#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11133054
use warnings;

$_ = '43:1:1; 43:1:2; 43:1:3; 43:1:4; 43:1:5; 43:1:6; 27:3:7; 27:3:8; 
+27:3:9; 65:1:4; 65:1:18';
print "$_\n";

s/\b(\d+:\d+:)(\d+)\K(; \1((??{$+ + 1}))\b)+/-$+/g;

print "$_\n";
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Outputs:

43:1:1; 43:1:2; 43:1:3; 43:1:4; 43:1:5; 43:1:6; 27:3:7; 27:3:8; 27:3:9
+; 65:1:4; 65:1:18
43:1:1-6; 27:3:7-9; 65:1:4; 65:1:18
[download]