0-9a-f]{1,4}         # 1::8               1:2:3:4:5:6::8   1:2:3:4:5:6::8
  |(?:[0-9a-f]{1,4}:){1,7}   :                      # 1::                                 1:2:3:4:5:6:7::
  |:(?:(?::[0-9a-f]{1,4}){1,7}|:)                   # ::2:3:4:5:6:7:8    ::2:3:4:5:6:7:8  ::8       ::
)