Validation rule required on multiple select field always fails

My bad, I meant 'rules' => 'required' in the example PHP code (I edited my initial question).
Any how I found the solution to my problem, I had to change the name of the field in the rules. So instead of usings 'groups'

Code:

$rules = [        
   ['field' => 'groups', 'label' => 'groups', 'rules' => 'required']
];

is used 'groups[]'

PHP Code:

$rules = [        
    ['field' => 'groups[]', 'label' => 'groups', 'rules' => 'required']
]; 


and it worked. The strange thing using 'groups' seemed to worked for CI2.