Error when registering user that was registered already

[jpietrowiak]

Here is the error that shows up when I try to register a user that already registered. The page looks like it refreshes and the error below shows up on the top of the page. ANy idea what could be the problem? Its a new vbulletin setup and was imported from Punbb. version 3.8

Warning: preg_match() [function.preg-match]: Compilation failed: regular expression is too large at offset 51953 in [path]/includes/functions_user.php on line 849

Then I get this when I do register a new member- in addition to the one above

Unable to add cookies, header already sent.
File: /home/cncmach/public_html/cncforums/includes/class_core.php
Line: 3254

Thanks
Jamey