I don't see how a person was supposed to solve this without doing some crypto analysis. Could you explain that please? I read your spoiler and thought it'd be trivial to write a regex to extract the solution. Here it is. It's really slow. It'd be faster if I could use backreferences in negated character classes. I could have written this using a regex that generates another regex to incorporate the negated backreference but that seemed like a whole lot of work and I guessed it wouldn't be . It's pretty as-is.
[Updated: Added an expensive and less wasteful version. I'll be adding timing info when my computer has finished running these. They succeed within a few minutes but take a lot longer to finish failing.]