What is special about the regexp "/"?

-----
kk@kundera:~/Kunden/hobby $ ~/bin/php -v
3.0b3-dev
kk@kundera:~/Kunden/hobby $ ~/bin/php   
<?php
$p = "/de/lall/laber";
$q = split("/", $p);
Content-type: text/html
 
<br>
<b>Warning:</b>  bad regular expression for split() in <b>standard input</b> on line <b>3</b><br>
kk@kundera:~/Kunden/hobby $ 
-----

$q = split("\/", $p) and $q = split('/', $p); fail as well.

I'm back to using explode now, but I really need to do more complex things...

Kristian