Restrict usernames to only letters, numbers, and -

[Arkaeriit]

This ensures that no characters from the username
get interpreted by the markdown engine.

[Copilot]

Pull request overview

This PR strengthens username validation by restricting usernames to only contain ASCII letters, numbers, and hyphens. The change prevents special characters in usernames from being interpreted by the markdown rendering engine.

Key Changes:

• Replaced character-by-character ASCII printable filtering with a regex-based approach that explicitly allows only [a-zA-Z0-9-]
• Updated the function comment to accurately reflect the new behavior
• Removed trailing whitespace on line 164

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The regex is being compiled on every function call, which is inefficient. Consider declaring this as a package-level variable using var invalidCharactersRegex = regexp.MustCompile(...) outside the function to compile it only once at initialization time. This will improve performance, especially since cleanName is called for every username validation and change.

[Arkaeriit]

I'm not sure if the performance boost would be worth making the code less readable...