[kh99]

OK Try this:

1. Create a phrase for your error message text and for Phrase Type choose "Error Messages". Let's say the varname is must_use_hide (or if you use something else, also change it in the code below).

2. Create a plugin using the newpost_process hook with this code:

PHP Code:

if (!preg_match('|\[HIDE].+\[/HIDE]|', $post['message']))
{
   $dataman->error('must_use_hide');
} 


I tested it a little but I hate doing regular expressions so it may not be 100% correct, so maybe BirdOfPrey has a good way to check for HIDE tags.