1. a
  2.  
  3. ^[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9]$
  4.  
  5. ^[[:alnum:]](?:[[:alnum:]-](?!-$))*$
  6.  
  7. ^(?:[[:alnum:]]+(?:-(?!$))?)+$
  8.  
  9. ^(?:[a-zA-Z0-9]+(?:-(?!$))?)+$
  10.  
  11. ^[a-zA-Z0-9]([a-zA-Z0-9-]*[a-zA-Z0-9])?$