Regex and cPanel Account Filtering - regex

Question

after countless hours googling and trying to contact my webhost (with no positive results) I wanted to jsut 'throw my question out there' and get better expertise with my issue. I really do believe, that this will be helpful to a lot of people as well, stuck asking the same question!
Just to keep things short, we have hosted our email solution with a webhost using cPanel and I have a big requirement. Basically, I need an account level filter to block certain mail addresses from sending out to other mail servers. For example;
lets say we use example.com
user1#example.com can send mail to anyone, anywhere
user2.int#example.com is only allowed to send mail to example.com address but not to any other address, for example gmail.com, yahoo.com, etc.
Out of the options given to me at account level filtering, I thought the best to use is regex.
I'm suspecting that EXIM (default mta for cpanel) uses PCRE like regex expressions, please correct if im wrong.
The syntax i wrote and need help with is the following:
^(?!.+\#example\.com$).*$
With this, all example.com addresses should not match and all other addresses should.
The testing tools I used is https://www.debuggex.com/
Guys, please help and let me know what I am doing wrong. cPanel is letting mail go through and is not blocking it.

Answer 1

The regex:
^(?![^#]*?#example\.com)
should do the trick
How it works
^: Find the beginning of the string/line
(?!...) Assert that it is impossible to find the following regex:
[^#]*? Match all the characters that are not an at symbol (#)
#example\.com Match the exact string '#example\.com'
For a more in-depth explanation see this