Not a regex, but it works fine for decimal encoded IPs.

sub validIP{ 
    use List::Util qw[ reduce ];
    $a = $b;
    $_[0] =~ m[^[\d\.]+$]
    and 2**32 > reduce{ 
        $a * 256 + ($b < 256 ? $b : 1e99) 
    } reverse split '\.', $_[0]
}
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