$post_content = preg_replace("/\r\n/", "\n", $post_content);
$post_content = preg_replace("/\n\r/", "\n", $post_content);
$post_content = preg_replace("/\r/", "\n", $post_content);
$post_content = preg_replace_callback(
    '/<pre>((?:.|\n)*?)<\/pre>/',
    create_function(
        '$match',
        'return "[code]" . preg_replace("/ /", "&nbsp;", preg_replace("/\t/", "    ", preg_replace("/\n/", "<br>", $match[0]))) . "[/code]";'
    ),
    $post_content
);
$post_content = preg_replace("/\s+/", " ", $post_content);
$post_content = preg_replace("/&nbsp;/", " ", $post_content);
$post_content = preg_replace("/(?!^)(?:\s*<\/?p(?: .*?)?>\s*)+/", "\n\n", $post_content);
$post_content = preg_replace("/<br(?:\/| .*?)?>/", "\n", $post_content);
$post_content = preg_replace("/<script(?: .*?)?>(?:.|\n)*?<\/script>/", "", $post_content);
$post_content = preg_replace("/<a [^>]*?href="?([^ ">]+)"?.*?>((?:.|\n)*?)<\/a>/", "[url=$1]$2[/url]", $post_content);
$post_content = preg_replace("/<img [^>]*?src="?([^ >"]+)"?.*?>/", "[img]$1[/img]", $post_content);
$post_content = preg_replace("/<blockquote>((?:.|\n)*?)<\/blockquote>/", "[quote]$1[/quote]", $post_content);
$post_content = preg_replace("/<li>((?:.|\n)*?)\s*(?=(?:<\/ul>|<\/ol>|<li>))/", "[*]$1\n", $post_content);
$post_content = preg_replace("/<ul>((?:.|\n)*?)<\/ul>/", "[list]$1[/list]", $post_content);
$post_content = preg_replace("/<ol>((?:.|\n)*?)<\/ol>/", "[list=1]$1[/list]", $post_content);
$post_content = preg_replace("/<(strong|b)>((?:.|\n)*?)<\/\1>/", "[b]$2[/b]", $post_content);
$post_content = preg_replace("/<(em|i)>((?:.|\n)*?)<\/\1>/", "[b]$2[/b]", $post_content);
$post_content = preg_replace("/<u>((?:.|\n)*?)<\/u>/", "[u]$1[/u]", $post_content);
$post_content = preg_replace("/<.*?>/", "", $post_content);
$post_content = html_entity_decode($post_content);