I'm not an Ipv6 expert but I think you can get a pretty good result more easily with this one:
^([0-9A-Fa-f]{0,4}:){2,7}([0-9A-Fa-f]{1,4}$|((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\.|$)){4})$
to answer "is a valid ipv6" it look like ok to me. To break it down in parts... forget it. I've omitted the unspecified one (::) since there is no use to have "unpecified adress" in my database.
the beginning:
^([0-9A-Fa-f]{0,4}:){2,7} <-- match the compressible part, we can translate this as: between 2 and 7 colon who may have heaxadecimal number between them.
followed by:
[0-9A-Fa-f]{1,4}$ <-- an hexadecimal number (leading 0 omitted)
OR
((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\.|$)){4} <-- an Ipv4 adress
