Error message

Hi,

I have done all of the updates, but my gallery is still showing an error message.

http://corporateartforce.com/project/catalog/nggallery/catalog/current-framed-inventory/

Warning: preg_match() [function.preg-match]: Compilation failed: unrecognized character after (?< at offset 19 in /home/corpor16/public_html/wp-content/plugins/nextgen-gallery/products/photocrati_nextgen/modules/router/mixin.url_manipulation.php on line 161

How do I fix this issue? If it is not resolved soon I will have to remove it from my website.

http://wordpress.org/plugins/nextgen-gallery/