Email Local Part Only Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching email local part only, ported and verified for JavaScript. Identity and credential patterns need both correctness and safety, since they're frequent targets for adversarial input. The snippet below is ready to drop into your JavaScript project — whether you're validating in an Express middleware, a Next.js API route, or a client-side form.

Javascript Implementation

// Email Local Part Only
// ReDoS-safe | RegexVault — Identity & PII > Email Address

const emailLocalPartOnlyRegex = /^(?![^@]*\.\.)[a-zA-Z0-9][a-zA-Z0-9._%+\-]{0,62}[a-zA-Z0-9]$|^[a-zA-Z0-9]$/;

function validateEmailLocalPartOnly(input: string): boolean {
  return emailLocalPartOnlyRegex.test(input);
}

// Example
console.log(validateEmailLocalPartOnly("user")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Identity & PII > Email Address category and carries a ReDoS-safe certification. That matters for JavaScript developers because especially critical in long-running Node.js event loops where a ReDoS vulnerability can block the entire process. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

Local part max length is 64 chars per RFC 5321. Cannot start or end with a dot. Consecutive dots are prohibited in practice (though technically allowed in quoted strings). Common limit is 64 characters total.