Wordpress / Gravity Forms - custom validation in specific field

Job ID: 33245796

Budget: $10 – $30 USD

I need to implement a custom validation for one of the form fields supported by Grativy Forms in Wordpress

Validation is to check whether the entered number is the correct Polish NIP number.

Actually, I have a feature ready, but to apply to a different plugin:
https://gist.githubusercontent.com/swoboda/446ded1dad928c9baf436140e5e9a537/raw/467d6ceeb73bc94b8ac0191b261e0e952e5a46da/fcf-wlasna- validation-nip.php

Now I need to transfer the assumptions of the above validation to a function compliant with the gravity forms guidelines here: https://docs.gravityforms.com/gform_field_validation/

So the task is to rewrite one function to another.
The result is to be ready code to be implemented in the functions.php file
Related categories: PHP WordPress Process Validation Gravity Forms