wordpress中的preg_replace在html标签处停止

I have written a preg_replace in my functions.php for my wordpress blog for my girlfriend to use a "shortcode" like the following:

[spoiler id="1" show="show me the list" hide="hide the list"]test1
test2<ul>test3
  <li>list item 1</li>
  <li>list item 2</li>
</ul>
[/spoiler]

My preg_replace in the functions.php is the following:

function BUFU_add_spoilers ($content)
{   global $post;
    $pattern = "/\[spoiler id=\"(.*?)\" show=\"(.*?)\" hide=\"(.*?)\"\](.*?)\[\/spoiler\]/is";
    $replacement ='<br><div align="right" style="margin: -58px 0 0 0"><a id="button_show_%SPOILID%_$1" onclick="document.getElementById(\'spoiler_%SPOILID%_$1\').style.display=\'block\'; document.getElementById(\'button_hide_%SPOILID%_$1\').style.display=\'\'; document.getElementById(\'button_show_%SPOILID%_$1\').style.display=\'none\';"><h5>[$2]</h5></a></div><div align="right"><a id="button_hide_%SPOILID%_$1" style="display: none" onclick="document.getElementById(\'spoiler_%SPOILID%_$1\').style.display=\'none\'; document.getElementById(\'button_hide_%SPOILID%_$1\').style.display=\'none\'; document.getElementById(\'button_show_%SPOILID%_$1\').style.display=\'\';"><h5>[$3]</h5></a></div><span id="spoiler_%SPOILID%_$1" style="display: none" class="spoilerBox">$4</span>';
    $content = preg_replace($pattern, $replacement, $content);
    $content = str_replace("%SPOILID%", $post->ID, $content);
    return $content;
}
add_filter('the_content', 'BUFU_add_spoilers', 5);

All works just fine if there is only text in between the "shortcodes", but it somehow does not work for the example above. The spoiler will include everything up to the opening "<ul>" (or an opening "<table>" for that matter), so test1 and test2 are in the spoiler, test3 is NOT. I just have no idea why. I tested the preg_replace online and it worked just fine.

I reckon it has something to do with the way wordpress adds filters and stuff to work on the text, but I can't figure it out... The result is just an empty ... class="spoilerBox"></span> followed by the list..

If anyone can help me with this, it would be greatly appreciated! I'm really going nuts over this problem!

Thanks in advance,

BUFU