Hi Ranadeep,
The extension validation is an add-on for the jQuery validation library and it's not included in Meta Box.
In order to use it, please download the extension.js file from Github:
https://github.com/jquery-validation/jquery-validation/blob/master/src/additional/extension.js
And enqueue it like this:
add_action( 'rwmb_enqueue_scripts', function() {
wp_enqueue_script( 'jquery-validation-extension', 'url to the js file', '', '', true );
}
Then you can use the extension rule.
.
