Another approach:

>perl -wMstrict -le
"my $s = 'dsmgt dsmgt_ dsmgt_9 dsmgt99 dsmgt__99 dsmgt_999 '
       . 'dsmgt_12_34_56dsmgt_12_34 dsmgt_12';
 print qq{'$s'};
 ;;
 my $rx_dsmgt = qr{ (?<! [[:alpha:]]) dsmgt (?: _ \d\d)+ (?! \d) }xms;
 ;;
 my @pieces = $s =~ m{ $rx_dsmgt }xmsg;
 printf qq{'$_' } for @pieces;
"
'dsmgt dsmgt_ dsmgt_9 dsmgt99 dsmgt__99 dsmgt_999 dsmgt_12_34_56dsmgt_
+12_34 dsmgt_12'
'dsmgt_12_34_56' 'dsmgt_12_34' 'dsmgt_12'
[download]