It's really subject-specific. What I usually see is a small problem easily solved by Regex, that a few months later down the line we need to expand the regex a little bit, but that's ok it's still readable... repeat this a bunch of times and now you have a critical regex line that nobody really understands how it works and nobody ever wants to touch without it being unit-tested to death.
I am a huge, huge fan of https://regexper.com/ - paste pretty much any regex, and it will generate a visualization of what it does. It's been invaluable even just as a sanity check, it just makes it so easy to trace the flow of what a pattern is doing.
Back when I used Windows, I had an app called Regex Buddy and you could paste in any regex and it would generate a comment block that broke down what was going on. Was super useful when you come back a few months later and need to understand a complex regex
It's just a riff on tech-debt, imho