making a plugin: replace custom bbcode with an image

[tassoman]

Hi i'm developing a small plugin who should insert an image taken from filesystem inserting a bbcode, but i can't get the image rendered, only

[ img ]/url/of/img.png[ /img ] (without spaces in brackets)

here there is the plugin code:

$vbulletin->GPC['message'] = preg_replace('/\[1d20\]/i','<img src="images/dices/dice_20_faced_'.mt_rand(1,20).'.jpg" alt="" />',$vbulletin->GPC['message']); 

what's wrong?