Allowing only question marks in input

[eluser]ModernNinjaX[/eluser]
Hey guys, I'm new to Codeigniter but I'm loving it so far... here's a question I have though

I have a controller method that needs to accept an external API key, which could possibly have a question mark in it.

How would I allow *only* this character in URLs? Codeigniter gives it a 404 immediately, even when I don't use the default routing (index.php/controller/method/api?key)

Thanks in advance

EDIT: By 'only' I mean that this is the only special character I want to allow (if CI is even designed to handle question marks)