In general, I agree with AppleFritter's comment; a regular expression is developed according to the environment in which it must operate. With regard to the code you've posted, I wouldn't think

([0-9]?\d\d?|2[0-4]\d|25[0-5])
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would be a good way to describe an IP address because [0-9] is (barring Unicode) equivalent to \d. Therefore, your first term swallows your following two. You probably want something more like

(1?\d\d?|2[0-4]\d|25[0-5])
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but this still allows 00, which you may or may not care about.

So how about

perl -nle 'print if s/((1?\d\d?|2[0-4]\d|25[0-5])\.){3}(?=\d+)/xxx.xxx
+.xxx./g'
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It misses spec on match x-counts, but you should probably be scrubbing that too, if you are scrubbing.