1. REGEX regex /^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}$/i This is ready to use in JavaScript as a regular expression literal. 2. HOW IT WORKS Pattern: regex ^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}$ with the i flag for case-insensitive matching. Part Meaning ^ Start of the string. Ensures we match the whole input, not just part of it. [A-Z0-9._%+-]+ Local part (before @). Allows one or more letters, digits, ., _, %, +, and -. @ Literal @ separator. [A-Z0-9.-]+ Domain name. Allows letters, digits, dots, and hyphens. \. A literal dot before the top-level domain. [A-Z]{2,} Top-level domain (TLD): at least two letters (e.g. com, uk, museum). $ End of the string. Prevents extra characters after the email. i Case-insensitive flag. TEST@EXAMPLE.COM matches just like test@example.com. Assumption This regex aims for practical email validation, not full RFC 5322 compliance. That's the most common interpretation for web forms in JavaScript. 3. TEST RESULTS Must match Input Result test@site.com ✅ Pass a.b@example.co.uk ✅ Pass Must NOT match Input Result plain text ✅ No match @nope.com ✅ No match (missing local part) user@ ✅ No match (missing domain) a@b ✅ No match (no .TLD) All required examples behave as requested. 4. EDGE CASES This regex intentionally keeps things readable, so there are some trade-offs. It accepts user+tag@example.com ✅ USER@EXAMPLE.COM ✅ (because of i) first_last@example.org ✅ user-name@example-domain.com ✅ It also accepts (although technically invalid) user..name@example.com .user@example.com user.@example.com user@example..com user@-example.com These are invalid according to the email specification, but preventing them makes the regex considerably more complex. It rejects a@b ❌ (no valid TLD) user@localhost ❌ user@[192.168.1.1] ❌ Internationalized email addresses like 用户@example.com ❌ If you need any of these, you'll need a different pattern or dedicated email validation. 5. VARIATIONS Stricter version Prevents leading/trailing dots in the local part, consecutive dots there, and requires each domain label to start and end with an alphanumeric character. regex /^(?!.*\.\.)[A-Z0-9_%+-]+(?:\.[A-Z0-9_%+-]+)*@(?:[A-Z0-9](?:[A-Z0-9-]*[A-Z0-9])?\.)+[A-Z]{2,}$/i Compared to the basic version, this rejects examples like: .user@example.com user.@example.com user..name@example.com user@example..com user@-example.com Looser version Allows almost anything except whitespace and requires only an @ and a dot after it. regex /^\S+@\S+\.\S+$/i This is useful when you want minimal validation and plan to verify addresses another way (for example, by sending a confirmation email). It may accept some malformed addresses that the stricter patterns reject. JavaScript notes The pattern is written as a JavaScript regex literal: /.../i. The i flag provides case-insensitive matching. The ^ and $ anchors ensure the entire string must be a valid email address, satisfying the "match the whole string" requirement.