> PS: beautifying with /x modifier and adding missing ; in replacement left for the interested reader. :)

well since my editor has a (mostly functioning) cperl-beautify-regexp

use v5.12;
use warnings;
use Test::More;


my $str = '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';
my $exp = '43:1:1-6; 27:3:7-9; 65:1:4; 65:1:18';

my $got = $str;

$got =~ s/
             (
                 \d+:\d+:
             )                      # $1
             (
                 \d+                # (updated +)
             )                      # $2 = first $^N
             ;
             (?:                    # group w/o match
                 \s
                 \1
                 (
                     (??{ $^N+ 1 }) # include last_match + 1
                 )                  # $3 = next $^N
                 ;
             )+                     # repeat
         /$1$2-$3;/xg;


is($got, $exp, "fits");

done_testing;

[download]

Worth noting that the OP was wrong with his expectation, it's 43:1:1-6; not 43:1:1-5;

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

PS: still not perfect, because the input isn't terminated with a semicolon, but I'm tired now. :)

In reply to Re^3: Regexp substitution on variable-length ranges with embedded code? ( $^N ) by LanX
in thread Regexp substitution on variable-length ranges with embedded code? by Polyglot

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