Ugh. Ya've gotta be so careful with terms when talking about regexes. By "multi-line matching" I didn't mean a match that spans across two lines of data. I meant an expression that would match different 'types' of lines. My apologies for the confusion. That's what I get for trying to put all the particulars in the subject.
So no, that kind of false positive is not an issue. The lines (log files) I'm matching against are pretty strongly formatted. Plus, these are remarkably oversimplified examples, so that kind of clash would be very very tough to conjure.