The question is far from being clear, and I don't understand if you want to keep or not a line starting with #.

Assuming you just want to keep all the lines which have an IP address, or something very much looking an IP address, you might try something like this: 

perl -ne 'print if /(\d{1,3}\.){3}\d{1,3}/;'

[download]
Of course, if you want to be more selective, you could check that the captures are smaller than 256: 
perl -ne 'print if /(\d{1,3}\.){3}(\d{1,3})/ and $1 < 256 and ... and 
+$4 < 256;'

[download]
 It really depends on your data. In many cases, the first simple regex is just sufficient, in others, you really need to be sure that you don't keep something like "345.765.5.34", which is obviously not an IP address, whatever it is.

Update: my code line above is wrong, as explained and shown below by AnomalousMonk: capture groups don't change their numbering under a counted quantifier.

It would have to be something like this: 

perl -ne 'print if /(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})/ and $1
+ < 256 and ... and $4 < 256;'

[download]
but that's probably getting a bit too convoluted for a one-liner.

In reply to Re: Seeker of Regex Wisdom (strings which don't form specific patterns) by Laurent_R
in thread Seeker of Regex Wisdom (strings which don't form specific patterns) by ilcylic

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