... the code extracts everything prior to the first dash. I need the entire expression ...

I don't know if | Evidently you're still stuck on the "extraction" part, but if so, something like this might work:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $line = qq{foo \n ACCESSION NUMBER: 0001144204-09-017358 bar};
 print qq{[[$line]]};
 ;;
 my $rx_acc_num = qr{ \d+ (?: - \d+)* }xms;
 ;;
 my $acc_num;
 if ($line =~ m{ ^ \s* ACCESSION \s* NUMBER: \s* ($rx_acc_num) }xms) {
   $acc_num = $1;
   $acc_num =~ tr/-//d;
   }
 print qq{'$acc_num'};
"
[[foo
 ACCESSION NUMBER: 0001144204-09-017358 bar]]
'000114420409017358'
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Update: Please see perlre, perlretut, and perlrequick.