The single example you give is trivial to solve*, so I think you haven't described the task completely. What do you want your output to look like? How large are your input files? If there are other calls in between "FunctionZ" and "FunctionY", should it still match? Also I don't understand your point #3 - how do you expect to match function names when they're missing?

* e.g. /^ENTRY:.*\b\Q$f1()\E\nENTRY:.*\b\Q$f2()\E$/m