I like this one using quoted reg ex

my $ip = '192.168.255.254'; # for example
# Set up the reg ex
my $ipno = qr/
    2(?:5[0-5] | [0-4]\d)
    |
    1\d\d
    |
    [1-9]?\d
/x;
# To test
if ( $ip =~ /^($ipno(?:\.|$)){4}/ ){
    print "IP OK\n";
};
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I went for an interview at Arm in Cambridge, UK. They asked me to write one on the board and I wrote some lame one .. and later ... brooding on my poor attempt set out to make a better one. Driven by failure? Or maybe just really irritated by it. I still got the job :)