How to set custom error message with form_validation And CodeIgniter

[atoz]

i am newbie in CodeIgniter...and i am trying to do form validation for array input...the array name is pages[].and i wrote:\[code\]$this->form_validation->set_rules('pages[]','','required');\[/code\]if i use that:\[code\]$this->form_validation->set_message('required', 'you not selected pages.');\[/code\]it will not change the other "required" validation input params?So how can i set error message only for one validation?