>perl -wMstrict -le
"use feature 'state';
 ;;
 my $s = '2,3,5,6,7,9,11,12,13,14,16,17,19';
 print qq{'$s' \n};
 ;;
 my $sep = qr{ \s* , \s* }xms;
 my $n   = qr{    \d+    }xms;
 ;;
 sub order {
   state $p = 0;
   my $t = $p;
   $p = $_[0];
   return $_[0] - $t == 1;
   }
 ;;
 use re 'eval';
 $s =~ s{
   (?<! \d)   ($n)   (?{   order($^N) })
   (?=   $sep ($n) (?(?{ ! order($^N) }) (*F))
     (?: $sep ($n) (?(?{ ! order($^N) }) (*F)))+
     )
   .+? \3 (?! \d)
   }
   {$1-$3}xmsg;
 ;;
 print qq{'$s' \n};
"
'2,3,5,6,7,9,11,12,13,14,16,17,19'

'2,3,5-7,9,11-14,16,17,19'