CodeIgniter: Validate form with multidimensional POST data

Question

So the framework is CodeIgniter 2.0.2. I have a form that has groups of fields that correspond to rows in a database. The names of the fields are in the format:

opt[0][foo]
opt[0][bar]
opt[1][foo]
opt[1][bar]
etc...

The index (1,2,etc...) does not correspond to row IDs in the database, it is simply a way to split up the groups of fields. There may be gaps in the index as users are able to add and remove an arbitrary number of the field groups. All groups are identical, that is, they contain exactly the same set of fields with the same second level names.

I want to be able to use CodeIgniter's validation library to validate the form and (p)re-populate as necessary. I've found plenty of posts (in addition to the excellent CI user guide) on the pre-populating and I know how to get the working with the re-populating in general. However, this is the first time I've had to try it with the indexed field names as above. I've tried the below and it doesn't work:

array(
    'field' => 'opt[][foo]',
    'label' => 'Foo',
    'rules' => 'required'
)

I'm guessing I was just hoping for too much and CodeIgniter doesn't support what I need it to do. Extending the existing form validation library is an option so if anyone has been in the same situation and can provide some tips that would be very welcome.

UPDATE:

Just a little extra info, I've also tried validating a specifically indexed field (see below) and that also didn't work... As I understand it multidimensional validation should work in the specific case:

array(
    'field' => 'opt[0][foo]',
    'label' => 'Foo',
    'rules' => 'required'
)

Answer 1

The following controller code works for me on CI 2.0.2

public function test() {

        $this->load->library('form_validation');
        $this->load->helper('form');

        $this->form_validation->set_rules('test[test1][test2]', 'Test', 'required|valid_email');

        $this->form_validation->run();  

        echo validation_errors();

        echo form_open($this->uri->uri_string());
        echo form_input('test[test1][test2]', set_value('test[test1][test2]'));
        echo form_submit();
        echo form_close();

    }