about "Username contains special characters"

hi everyone.

I try to create an account with name "wb123456" or "web7350484993" via API.
but when i try to post the api params I get "Username contains special characters".
whats the restrictions or rules in "USERNAMES" I couldn't find a document.

and Can I use 12 or 15 char in username. I try to increase username limit but it did not affect

Thank you.