This has been bothering me for a long time. I don't know if this is a bug or not, but I can't get proper results using the source metatag. It used to work some years ago, but now I can't get ALL Pixiv posts using source and/or -source no matter what I search for.
I think it's better if I use some examples. Searching for sasaame's posts from Pixiv:
First, when not using asterisk. I'm not sure how it works. According to the Cheat Sheet, it searches for a source "starting with" something. So I believe it's the same as putting a * at the end. However, I had used "source:pixiv" in the past and it used to work perfectly.
sasaame source:pixiv
Finds pics with a *pixiv.net* url, but does not show pics with a *pximg.net* url (even though both show as pixiv.net/member_illust... in the other source)
sasaame source:pximg
No results.
sasaame source:http://pixiv.net
No results. This is actually expected, since urls are usually i3.pixiv.net or i.pximg.net and similar.
By the way, this is the example that's shown in the Cheat Sheet, I believe an update is in order.
sasaame source:http://i3.pixiv.net
Here things start to get odd. This should yield results, but there's nothing there but them chickens. I also tried https://i3.pixiv.net, https://pixiv.net, i3.pixiv.net, pixiv.net, but it's the same.
sasaame source:https://i.pximg.net
For some reason, this is the only one that actually worked properly. You can even remove stuff after https://i and it still works.
sasaame source:http://i
if https://i finds https://i.pximg.net, http://i finds http://i3.pixiv.net, right? Nope, no results.
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Now, using asterisk. I got better results with it, but could only get EITHER pixiv or pximg. There are still many cases where it should work, but it doesn't.
I should also mention that using asterisk, at least until 1-2 years ago, can sometimes cause the search to timeout, and I believe it takes longer.
sasaame source:*pixiv*
Same as source:pixiv
sasaame source:*pximg*
Same as source:https://i.pximg.net and source:https://i, etc
sasaame source:*p*x*.net*
This should find both, but it only finds pximg. It's really like Source hates pixiv.net.
sasaame source:*p*x*
Finally, it finds both. Hardly usable though, as anything with a P and X will be shown, even if it's not from Pixiv. At least now I know that .net in the search ignores posts with pixiv.net.
sasaame source:*img-original*
Just for testing, I searched for something that was in both kinds of url (not always), /img-original/. There was no .net this time, and it still ignored pixiv.net.
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I also like to search for posts that are not from Pixiv. If I can find either one separately, how about I try to exclude both?
sasaame -source:*pixiv* -source:*pximg*
Only hides pximg. Maybe 2 tags with asterisk is screwing something up, so let's try something different...
sasaame -source:pixiv -source:*pximg*
This seems better, but it's still not quite right. It shows pixiv.net up to a certain point (pixiv.net posts are older), stopping exactly at the end of a page, which is really suspicious. You know what's even weirder? Add limit:150 to both searches above; now they have the same results.
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TL;DR source metatag is busted and I humbly request a source:pixiv or similar that works properly and can be used with -source as well.
Maybe a metatag that searches in the "other" source (the one with medium&illust)?