form_validation->run not working as expected

[eluser]Unknown[/eluser]
I　ｗａｓ　ｔｒｙｉｎｇ　ｔｏ　ｍａｋｅ　ｏｎｌｙ　ｓｏｍｅ　ｏｆ　ｔｈｅ　ｒｕｌｅ　ｒｕｎ　ｆｏｒ　ｆｏｒｍ_ｖａｌｉｄａｔｉｏｎ　ｂｙ　ｓｐｅｃｉｆｙｉｎｇ　ｗｈａｔ　ｔｏ　ｒｕｎ.　Ｆｏｒ　ｅｘａｍｐｌｅ:

Code:

$this->form_validation->set_rules($config);
  if (($this->form_validation->run('username') == FALSE))
  {
   $this->load->view('templates/header');
   $this->load->view('teacher/test_regi');
   $this->load->view('templates/footer');
  }

ｗｈｅｒｅ　Ｉ　ｓｐｅｃｉｆｉｅｄ　ｏｎｌｙ　ｃｈｅｃｋ　ｆｏｒ　ｔｈｅ　ｕｓｅｒｎａｍｅ　ｆｉｅｌｄ.
Ｗｈｅｎ　ｔｈｉｓ　ｃｏｄｅ　ｉｓ　ｐｕｔ　ｉｎｔｏ　ｐｒａｃｔｉｃｅ, all of the rules specified in the $config array is checked. Did I do something wrong?
Many thanks for the help

PS: the $config array is here:

Code:

$config = array(
  array(
            'field'   => 'username',
            'label'   => 'Username',
            'rules'   => 'trim|required|xss_clean|min_length[4]|max_length[20]|alpha_dash'
        ),
  array(
            'field'   => 'password',
            'label'   => 'Password',
            'rules'   => 'trim|required|xss_clean|min_length[4]|max_length[20]|alpha_dash|matches[passconf]'
        ),
  array(
            'field'   => 'passconf',
            'label'   => 'Password Confirmation',
            'rules'   => 'trim|required'
        ),
  array(
            'field'   => 'firstname',
            'label'   => 'Firstname',
            'rules'   => 'trim|required'
        ),
  array(
         'field'   => 'lastname',
         'label'   => 'Lastname',
                     'rules'   => 'trim|required'
                  )
)