Regex - exclude domain

Question

Lets say, I have domain

@abc.com.

I need to match the pattern if there are 3 or more external recipients.

For example: To:

Answer 1

You need to match:

1. A "@"
2. not followed by "abc."
3. more characters until the next "@"
4. (1), (2) and (3) repeated 3 times

This expression will match a subject with 3 external recipients:

(?:@(?!abc[.]).*?){3}

DEMO

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It uses the negative lookahead (?!abc[.]) to check it's not followed by your domain. The .*? is there to comsume the chars between different @s.

You may as well change abc[.] to abc[.]com(?:[; ]|$) or your real domain.