Backing up to the start of this thread, I too like Peter's suggestion
that a different set of encapsulation characters could be great for
setting apart version syntax. However bringing up square brackets
immediately makes me think of RFC 2732
(http://www.ietf.org/rfc/rfc2732.txt).
It is notable because it explicitly updates RFC 2396 to move square
brackets from the "unwise" character set to the "reserved" character set
and states that RFC 2732 "defines a syntax for IPv6 addresses and allows
the use of "[" and "]" within a URI explicitly for this reserved
purpose",
Examples (from RFC 2732):
http://[FEDC:BA98:7654:3210:FEDC:BA98:7654:3210]:80/index.html
http://[1080:0:0:0:8:800:200C:417A]/index.html
http://[3ffe:2a00:100:7031::1]
http://[1080::8:800:200C:417A]/foo
http://[::192.9.5.5]/ipng
http://[::FFFF:129.144.52.38]:80/index.html
http://[2010:836B:4179::836B:4179]
