unreadable once written (to me anyway). (…) there might be more inputs upon which it has unintended effects.

https://regex101.com can explain your regex back to you, and allows you to test it with more inputs.

Though I’m not trying to convince you to always use regular expressions, I agree with GP:

Obviously regexes aren't the right tool for every job, and they can certainly be done poorly; but in the right place at the right time they're the simplest, most robust, easiest to understand solution to the problem.