Discrete Mathematics with Applications - Metric Edition (Epp) Chapter 12
https://runestone.academy/ns/books/published/ads/chapter_14.html
https://runestone.academy/ns/books/published/ads/s-Monoids.html
https://runestone.academy/ns/books/published/ads/s-free-monoids-and-languages.html
https://runestone.academy/ns/books/published/ads/s-automata-finite-state-machines.html
https://runestone.academy/ns/books/published/ads/s-monoid-of-fsm.html
https://runestone.academy/ns/books/published/ads/s-machine-of-monoid.html
https://en.wikipedia.org/wiki/Regular_expression
https://en.wikipedia.org/wiki/Finite-state_machine
https://en.wikipedia.org/wiki/Automata_theory