That's another absolutely huge one, and in fact, Perl has dictated its own widely-used standard: "Perl Compatible Regular Expression" (PCRE). I agree with the typewriter monkey if not used well.
One distribution I've contributed to a few times over the years is Email::Valid. Check out this regex that's part of it. By far, it is not poorly written, but in the past in a ticket or a patch, another contributor stated (paraphrased as I can't recall the exact wording), 'I'm not touching that regex; it's a thing that is made of nightmares' :)
Another thing, Perl is the "Practical Extraction and Reporting Language".
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...and that, because in a past life I knew nearly the whole RFC by heart, is one damned hard thing that became possible in literally, a single scalar variable.
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