How to Validate Input with Regex (and Where It Falls Short)

Step-by-step

1. Define exactly what a valid value looks like, including length and allowed characters.
2. Write a pattern with ^ and $ so the whole value must match.
3. Test it with valid values, invalid values and edge cases in the Regex Tester.
4. Start from a ready-made pattern in the Regex Generator for common formats, then adjust it.
5. Use the pattern in the browser for quick feedback and repeat the check on the server.

Shape versus truth

A pattern can confirm that a value looks like name@domain.tld, but not that the mailbox exists. A date pattern can check YYYY-MM-DD and month ranges, but not that 2026-02-31 is impossible. Use a library or a confirmation step for those cases.

Keep patterns readable

Long patterns are hard to maintain. Prefer a simple pattern plus a separate check over one giant expression, and add a comment explaining what each part allows.

Accessibility and clarity

Tell people the expected format before they type it, and give a clear message when a value is rejected. Avoid rejecting valid input, such as email addresses with a plus sign or names with apostrophes.