PCRE actually provides this information. There's a feature called backtracking control verbs, and one of those is (* MARK:NAME). You could put a (* MARK:A), (* MARK:B), etc at the end of every path and PCRE would tell you which of the paths was taken. However PHP does not expose this information - just like it does not expose the (?C) callout feature :(

Maybe I should implement those two things.

Note: There is no whitespace between * and MARK, but if I remove it HN formats it as an italic string.