This pattern covers the email shapes that actually show up in practice: a username of letters, numbers, and ._%+-, an @, a domain, and a TLD of 2+ letters.
What it's not: genuinely full RFC 5322 compliance — quoted local parts ("john doe"@example.com), comments, and IP-literal domains (user@[192.168.1.1]) are technically legal email addresses this pattern rejects. For form validation, that's usually the right tradeoff; for a mail-parsing library, use an actual RFC 5322 parser instead of a regex.