For efficiency you should avoid the capture of the parentheses like /^(?:\d{1,3}\.){3}\d+/ as you don't use the result. Here is a simple more accurate way to look for IPs. We just capture each of the 4 bytes into $1-$4 and can then test them however we like. Here we just see they are <256 but you could require say $1 to be 10 or whatever.

while(<DATA>) {
    if (/(\d+)\.(\d+)\.(\d+)\.(\d+)/) { 
        print "$1.$2.$3.$4 ";
        if ($1<256 and $2<256 and $3<256 and $4<256) {
           print "is IP\n";
        } else {
           print "is not an IP\n";
        }  
    }
}
__DATA__
0.0.0.0
255.255.255.255
1.2.3.4
256.1.1.1
[download]

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

You are very correct, there is no reason to capture the result via parentheses in my version.