c:\@Work\Perl\monks>perl -wMstrict -le
"my @strings_to_be_matched = qw(foo bar wibble wobble fee_fie foe fum);
 ;;
 my ($ur) =
   map  qr{ $_ }xms,
   join q{ | },
   map  quotemeta,
   reverse sort
   @strings_to_be_matched
   ;
 print qq{\$ur: $ur};
 ;;
 my $reg3 = qr/extern.+\b$ur\b\s*/i;
 print qq{\$reg3: $reg3};
"
$ur: (?^msx: wobble | wibble | fum | foo | foe | fee_fie | bar )
$reg3: (?^i:extern.+\b(?^msx: wobble | wibble | fum | foo | foe | fee_fie | bar )\b\s*)