Single letter domain in DNS Zone Wizard

Hello,

I am trying to add a single letter domain (x.xx) to the dns wizard but it leaves me with the following error message:
"Domain contains invalid characters."
I edited the validate_domain.inc.php file as suggested here, and it allowed me to add the domain to the websites but it does not fix the problem inside the dns zone wizard. Any suggestions?

I run ISPConfig 3.1.6 on Debian 9

 

You have to change the regex in web/dns/dns_wizard.php

 

Thank you!

 

Which file handles the regex when I'm editing a DNS Zone? I get the following error message when I click on Records:
"1. Zone has a invalid format."

 

I did a grep and found it on line 99 in "dns_soa.tform.php", if anyone else has the same problem.