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Username regex tester

Test username length, allowed characters and separators. Pick a generic, GitHub, Instagram or Twitter/X style preset, paste usernames one per line, and check syntax and reserved names in your browser.

Username rules and reserved names

The generic preset allows 3-16 ASCII letters, digits, dots, underscores and hyphens, with a letter or digit at each end. Choose no double separators to also reject adjacent punctuation. Platform-style presets use their own length and separator rules.

The tester separately blocks admin, root, support, api, www, null, undefined and system regardless of case. Regex cannot check account availability; enforce uniqueness on the server.

Recommended pattern

Default preset: ^[a-zA-Z0-9](?:[a-zA-Z0-9._-]{1,14})[a-zA-Z0-9]$. Requires 3-16 characters and a letter or digit at both ends. Reserved names are checked separately.

HTML username input

<label for="username">Username</label>
<input id="username" name="username" type="text" autocomplete="username" autocapitalize="none" spellcheck="false" pattern="[a-zA-Z0-9](?:[a-zA-Z0-9._\-]{1,14})[a-zA-Z0-9]" title="3-16 letters, digits, dots, underscores or hyphens; start and end with a letter or digit" minlength="3" maxlength="16" required>

Use autocomplete="username" for autofill. The pattern attribute matches the whole value with the v flag; escape literal hyphens. Pair it with required. HTML pattern checks syntax; add reserved-name checks on the server.

Validate on the server too

const USERNAME_RE = new RegExp("^[a-zA-Z0-9](?:[a-zA-Z0-9._-]{1,14})[a-zA-Z0-9]$");
const RESERVED = new Set(["admin", "root", "support", "api", "www", "null", "undefined", "system"]);
const ok = USERNAME_RE.test(value) && !RESERVED.has(value.toLowerCase());

$username = $_POST['username'] ?? '';
$reserved = ['admin', 'root', 'support', 'api', 'www', 'null', 'undefined', 'system'];
$ok = is_string($username)
    && preg_match('/\A[a-zA-Z0-9][a-zA-Z0-9._-]{1,14}[a-zA-Z0-9]\z/', $username) === 1
    && !in_array(strtolower($username), $reserved, true);

import re

USERNAME_RE = re.compile(r"[a-zA-Z0-9][a-zA-Z0-9._-]{1,14}[a-zA-Z0-9]")
RESERVED = {"admin", "root", "support", "api", "www", "null", "undefined", "system"}
ok = USERNAME_RE.fullmatch(username) is not None and username.lower() not in RESERVED

These examples use the generic preset plus reserved-name checks. Use full-string matching and enforce a unique database constraint for account creation.

Username regex FAQ