I have reviewed the documentation and FAQ and confirm that my issue is NOT addressed there.
on
I have searched this support forum (open and closed topics) and confirm this is not a duplicate.
on
I understand that topics in this category are triaged by a bot first but a human reply will follow up. If I’d prefer a human-only reply, I’ll add the no-ai tag to my topic.
The doc is actually correct as written. That block is a JSON object (submitted via the JMAP API or WebUI, not a raw expression string), so the regex has to be JSON-escaped: to represent a single backslash character (for \. in the regex) inside a JSON string you need \\.. If you used the single-backslash version you posted, the JSON parser would interpret \. as an invalid escape sequence.
So "matches('^([^.]+)\\.([^.]+)$', rcpt)" in the JSON snippet does correctly encode the regex ^([^.]+)\.([^.]+)$ once JSON-decoded, matching a local part like alias.user and capturing user as group 2. You can see this pattern on the Subaddressing docs page.
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