hi,
after a fresh install of J!4 and CBSubs, you can edit and save existing country entries in Geographic Zones, but when you try to add or copy an entry in "edit entries in Geographic Zones" (e.g. because Croatia is missing in EU Zone) you get an error and the message:
An error has occurred.: 1364 Field 'zip_code_regexp' doesn't have a default value, and redirect to Joomla! dashboard

everything works correctly after setting the table structure manually, e.g. in phpMyadmin
`poaa_cbsubs_geo_zones_entries` -> `zip_code_regexp` ->DEFAULT NULL;

this problem does not occur in J!3.10, even though the default value of zip_code_regexp is not set there either, you can not only edit, but also add or copy entries without any problems.

tested on:
CB 2.8.0+build.2023.01.27.16.59.55.3636e2d5a
CBSubs 4.6.1+build.2023.01.18.18.35.11.074602ba8
J!4.2.6
php 8.0.25, mysql DB 10.4.27-MariaDB