Regex to allow single hyphen and underscore but not at beginning or end of string

Question

I have this Regex I modified to allow underscores, hyphen, letters and numbers. I am trying to modify it further so that it has the following properties:

1. Allow

Answer 1

^([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9_-]*[a-zA-Z0-9])$

Either one of the three possibilities:

• [a-zA-Z0-9]
• [a-zA-Z0-9][a-zA-Z0-9]
• [a-zA-Z0-9][a-zA-Z0-9_-]*[a-zA-Z0-9]