You don't need to be precise. Just give it an example string and tell it what information you want to extract from it and it usually works. It is just way faster than doing it manually.
Writing regexes by hand is hard so there will always be some level of testing involved. But reading a regex and verifying it works is easier than writing one from scratch.
My overly snide point about regexes was that most of the time "verifying it works" is more like finding and fixing a few more edge cases on the asymptotic journey towards no more brokenness.
