Form Validation Config File

Hello, I am new on this forum
I have problem with creating a form_validation.php file for entire application validation
I want custom error message for regex_match but. I got Default Error Validation Message

PHP Code:

//form validation regular expression
defined('NAME_EXP') OR define('NAME_EXP', '[a-zA-Z .\-]+$');

'applicant' => [
    'field' => 'full_name',
    'label' => 'Applicant Name',
    'rules' => 'trim|required|max_length[255]|min_length[3]|regex_match[/' . NAME_EXP . '/]',
    'errors' => [
        'regex_match[/' . NAME_EXP . '/]' => "The %s field may only contain Alphabetic characters, Fullstops(.), Hyphens(-) and spaces( )."
    ]
]