Okay, I'll quote some more from the question: > […] for the sake of the challenge, let the expression be delimited (starting symbol, expression, ending symbol

Okay, I'll quote some more from the question: > […] for the sake of the challenge, let the expression be delimited (starting symbol, expression, ending symbol ex: /fancypantpattern/ or @[^2048]@), if you want to argue quotes as your delimiter, so be it. I think given the apparent difficulty of this problem it won't make much of a difference.