Don't parse IP addresses with regexen in real code. The OP makes an amusing puzzle (one which I have admittedly not figured out yet), but nothing more. There are far more valid formats for IPv4 than the traditional dotted-quad one. I don't even want to think about all the formats available for IPv6.

Look at Valid IP? for more info.

You can match an IP-address with a regexp, just don't try to roll your own, do  {use Regexp::Common; /$RE{net}{IPv4}/} instead.

From the Regexp::Common::net docs:

$RE{net}{IPv4}

Returns a pattern that matches a valid IP address in "dotted decimal"

So it only matches the traditional dotted-quad form of IPv4 addresses. That's just one format IPv4 can come in (albeit the most common). There are modifiers Regexp::Common allows for some other formats (like "dotted hex"), but not all possible formats (a simple 32-bit number, for example).

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Note: All code is untested, unless otherwise stated