Form validation not running callback function

[eluser]benshepherd283[/eluser]
Hello,

Before I state my problem, I would like to say that I started learning CodeIgniter yesterday so apologies in advance if I make myself look stupid

I have a model that I am using to check inputted information when a user signs up and I would like a callback function that checks if the username contains symbols but for some odd reason, I can't seem to get the callback function to work. I can't get it to be called
Here's my code:

Code:

public function create() {
  
   $this->init();
  
   $this->load->library('form_validation');
   $this->form_validation->set_rules('username','Username','required|min_length[3]|max_length[16]|is_unique['.$this->table.'.username]|trim|xss_clean|callback_validate_username');
   $this->form_validation->set_rules('password','Password','required|min_length[6]');
   $this->form_validation->set_rules('passconf','Password confirmation', 'required|min_length[6]');
  
   return $this->form_validation->run();
  
  }
  
  public function validate_username($username) {
  
   die("DEBUG"); //so I can see if it has been called
  
   if(!preg_match("/^[a-zA-Z0-9]+$/", $username)) {
    
    $this->form_validation->set_message('validate_username', 'Username must not contain symbols');
    return false;
    
   }
  
   return true;
  }