my ($skip, $re, $expr, $term);

   $skip = qr! \s* !x;

   $re   = qr! ^ (??{ $expr }) (??{ $skip }) \z !x;

   $expr = qr! (??{ $term })
               (?:
                  (??{ $skip }) [-+*/] (??{ $term })
               )*
   !x;