Assuming you want to stick to regular expressions, for readability's sake you might want to do this:

$pattern = '[01]?[0-9]?[0-9]|2[0-4][0-9]|25[0-5]';

/(?:(${pattern})\.){3}($pattern)/;  # "$1.$2.$3.$4" = IP if matched
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Due the complexity, though, it would probably be tons more efficient to do as other posters suggest and simply loop through the 4 octets:

sub get_octets {
    my @octets = split(/\./, shift);
    return unless @octets == 4;

    foreach (@octets) {
        $_ += 0;   # Perhaps unnecessary, but forces the "octet" to be
+ numeric or zero
        return unless $_ >= 0 && $_ <= 255;
    }

    return @octets;
}
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