preg_replace('/\s\s+/i', ' ', strip_tags(trim($keyword))); // filter
foreach(array_unique(explode(' ', $keywords)) as $keyword)
{
    $str = str_ireplace($keyword,"<b>".$keyword."</b>",$str);
}