I'd have to strongly second the answer from Frank Krueger.
Whilst you say you need a regular expression to match an IPv6 address, I'm assuming what you really need is to be able to check if a given string is a valid IPv6 address. There is a subtle but important distinction here.
There is more than one way to check if a given string is a valid IPv6 address and regular expression matching is only one solution.
Use an existing library if you can. The library will have fewer bugs and its use will result in less code for you to maintain.
The regular expression suggested by Factor Mystic is long and complex. It most likely works, but you should also consider how you'd cope if it unexpectedly fails. The point I'm trying to make here is that if you can't form a required regular expression yourself you won't be able to easily debug it.
If you have no suitable library it may be better to write your own IPv6 validation routine that doesn't depend on regular expressions. If you write it you understand it and if you understand it you can add comments to explain it so that others can also understand and subsequently maintain it.
Act with caution when using a regular expression whose functionality you can't explain to someone else.
