php如何用正则给文章加导航目录

$str = '<h2 class="archt"><strong>我是里面的内容</strong></h2><h2 class="archt2">我是第2个内容</h2><strong>我没有h2包在里面</strong>';
preg_match_all('/<h2.*?>.*?(<[^>]+>)?([^<]+)(<\/[^>]+>)?<\/h2>/s', $str, $arr);
print_r($arr);

 Array(
    [0] => Array
        (
            [0] => <h2 class="archt">
<strong>我是里面的内容</strong></h2>
            [1] => <h2 class="archt2">我是第2个内容</h2>
        )

    [1] => Array
        (
            [0] => <strong>
            [1] => 
        )

    [2] => Array
        (
            [0] => 我是里面的内容
            [1] => 我是第2个内容
        )

    [3] => Array
        (
            [0] => </strong>
            [1] => 
        )

)

function article_index($content){  
    $matches = array();   
    $ul_li = '';    $r = '/<h2.*?>.*?(<[^>]+>)?([^<]+)(<\/[^>]+>)?<\/h2>/s';    if(preg_match_all($r, $content, $matches))
    {        foreach($matches[2] as $num => $title)
        {            $ul_li .= '<em>'.$title." </em>";
        }   
    }   
    return $ul_li;
}