I copied and pasted them in my farewell.php and I tried 2 different posts.
In the first one I said "I'm leaving" and the hack kicked in. And in the other one I said "blah blah leaving" and nothing happened. But I did not get any errors either. I am not sure why you are getting that error. Maybe you can tell me what exact phrase causes that error.
On a different note, you don't need various upper/lower case combination of words. The
/i makes the regex case insensitive.