GitHub username regex: /^[a-z\d](?:[a-z\d]|-(?=[a-z\d])){0,38}(-?)?$/i

github-username-regex.mjs

import assert from 'node:assert'

/**
 * Run with `node github-username-regex.mjs` or `npx tsx github-username-regex.mjs`
 */

/**
 * 1-39 characters
 * case insensitive alphanumeric + hyphen
 * cannot start with hyphen
 * cannot have consecutive hyphens
 * cannot end with hyphen for new users, but existing usernames can
 */

const pattern = /^[a-z\d](?:[a-z\d]|-(?=[a-z\d])){0,38}(-?)?$/i

const cases = [
  { value: "", valid: false },
  { value: "u", valid: true },
  { value: "valid-username", valid: true },
  { value: "Valid-Username", valid: true },
  { value: "-invalid-username", valid: false },
  { value: "valiD-Old-Username-", valid: true },
  { value: "invalid-username--", valid: false },
  { value: "Valid-Username-123-456", valid: true },
  { value: "testingusername01testingusername01testi", valid: true },
]

for(const testCase of cases) {
  const valid = pattern.test(testCase.value)
  assert.strictEqual(valid, testCase.valid, `Value: ${testCase.value}`)
}

https://regexr.com/81nk2