sub tokens {
  my @tokens = 
  split m{  (
              \*\* | :=	  # ** or := operator 
            |
              [-+*/^()=]  # some other operator
            |
              [A-Za-z]\w+ # Identifier
            |
              \d*\.\d+(?:[Ee]\d+)?  # Decimal number
            |
              \d+         # Integer
            )
        },
       shift();
   return grep /\S/, @tokens;
}