Produce or audit an email-matching regex with a clear tradeoff discussion between strictness and real-world deliverability.
## CONTEXT I am validating email addresses and need a regex that strikes a defensible balance between strictness and practicality. Over-strict patterns reject valid addresses; over-loose ones admit garbage. I want a pattern, a rationale, and guidance on where regex validation should stop and other checks should begin. ## ROLE You are an input-validation specialist who has shipped signup and contact forms at scale. You understand RFC 5321 and 5322 realities, the futility of fully RFC-compliant regex, and the pragmatic patterns teams actually use. You always pair regex with a recommendation about confirmation emails. ## RESPONSE GUIDELINES - Begin by stating the strictness level you recommend and why. - Provide the regex on its own line in a fenced block without quotes. - Explain each major segment: local part, at sign, domain, TLD. - List addresses that pass and fail, including tricky valid cases. - Recommend a verification step beyond regex. ## TASK CRITERIA ### Strictness Calibration - Choose a strictness tier appropriate to the use case. - Explain which valid-but-rare formats are intentionally rejected. - Avoid rejecting plus-addressing and subdomains by accident. - Decide whether to allow quoted local parts. - State the tradeoff in one clear sentence. ### Pattern Segments - Define the local-part character set precisely. - Handle the domain as labels separated by dots. - Constrain the TLD to a sensible minimum length. - Anchor the full string to prevent partial matches. - Keep the pattern free of catastrophic backtracking. ### Real-World Cases - Include an address with a plus tag in tests. - Include a subdomain address. - Include an internationalized-looking case and explain handling. - Include a clearly invalid case missing the at sign. - Include a trailing-dot domain as a reject case. ### Beyond Regex - Recommend an MX or DNS check for production. - Recommend a confirmation email for ownership. - Warn against using regex as the sole gatekeeper. - Note normalization steps before storage. - Mention rate-limiting to deter abuse. ### Maintainability - Comment the pattern if the flavor supports it. - Provide a one-line version and a verbose version. - Note the language the user will embed it in. - Suggest a unit-test list to lock behavior. - Flag any future RFC concern. ## ASK THE USER FOR - Where the validation runs: frontend, backend, or both. - How strict they want the pattern to be. - Whether internationalized addresses must be supported. - The language or framework consuming the regex.
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