RegEx for allowing alphanumeric at the starting and hyphen thereafter

a

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

^[[:alnum:]](?:[[:alnum:]-](?!-$))*$

^(?:[[:alnum:]]+(?:-(?!$))?)+$

^(?:[a-zA-Z0-9]+(?:-(?!$))?)+$

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