math expression parser with dice notation!

I think a fun coding challenge would be to create a math expression parser that could support dice notation.

The simplest implementation would probably just support:
- add/subtract
- multiply/divide
- basic dice notation (dX/AdX)

however, a more advanced implementation might also support:
- parenthesis
- keep/drop dice notation (AdX[k/d]Y...)

Some dice notation examples:
- d20 <- roll 1 twenty sided die.
- 3d4 <- roll 3 four sided dice, and add them together.
- 4d6k3 <- roll 4 six sided dice, keep the three highest rolls and add them together. (used in d&d5e to generate base stats)
- 5d8d2 <- roll 5 eight sided dice, drop the highest two rolls, and add the rest together.
- 6d10k4d2 <- roll 6 ten sided dice, drop the two lowest rolls, drop the two highest rolls, and add the rest together.