(?= (??{ qr!.{$a[2][0]}! }) 
   ( # start $2
      (?= (??{ qr!.{${\($a[3][0]-$a[2][0])}}! })                                        
          ((??{ qr!.{$a[3][1]}! })) ) # $3 nested
      (??{ qr!.{$a[2][1]}! })
   ) # $2 ended
)