Routing optional paremeter

[eluser]Jagar[/eluser]
How can I make a parameter optional in routing.

For example I have the following link

Code:

admin/admin_manage/index/(optional param}

into this

Code:

admin/manage/(optional param)

I'm doing the following:

Code:

$route['admin/manage/?(:any)'] = 'admin/admin_manage/$1';

but does not work

Thanks