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Postings from August 2010
Ben Morrow wrote: > Quoth public@khwilliamson.com (karl williamson): >> Ben Morrow wrote: >>> Quoth public@khwilliamson.com (karl williamson): >>>> But there must be some reason that xism are always shown in the >>>> stringification, whether they are in effect or not in effect. But I >>>> can't think what it might be. It would have been simpler for the >>>> original design to only output them in the stringification when >>>> different from the default. So there must be a reason why the plus or >>>> minus of them is always output. Does that reason apply to these new >>>> modifiers? >>> The reason is so that interpolation under different flags works >>> properly. Currently, if I have >>> >>> my $rx = qr/f./s; >>> >>> is stringifies to (?s-xim:f.), which means that when I interpolate it >>> into a pattern with different flags >>> >>> "AFx" =~ /a$rx/i; >>> >>> the /i is 'cancelled' for the duration of the interpolation (and /s >>> activated). If it were changed to stringify to (?s:f.) only, the match >>> above would change from failing to passing. >> I still don't understand. I had considered interpolation, and it still >> seems to me that the absence of a modifier means use the underlying >> default for the duration of that interpolation, no matter what the outer >> regex says. I don't see a flaw in that thinking. > > Um, it's not what actually happens :)? > > ~/src/perl% perl -E'say "F" =~ /(?s:f)/i' > 1 > ~/src/perl% perl -E'say "F" =~ /(?s-ixm:f)/i' > > ~/src/perl% > > Complete absence of a modifier means 'leave this modifier as it was > outside this group'. I don't think that can (or should) be changed now. > > As Avar said, this could potentially be changed for the simple case of > direct interpolation, by looking up the flags on the interpolated regex > directly. However, a lot of code assumes it can stringify a qr//, write > it to a file/pass it across the network/whatever, and re-qr// to get an > equivalent pattern back. > > My alternative suggestion was to introduce a new grouping construct, > which I tentatively called (?~sixm:) (I don't much like that, but there > aren't many alternatives at this point), which *does* do what you > expect; and use that for stringification instead. That way we change the > stringification once, now, and then never again. > > Ben > I think I like your idea. Let me make sure I understand. A tilde immediately following '(?' means that the clustering group is to have the default modifiers, subject to any flags that come after it. Thus (?~:abc) means all default flags. (?~x:abc) means the defaults except use the x modifier, and (?~-x:abc) means the defaults without the x modifier, which is a somewhat more complicated way of writing the equivalent (?~:abc). regex stringification could change to output just the non-defaults, or we could leave them as-is, and just add the tilde. Since we would already be breaking compatibility, I would think we might as well remove them. A tilde is not the ideal choice because it can easily be confused with a minus sign. I don't know if there are other characters that could be used instead.Thread Previous | Thread Next