Regex in the context of finite state automata? The latter can be useful for many other programming tasks. I sometimes find it useful to visualise program flow using these.

See http://www.zsoltnagy.eu/visualizing-regex-execution-using-finite-state-machines/

and two online "visualisers" https://regexper.com/ and http://ivanzuzak.info/noam/webapps/fsm2regex/