mod rewrite: 500 error if user enters invalid characters in URL

I am using mod_rewrite on all queries on my site. I am trying to secure it so that anytime a url that is not found in the database it throws a 404.

The problem I am having is mod_rewrite gives me a 500 error if special characters are entered. I am only using lowercase letters, numbers and hyphens in all URLs on my site.

Here is the break down,
test/index.php

 $url = $_GET['p'];
 $url2 = strtolower(preg_replace('/[^a-z¦0-9¦-]/i','',$url));;
 
 if ($url != $url2) { 
  echo "404"; 
 } 
 else {
  echo "url is fine, fetching from database...";
 }

.htaccess

DirectoryIndex index.php
ErrorDocument 404 errors/404.php
<IfModule mod_rewrite.c> 
RewriteEngine on 
RewriteRule ^([a-z¦0-9¦-]+)$ index.php?p=$1 [L]
</IfModule>

this is the error i get in the browser if i enter test!
this works file with test or te-st

Not Found
The requested URL /test/scratch/test! was not found on this server.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
Apache/1.3.33 Server at 127.0.0.1 Port 80

error_log

[Mon Feb 25 20:48:59 2008] [error] [client 127.0.0.1] Invalid error redirection directive: errors/404.php

how can i just have the custom 404 page displayed if the user enters any characters that should not be in the url at all?