Especially for firewall configuration, it's very much useful to be able to allow or block subnets (e.g. I want to allow all traffic originating on trusted part of my home network). Both UFW and firewalld to accept IP addresses with netmask, so it is just about allowing them in the input's validtors.
I decided to replace the regular expressions with a custom QValidator subclass, which uses lib'c inet_pton() to validate the entered IP address and can also handle optional netmasks.
FEATURE: 434317 CHANGELOG: Allow IP addresses with netmask in advanced rule editor