too long

I need to display an error message for a custom validation rule, but I can't get to do it.

This is the validation rule:

    $config = array(
             ....,
            array(
                    'field' => 'general_sales_subaccount',
                    'label' => 'General Sales Subaccount',
                    'rules' => array(
                            'required',
                            'numeric',
                            array(
                                    $this->subaccounts_model,
                                    'is_valid'
                            )
                    ),
            )
    );

    $this->form_validation->set_rules($config);

And now this is the referenced model method:

    public function is_valid($subaccount)
    {
        $subaccount_num_digits = $this->preferences->get('subaccount_num_digits');

        if (strlen($subaccount) != $subaccount_num_digits ) {
            $this->form_validation->set_message('is_valid', "The number of digits in %s doesn't match the length set to " . $subaccount_num_digits);
            return false;
        }       

        return true;
    }

The rule seems to work, but it displays this error message:

Unable to access an error message corresponding to your field name (Anonymous function).

Any ideas?

You can't get an error message because you don't setup functions name. You may change your rule function like below:

$config = array(
         ....,
        array(
                'field' => 'general_sales_subaccount',
                'label' => 'General Sales Subaccount',
                'rules' => array(
                        'required',
                        'numeric',
                        array( //you may get all in another array
                          'is_valid', // and tell codeigniter your functions name
                          array(
                                $this->subaccounts_model,
                                'is_valid'
                        )
                    )
                ),
        )
);

$this->form_validation->set_rules($config);