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>perl -wMstrict -le "use feature 'state'; ... '2,3,5,6,7,9,11,12,13,14,16,17,19' '2,3,5-7,9,11-14,16,17,19'
$s =~ s{ (?<! \d) ($n) (?{ order($^N) }) (?: $sep ($n) (?(?{ ! order($^N) }) (*F))){2,} } {$1-$2}xmsg;
sub order { state $p = 0; ... (?: $sep ($n) (?(?{ ! order() }) (*F))){2,} } {-$^N}xmsg;