It is not that it is hard to understand the syntax or even write them, it is hard to read and change them. Here is an example I just grabbed off a random webpage:

^(?=.\d)(?=.[a-z])(?=.[A-Z])(?=.[!@#$^&()_-]).{8,18}$

That is hard to quickly* read and understand what it is. Sure a comment would help, but if someone said it isn't working, I have to work all that out and then fix it.

There is a reason Perl lost to Python. It just seems like we can do better.