I'll quote the WHATWG HTML spec, which that Ruby code cites as its source (https://html.spec.whatwg.org/multipage/input.html#valid-e-mail-address https://html.s

I'll quote the WHATWG HTML spec, which that Ruby code cites as its source (https://html.spec.whatwg.org/multipage/input.html#valid-e-mail-address https://html.spec.whatwg.org/multipage/input.html#valid-e-ma...): > This requirement is a willful violation of RFC 5322, which defines a syntax for email addresses that is simultaneously too strict (before the "@" character), too vague (after the "@" character), and too lax (allowing comments, whitespace characters, and quoted strings in manners unfamiliar to most users) to be of practical use here. Personally, I say if it's good enough for Web browsers, it's probably good enough for your app.