I want the first 2 octets of other string should also be X and X How to do that?
Your first step would be to remove the ^ anchoring your regex to the start of the line but that will cause every pair of numbers separated by a dot, (actually, given your regex, any character, you should escape the '.'), which would be all of them result in in everything becoming 'xx.xx.xx.xx';
So you need to ensure that only the first pair of each quad get modified, so try: s/(?<![0-9.])\d+\.\d+/X.X/g;
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