What it emitted accepts a large number of invalid addresses (due to things like not checking dot placement, and the inexplicable (…)+ wrapping around the entire thing), and doesn’t accept a large number of valid addresses (some comparatively esoteric, like local parts containing any of !#$%&'*/=?^`{|}~ or IP addresses for the domain name, and some very reasonable, like TLDs of more than six characters, or internationalised TLDs even in Punycode form).
The description it emits does not match the regular expression at all well, either.
The second regex it emits is even worse than the first, unnecessarily uses PCRE-specific syntax, and is given with a nonsensical description. (Note: the asterisks got turned into italics, backslash-escape them here on HN. With this fixed, the regex was \A[\w.]*@[\w.]*\.\w{1,4}\z.)
> on the very surface scan seems OK
And there’s the danger of this stuff. As a subject-matter expert on regex and email, I glanced at the regular expression and was immediately appalled (… quite apart from the whole “here we go again, this is certain to be terrible” cringe on the prompt). But it looks plausible enough if you aren’t.