I don’t believe this is true. That’s why we do mathematical proofs, instead of only testing all the cases one can think of. It’s important to sanity-check one’s understanding with tests, but mere black-box testing is no substitute for the understanding.
in theory theory and practice are the same, in practice not really
in the context of regex, you have to know which dialect and programming language version of regex you’re targeting for example. its not really universal how all libs/languages works
