Matthew Barnett <[email protected]> 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.
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<https://bugs.python.org/issue23692>
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