The author is more than welcome to write the equivalent code themself, which is probably going to be more unreadable (if they derive the automaton themself) or much slower (if they are doing redundant tests or pre-processing that the automaton could have saved them from). Regular expressions are also more amenable to static analysis in both linting and optimisation than equivalent code, being more declarative in nature; a regex and an imperative matcher have a relationship like SQL and the query plan generated by a database.
Parser combinators are one of those rare REALLY great things that almost entirely replace something else (regex). Sadly, way too verbose in many languages.
