A non-Perl mailing list I'm on is tossing around Perl fragments. One, for determining whether a dotted quad IP address is valid, basically reads: 

sub isValidIP {
    my $ip = shift;
    if ( ($a,$b,$c,$d) = $ip =~ m/^(\d+)\.(\d+)\.(\d+)\.(\d+)$/ ) {
        if (($a|$b|$c|$d) < 256 ) {
            return 1;
        }
    }
    return 0;
}

[download]

Now, without running this code, can you see how and why this will run off the rails, and why a test case that includes "0.0.0.0", "127.0.0.1", and "255.255.255.255" won't demonstrate the problem?

So as not to give away the punchline and thus deprive people of a good puzzle, reply by posting a valid dotted-quad IP address that will demonstrate this routine failing. I'll post an explanation in 24 hours. (It doesn't have anything to do with \A and \Z.)

(Updated to correct a mistranscription, but one not related to the problem.)

In reply to Can you spot the problem? by dws

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