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。