Routing URLs with colon and numbers

Hello!
I'm trying to configure the routing file in code igniter, but I'm having problems with certain URLs. This route

$route['api2/(:any)'] = "api2/api2/$1/";

matches these URLs:

/api2/pictures/alpha_string
/api2/pictures/picture_id:alpha_string

but not this one:

/api2/pictures/picture_id:55

The difference is that I am using numbers after the ":". All URLs are valid, but cannot make the last one work. How can I make it match it?

Thanks