* expression. This is needed when the result of a match includes an empty group, to determine
* its position relative to other groups finishing at the same character position.
*/
private void computeNestingTable() {
nestingTable = new IntToIntHashMap(16);
UnicodeString s = regex;
int[] stack = new int[s.length()];
int tos = 0;
int group = 1;
int inBrackets = 0;