$n="1,2,3,4,9,10,11,12,14,16,17,18";
$n =~ s{
 (?<!\d) (\d+)
 (?:
  , ( (??{ 1 + $+ }) )
 )+
 }{$1-$+}gx;
print "$n\n";

$ perl cool_regex.pl 
1-4,9-12,14,16-18