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Postings from February 2020
Karl Williamson <public@khwilliamson.com> wrote:
:I have been looking at the code in regcomp.c in regpiece() that deals
:with qwuantifiers.
:
:After reordering things so that goto's don't cause it to jump back then
:forth, some anonmalies became clear. I also found some potential easy
:optimizations.
:
:I would expect that the results of parsing {1,} would be the same as
:'+', and they both do generate the PLUS regnode, but the flags passed to
:the higher level aren't set the same. This is true of the other
:shortcuts '*' and '?' as well.
:
:I then tried to figure out what the consequences of those differences
:are. Two of the flags WORST and SPSTART do not appear to ever be looked
:at. Should we remove them, or dig to find out how they used to be used,
:or might they come back again, and we should set them consistently?
I definitely think there's value in some digging, I'm happy to give that
a go, time permitting. But of those I'm sure at least WORST would be
from Ilya, quite likely SPSTART too, so digging is not guaranteed to
lead to light.
:regpiece assumes that any quantifier whose upper limit is non-zero
:causes the construct to not match the null string, and sets HASWIDTH.
:That simply isn't true when quantifying a zero-width assertion. I
:didn't look at what the optimizer does with that, but when I change that
:a higher level warning is emitted:
:
: "Quantifier unexpected on zero-length expression "
:
:Now to the optimizations: I believe the quantifier {1,1} can simply be
:optimized out. There are occurrences in our test suite of this; I
:believe from Abigail. And I can see machine generated or interpolated
:code ending up with this. So we don't need to create a loop that gets
:executed precisely once. But there is {1,1}+, that has to be
:considered; and that's easy to do.
:
:Generally, in {m,m}? the ? is a no-op and can be omitted.
I think it is worth trying to understand why \K{1,1} fails before
eliding the general {1,1} case, since I suspect there's something
fundamental going wrong there that will shed light on more than itself.
Hugo
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