qr{
   (?(DEFINE)
      (?<expr>
         (?&number) (?&expr_)
      )

      (?<expr_>
         (?:
            \s++
            (?:
               (?&expr) \s++ (?&binary_op) (?&expr_)
            |  
               (?&unary_op) (?&expr_)
            )
         )?
      )

      (?<number>
         ( (?&NUMBER) )
         (?{
            local @stack = @stack;
            push @stack, $^N;
         })
      )

      (?<unary_op>
         ( (?&IDENT) )
         (?(?{ !exists($unary_ops{$^N}) }) (*FAIL) )
         (?{
            local @stack = @stack;
            $unary_ops{$^N}->($stack[-1]);
         })
      )

      (?<binary_op>
         ( [-+/*^] )
         (?{
            local @stack = @stack;
            $binary_ops{$^N}->($stack[-2], pop(@stack));
         })
      )


      # Tokens.
      # Backtracking can't cause a token
      # rule to return something different.

      (?<NUMBER> \d++ (?: [.] \d*+ )?+ | [.] \d++ )
      (?<IDENT>  [_A-Za-z][_A-Za-z0-9]*+          )

   )

   (?{ local @stack = (); })
   \A \s* (?&expr) \s* \z
   (?{ $result = pop(@stack) })
}sx;