How can I validate an email address using a regular expression?

Validating an email address using a regular expression is a common task, but it’s important to note that email addresses can be surprisingly complex according to the full RFC standards. In most practical scenarios, a moderately strict pattern is sufficient. Below is a commonly used regex that works for many typical use cases:

^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$

Explanation

1. ^[a-zA-Z0-9._%+\-]+

   • Matches one or more characters allowed before the “@,” including letters, digits, “.”, “_”, “%”, “+”, and “-”.
   • The caret (^) ensures the match starts at the beginning of the string.

2. @

   • Requires the presence of the “@” symbol.

3. [a-zA-Z0-9.\-]+

   • Matches the domain name part (letters, digits, dots, and hyphens).

4. \.

   • Matches a literal “.” before the top-level domain (TLD).

5. [a-zA-Z]{2,}$

   • TLD must be at least two letters (e.g., .com, .net, .org, .io, etc.).
   • The dollar sign ($) enforces the match to end there.

Usage Example (JavaScript)

function isValidEmail(email) {
  const emailRegex = /^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$/;
  return emailRegex.test(email);
}

console.log(isValidEmail('test@example.com'));   // true
console.log(isValidEmail('invalid-email@com'));  // false

Recommended Resources

1. Grokking the Coding Interview: Patterns for Coding Questions

Caveats and Best Practices

1. No Regex is Perfect
   • The full specification for email addresses (RFC 5322) is extremely permissive. A single regex that catches every valid address and excludes every invalid one can become huge and unwieldy.
2. Practical vs. Theoretical
   • In typical scenarios (like web forms), the above regex or something similar will work well.
   • For strict compliance or specialized domains, consider more advanced validation or DNS checks.
3. Use Libraries
   • If possible, use vetted libraries (like validator.js) that have well-tested email validation functions.

By using a simple but effective pattern like ^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$, you’ll cover most common email formats in a concise manner without overcomplicating your regex. If you need robust validation for edge cases (uncommon TLDs, quoted local parts, etc.), consider adding checks beyond just a single regex approach.