Hello, please provide more details:
what led to this bug, enabled shippings, enabled payments

when you insert code or log message please use "Insert Code" in forum message editor.

To add additional data to email template, you should:
1. Add to php code variables with needed data. (Hooks, direct change)
2. Add variables to "Allowed Placeholders".

You can change tpl (for custom JS validation) or create extension with hook's to add php validation.

I try your regex - they work fine.
Provide more information: Abantecart Version, browsers for test, have you errors in browser console? errors in abantecart log or php? etc.

Best way - create extension with hook's in Storefront Category controller.
To prevent direct access to product you need hooks:
 Storefront Product controller, Storefront Search controller, Block controllers (if used)

Doc: https://abantecart.atlassian.net/wiki/spaces/DOC/pages/17793156/Hooks

You can create extension with Hook method like this:

public function onModelAccountCustomer_ValidateData()
{
$that = $this->baseObject;
 if ($this->baseObject_method !== 'validateRegistrationData') {
            return;
  }
if (!$that->request->is_POST()) {
return;
}
if ((mb_strlen($that->request->post['postcode']) !=5)) {
            $that->error['postcode'] = $that->language->get('error_postcode');   //Your custom error message here
     }
}

You can check exists orders in Cart Controller Hook: storefront/controller/pages/checkout/cart.php:26
for check email registration create hook for Registartion Controller: storefront/controller/pages/account/create.php:24