Grammar for IPv6 address

[Jeganatan, Srividhya]

Hi
I'm trying to define grammar rules for IPV6 addresses.
As defined in RFC3513, IPv6 addresses can be of three formats:
1. x:x:x:x:x:x:x:x
2. ::ipv4address or ::ffff:ipv4address
3. compressed form such as in x::x, x:x::, x::x:x

Each "x" can be of utmost 4 hex digits, for which I defined the token as:

HEXD [0-9a-fA-F]

{HEXD}{1,4} {action to return the two bytes from the 4 hex digits}

I'm not able to come up with the grammar for the third format. 
The problem lies in the positioning of the "::". It can appear anywhere
within the address format but only once.

I really appreciate any input/suggestions.

Thanks
SV