Question 1

Which email regex should I use?

Accepted Answer

Depends on your goal. The HTML5 (WHATWG) pattern is the pragmatic standard browsers validate with. The one-line "pragmatic" pattern (something@something.tld) is right when a confirmation email, not the regex, is your real check. The RFC 5322 pattern is technically complete and practically unusable — good for demonstrating why.

Question 2

Why do my test lines fail with the g flag?

Accepted Answer

Stateful: a regex with the g flag remembers lastIndex between exec calls, so line-by-line testing breaks. The tool silently strips g for you — this note exists so your production code does the same or creates a fresh RegExp per test.

Question 3

Should I validate emails with a regex at all?

Accepted Answer

Lightly. Reject obvious malformations (no @, spaces, double dots) and let the confirmation email do the real validation — it is the only test that proves the mailbox exists and belongs to the user. Over-strict regexes reject valid addresses like quoted local parts or new TLDs.

Question 4

Are the test addresses uploaded?

Accepted Answer

No — the whole tool runs locally in your browser. Load the sample list for instant edge cases, then paste your own.

Email regex tester

The three presets, honestly labelled