Routing problem

[eluser]cortina[/eluser]
I've got an issue with a URL containing a query string that's being appended by a third party payment processor when returning users to a site I'm building. The URL I want is:

Code:

http://www.mysite.com/checkout/complete

but the payment processor apends:

Code:

?oid=1234567

where 1234567 is an order number (always an integer).

According to the user guide I should be able to route this using a regex:

Code:

$route['checkout/(complete\?oid\=([0-9]+))'] = 'checkout/complete';

But I'm still getting 404's after trying a bunch of alternatives.
Does anyone have any suggestions?
TIA
Rob