Using 2.2.2rc2

Everytime I try to set the base-url through Director::setBaseURL ,SS throws an error :

FATAL ERROR: preg_match() [function.preg-match]: Unknown modifier ']'
At line 253 in /home/httpd/php/SilvWorldWW/sapphire/core/control/Director.php

This looks like a reg.ex bug ? If anyone can confirm this behavior I'll create a ticket for it.

Some more info :

Silverstripe is running in a subdir/alias... I'm using rewrite rules to make requests go to silverstripe :

e.g.
www.mydomain.com/helpme would be rewritten to www.mydomain/silverstripe/helpme ...

All works great except I can't fool Silverstripe into producing the correct links by itself ; it always produces links to the real location instead of the location I want. That has to do with SS using the wrong Base URL ....

If I try to manually set my base URL to "/" or "www.mydomain.com" , it throws the error I mentioned in the post above..

In the site itself I can manually fix this by building links by concatenating a "/" and
$URLSegment , but the sitemap.xml is all screwed up.

Any solutions ?