There's little I can offer to make it more understandable: the syntax of email addresses is complex. Just be glad that we're living in a post-5.10 world: now we can use rules and recursion which allows us to, almost mechanically, translate BNF grammars to regular expressions. In one (both?) of the editions of "Mastering Regular Expressions", Jeffrey Friedl gives a pre-5.10 regular expression to match email addresses. That one is far, far more complex (and doesn't allow nested comments below a certain depth (2, IIRC)).

You may want to look at RFC 822, or one of its descendants, for the grammar of email addresses. It's my understanding, the regexp I gave was constructed based on the grammar given in one of the RFCs. (I don't recall which one, and the file t/re/reg_email.t doesn't say where it comes from).

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