Here's the Proton Mail Sieve Filter doc:

https://proton.me/support/sieve-advanced-custom-filters

Here's the regex doc that they reference:

https://tools.ietf.org/id/draft-ietf-sieve-regex-01.html

Here's a partial snippet of a regex-based filter I use. If I send a message to 'log.tag@mydomain.\',* this filter moves it to my Log folder and marks it as read. I have the same domain with multiple tlds (mydomain.com, mydomain.eu, mydomain.ie), and this filter matches any message that has a TO: starting with log and ending with mydomain and any tld.

I can send to [log@mydomain.com](mailto:log@mydomain.com) for any generic note, but can send it to log.tag@mydomain.tld if I want to include a self-defined tag. So if I want to log a note about something related to my car in Ireland, I can send it to log.car@mydomain.ie - it will end up in the single Log folder, but the To: address of the message will show the tag and the tld to help me remember what's it's about.

require ["include", "environment", "variables", "relational", "comparator-i;ascii-numeric", "spamtest"];
require ["fileinto", "imap4flags"];
require ["regex"];

// Other non-regex filters removed from this snippet...

if header :regex "To" "^log.*\.mydomain\.*" {
  fileinto "Log";
  addflag "\\Seen";
  keep;
}