When I proposed to use a regular expression, I was told that it was not the point of the exercise, despite it being asymptotically identical in time complexity and easier to read and execute in parallel. I would have preferred a more difficult question where I didn't have to reimplement some basic construct in a more bespoke fashion. Design interview was too broad in scope, and not particularly interesting, and too much focused on the actual stack rather than abstracting those details and focussing on the actual constraints and requirements.