The debugging util java.util.regex.PrintPattern is used to display the internal representation structure of a compiled regex pattern.

For a node that is followed by already printed node it is supposed to print a link to that node, but fails:

./java java.util.regex.PrintPattern "(?:\\d?){1}"

   Pattern: (?:\d?){1}
     0: <Start>
     1: <Prolog>
     2: <Loop {1, 1}>
     3: <Group.head 0>
     4: <Ques GREEDY>
     5: <Single "ASCII.DIGIT">
             </Ques>
     6: </Group.tail 0> < (=>%d)>
                                         ^^^ was supposed to print 2
         </Loop>
     7: <END>