They can get ridiculously convoluted as in the case above and, according to the specification, are often too strict anyway.
If you actually check the Google query I linked above, people have been writing (or trying to write) RFC-compliant regular expressions to parse email addresses for years.
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At this point, why keep parsing email addresses for their format?
Think about it this way: I register for your website under the email address . That’s probably going to bounce off of the illustrious mail daemon, but the formatting is fine; it’s a valid email address.
To fix this problem, you implement an activation system where, after registering, I am sent an email with a link I must click.
A complex regex validation on the email address doesn’t introduce an additional solution, it introduces an additional problem.
We can validate email address at client side and server side.
Instead, the regular expression merely verifies that the top-level domain name consists of between two and twenty-four ASCII characters, with alphanumeric first and last characters and the remaining characters being either alphanumeric or a hyphen (-). Try 'Examines the domain part of the email and normalizes it.