Re^2: Parenthesis grouping into regexes.

Regexp::Common doesn't support IPv6 addresses though. Regexp::IPv6 does, however it creates quite a monster just to match the third form from RFC 4291 section 2.2 (mixed hex/decimal) that I've never seen used anywhere. This is (supposedly, I haven't tested it) a much shorter version:

qr/^(((?=(?>.*?::)(?!.*::)))(::)?(([0-9A-F]{1,4})::?){0,5}|((?5):){6})(\2((?5)(::?|$)){0,2}|((25[0-5]|(2[0-4]|1[0-9]|[1-9])?[0-9])(\.|$)){4}|(?5):(?5))(?<![^:]:|\.)\z/i

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