Regex: username

How it works

3–20 characters overall shape (1 + up-to-18 + 1): starts and ends alphanumeric, allows _ and - in the middle. The (?:…)? makes a single-character username also valid — tighten to your policy.

Honest caveats

Every service has its own rules (GitHub forbids consecutive hyphens, Twitter allows leading underscores). Treat this as a template: adjust the length bounds and allowed separators to your policy, and always check reserved names separately.

More patterns