Usernames and t.me
Official telegram.org Source
You can use a-z, 0-9 and underscores. Usernames are case-insensitive, but Telegram will store your capitalization preferences (e.g. Telegram and TeleGram is the same user).
Official Telegram Reference: https://telegram.org/faq
Part of the TGWAY.com Official Telegram Knowledge Base (98 Verified Q&As).