/^[a-z]       # Start with a letter. 
 (?:          # Start grouping.
   [^-]         # Anything other than a hyphen
    |           # Or
   -(?!-)       # A hyphen not followed by another hyphen
 )*           # End group. Match 0 or more of those groups.  
 (?<=[a-z])$  # The end, as long as it is preceded by a letter.
/ix           # Match case insensitively and allow comments.