Please, I am getting crazy with the pagination class :(

[eluser]Althalos[/eluser]
I didn't give you a solution so that you didn't have to think. I gave you an example, so that you could understand one way to do what you wanted to do. The username part had a purely explanatory purpose.

I understand why articles/page:2 is an improvement over articles/2, but articles/page/2 does exactly the same thing. It just looks a little bit different, but it is by far more common than separating values with :.

Maybe this is why the format I suggested is supported in the URI class whereas the format you suggest is not.

However, if you want to use your format you can still achieve this. It wouldn't be as beautiful a solution, but it'd work:
$pageId = str_replcae('page:', '', $this->uri->segment(2));