Regex to match a username

Question

I am trying to create a regex to validate usernames which should match the following :

• Only one special char (._-) allowed and it must no

Answer 1

I think you need to use ? instead of +, so the special character is matched only once or not.

^(?=(?![0-9])?[A-Za-z0-9]?[._-]?[A-Za-z0-9]+).{3,20}