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    @Quasímodo null RS doesn't cause the field separator to be a newline, it causes it to include a newline. It still also includes blank and tab (or whatever else the FS is set to if it's a single char). Commented Jan 6, 2021 at 15:09
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    Oh, that's it. Quoting verbatim: "The newline shall always be a field separator." So the newline being a field separator does not mean it is the only one. What a tricky wording in the specification! Commented Jan 6, 2021 at 15:13
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    Right and the current version of the POSIX spec is unfortunately wrong where it says a <newline> shall always be a field separator, no matter what the value of FS is but that's not quite true, it should end with ...if FS is a single char as that's how all awks actually berhave. I'm pretty sure I have a bug report open against the spec about that, let me check... Commented Jan 6, 2021 at 15:14
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    @Quasímodo Ah, now I remember - I had raised the issue with the gawk providers (see lists.gnu.org/archive/html/bug-gawk/2019-04/msg00029.html) and THEY were going to follow up with the standards folks to get it fixed there. Unfortunately at that point I lost interest and didn't pursue the standard change, I expect it is in the queue somewhere. Commented Jan 6, 2021 at 15:22
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    @Quasímodo I see that functionality and bug in the spec is now specifically addressed in the gawk manual (gnu.org/software/gawk/manual/gawk.html#Multiple-Line): When RS is set to the empty string and FS is set to a single character, the newline character always acts as a field separator. This is in addition to whatever field separations result from FS....Note that language in the POSIX specification implies that this special feature should apply when FS is a regexp. However, Unix awk has never behaved that way, nor has gawk. This is essentially a bug in POSIX. Commented Jan 6, 2021 at 15:53