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

You can hook ControllerCommonListingGrid via extension and change: $this->data[''colNames'']  $this->data[''colModel'']

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