This appears to do what you expect, although it replaces all octets but the last with '###' (rather than based on the number of digits in the octet): 

s/(([0-2]?\d{1,2}\.){3})([0-2]?\d{1,2})/###.###.###.$3/g;

[download]

Code to test regex: 

$ perl -le 'foreach my $c ( q{123.45.67.89 123.45.67.98}, qw/ 0.0.0.0 
+10.1.2.3 172.001.002.003 192.168.128.1 255.255.255.255 / ) { my $d = 
+$c; $d =~ s/(([0-2]?\d{1,2}\.){3})([0-2]?\d{1,2})/###.###.###.$3/g; p
+rint $c, q{ -> }, $d; }'

[download]

Test output: 

123.45.67.89 123.45.67.98 -> ###.###.###.89 ###.###.###.98
0.0.0.0 -> ###.###.###.0
10.1.2.3 -> ###.###.###.3
172.001.002.003 -> ###.###.###.003
192.168.128.1 -> ###.###.###.1
255.255.255.255 -> ###.###.###.255

[download]

Hope that helps.

In reply to Re: Filtering out IP addresses by atcroft
in thread Filtering out IP addresses by Anonymous Monk

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