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

Try this:

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

Or this, which will simply ensure that the string does not start with a hyphen or underscore:

^[a-zA-Z0-9][a-zA-Z0-9_-]*$