In the expression ((A)(B(C))), there are four groups: ((A)(B(C))), (A), (B(C)), and (C). Group zero stands represents the whole expression.

The captured characters in a group are the most recent sub-sequence that the group matched. All captured input is discarded at the beginning of each match. If a group is matched again, the previously captured value is retained if the second evaluation fails. For example, the matching the string "aba" in the expression

(a(b)?)+

changes group two set to

(B)

.

Groups that start with the expression

(?

do not capture text and do not count toward the group total.