 be used later in the expression, via a back reference, and
 may also be retrieved from the matcher once the match operation is complete.

 <p> The captured input associated with a group is always the subsequence
 that the group most recently matched.  If a group is evaluated a second time
 because of quantification then its previously-captured value, if any, will
 be retained if the second evaluation fails.  Matching the string
 <tt>"aba"</tt> against the expression <tt>(a(b)?)+</tt>, for example, leaves
 