if ( /(BA[ABC]{2})|(CA[CD]{2})|(DA[SC]{2})/ )
   my $first =
     ( defined( $1 ) ? 1
     : defined( $2 ) ? 2
     : 3
     );

##</code><code>##

local our $first;

/
   (?: BA[ABC]{2} (??{ $first = 1; })
   |   CA[CD]{2}  (??{ $first = 2; })
   |   DA[SC]{2}  (??{ $first = 3; })
/x;