On my way to add shy-groups (i.e. groups that do not have a number attached to them: you can't get their content with `match-{end,string,beginning}' or refer to them via backreferences and they do not impact the numbering of other groups), I came across a few bugs in regex.c that were sufficiently obscure to have lived with us until now. But they turned out to have disastrous effects in conjunction with shy-groups (although XEmacs people didn't bother to fix them, making their shy-groups rather erratic).