Win8 Strawberry 5.30.3.1 (64) Wed 05/26/2021 9:07:19 C:\@Work\Perl\monks >perl use 5.018; # need lexicals in regexes, regex extensions use strict; use warnings; my @Test = ( '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', '987:23:45; 987:23:46; 65:1:17; 65:1:19', ); for my $data (@Test) { print "'$data' \n"; my $rx_base = qr{ (?> \d+ : \d+ :) }xms; my $rx_tail = qr{ (?> \d+) }xms; my $rx_sep = qr{ (?> ;? \s*) }xms; my @run; $data =~ s{ ($rx_base) ($rx_tail) (?{ push @run, $^N }) (?: $rx_sep \1 ($rx_tail) (?{ push @run, $^N }) (?(?{ $run[-1] - $run[-2] != 1 }) (*F)) )+ } {$1$2-$3}xmsg; print "'$data' \n\n"; } ^Z '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' '987:23:45; 987:23:46; 65:1:17; 65:1:19' '987:23:45-46; 65:1:17; 65:1:19'