passing url vars to controller

building on what others have written

link:
http://www.example.com/category/books/mysteries/5

config/route:
$route['category/(:any)/(:any)/(:num)'] = "show/category/$1/$2/$3" ;

controller: Show

method: category($category=null,$subcat=null,$item=null){ // blah blah }

note: there are no numbers in the url except for the item number.
we don't reveal the controller name in the URL.
the URL is short and clear.