I like this. Only thing that I don't like is that many candidates will assume that they have to fix the code within the parser, given those instructions, even

I like this. Only thing that I don't like is that many candidates will assume that they have to fix the code within the parser, given those instructions, even if they know that a battle-tested library is how they would actually do it. I hope you accept an off-hand comment such as, "ew, why is this hand-rolled" as a sufficient indicator in favor of your solution.