How do I change words in post texts?

[kh99]

preg_replace returns NULL if there's an error, so my guess would be that you're passing it something that's not a valid pattern.

You could try something like this (for testing only):

$ret = preg_replace($pat, $rep, $this->post['message']);
if ($ret !== NULL)
   $this->post['message'] = $ret;
else
   $this->post['message'] .= '<BR /><BR />Pattern error: ' . preg_last_error();