I used to work for a ttrpg company. A dice parser is not a small undertaking. You’re basically writing a calculator with an embedded random number generator. It’s fun but not an easy first project. My advice would be to keep it simple to start with and have your command interface (repl) just accept simple roll commands like roll dex and that handler knows how to make a dex roll. Simple roll commands like roll d6 are also easy to parse out with just a regex. Honestly, I think you would be better off writing a gui app and give it a retro hacker look than going with some type of terminal. Typing on phones is a pain vs tapping buttons.