Matthew Barnett <pyt...@mrabarnett.plus.com> added the comment:
Suppose you had a pattern: .* It would advance one character on each iteration of the * until the . failed to match. The text is finite, so it would stop matching eventually. Now suppose you had a pattern: (?:)* On each iteration of the * it wouldn't advance, so it would keep matching forever. A way to avoid that is to stop the * if it hasn't advanced. The example pattern shows that there's still a problem. It advances if a group has matched, but that group doens't match until the first iteration, after the test, and does not, itself, advance. The * stops because it hasn't advanced, but, in this instance, that doesn't mean it never will. The solution is for the * to check not only whether it has advanced, but also whether a group has changed. (Strictly speaking, the latter check is needed only if the repeated part tests whether a group also in the repeated part has changed, but it's probably not worth "optimising" for that possibility.) In the regex module, it increments a "capture changed" counter whenever any group is changed (a group's first match or a change to a group's span). That makes it easier for the * to check. The code needs to save that counter for backtracking and restore it when backtracking. I've mentioned only the *, but the same remarks apply to + and {...}, except that the {...} should keep repeating until it has reached its prescribed minimum. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue23692> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com