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$ perl -nle 's/([0-9]?\d\d?|2[0-4]\d|25[0-5])\.([0-9]?\d\d?|2[0-4]\d|2 +5[0-5])\.([0-9]?\d\d?|2[0-4]\d|25[0-5])\.([0-9]?\d\d?|2[0-4]\d|25[0-5 +])/xxx.xxx.xxx.$4/; print' filename
$ perl -nle 's/\d+\./xxx./g; print' filename