いつもどう書くか迷うので有名どころで、簡単にエラーを表示できるサービスのを確認してみた。

GitHub

  • Username may only contain alphanumeric characters or dashes and cannot begin or end with a dash

Twitter

  • Invalid username! Alphanumerics only and not all numeric.

Yahoo

  • Your username must begin with a letter.
  • Your username cannot end in an underscore or a period.
  • Only letters, numbers, underscores, and one period are allowed.

GitHubのが1文に収まってていい感じ。