[exim] Finding Fallback DKIM Domain

Hi,

I’m trying to sign all mails with DKIM from all subdomains of certain
domains with the key of the „main“ dkim domain. Or in other words I
want to find the dkim fallback domain to know which key I need to use
for signing.

Some examples for what domains I need to find the key for:
foo.example.com -> example.com
foo.bar.example.com -> example.com
foo.example.co.uk -> example.co.uk
foo.bar.example.co.uk -> example.co.uk

I thought about using some regex substitution but with double domains
it’s not that’s easy.

Is there another way to achieve this?

Best,
Christian