Given a regex like
(a|b){2}\1
What will \1 refer to? The first capture, the last capture, or what?
Regex.IsMatch("aba", @"^(?:(a|b){2}\1)$") == False
Regex.IsMatch("abb", @"^(?:(a|b){2}\1)$") == True
Leads me to suspect it matches the last capture. I'm I correct in this assumption?
You guessed right. Java does this, too. Most (if not all) regex engines store the last capture for repeated groups.
See http://www.regular-expressions.info/brackets.html#repeat for a general description.
See http://msdn.microsoft.com/en-us/library/aa719621%28v=VS.71%29.aspx for a confirmation regarding .Net:
"[...] when a group makes multiple captures, a backreference refers to the most recent capture."
