I was asked that question about 7 years after reading Mastering Regular Expressions and about 10 years after taking Automata in college. It so happens that I am an expert in regex, have a patent in the field, regularly answer stackoverflow regex questions for fun, etc. Unfortunately, the question involved me actually drawing a DFA, trying to remember the notation, etc. At this point I don't even remember the particular question, only that I couldn't remember how to draw or read the diagrams.

Circles for states and arrows for transitions between states? Is there anything more to it than that?

I think you use "sausages" for initial and final states...

I too don't remember much.

Perhaps that's the point. A self-proclaimed expert in the field should know how to draw or at least read a DFA.

The interviewer told me this was a favorite question of his. Also I don't think I presented myself as a regex expert to them. Anyhow, I'm an expert in regex like a DBA would be an expert in SQL or particular databases. The interview was for an ordinary software engineer role.

Regardless, I only meant to make the point that I disagree with the comment "They're more interested in how you approach real world issues" and that this statement is false: "It's not like they get you in a room and ask you to draw a linked list or a binary tree"

edit: by "expert" I just mean something I'm particularly skilled in, not something I'm an authority in from theory to practice.