For my Comp Sci degree I wrote a regexp/automata library. It allowed you to create/modify/load/save/execute NFAs, and the same kinds of functions for a basic re

For my Comp Sci degree I wrote a regexp/automata library. It allowed you to create/modify/load/save/execute NFAs, and the same kinds of functions for a basic regexp language (character classes, ., Kleene star *, ?, etc) by first converting the regexp into an NFA (with epsilon transitions) as each regexp operation maps to a small NFA so they can be bolted together (with epsilon transitions). You could then (attempt to) convert NFAs into a DFA using Myhill-Nerode theorem to determinise it; I say attempt as it was possible to make it consume huge resources if you gave it the right/wrong kind of regexp. It also did minimisation and a few other bits and bobs. To say this gave me a good understanding of regular expressions is an understatement, and once you understand the basics then all of the parts of more complex regular expression languages become a lot easier! That was my enlightenment.