Digging more about IPv6 representation:

- It's possible to stick "::" in many places

- It's possible to have :0000: before or after "::"

- leading zeros (:0001: is the same as :1:)

- "Text Representation of Special Addresses", so the ::192.168.0.1 representation

- hex uppercase vs hex lowercase

see https://tools.ietf.org/html/rfc5952

Also due to the ambiguity of ports also using a colon delimiter, the IPv6 address may be in brackets:

    [::1:2:0:0:dead:beef]:443

And link-local addresses are mandatory and scoped per interface, so they need a zone id supplied as either an integer or interface name:

    fe80::1:2:0:0:dead:beef%eth0

that rfc says upper/lower doesn't matter? Am I wrong?

That's the point: because it doesn't matter it provides two representations for the same semantic thing.

Well, that's just like IPv4, then. They can still be canonicalized, if you need to compare them.

That's why there are library routines for this, inet_pton and inet_ntop. No one should need to parse these addresses themselves.