Inverse grep does not find all matching lines

Question

EDIT: I am making this too complicated. It's not about inverse grep. I get the same results using just grep -x -f stop.txt < in.txt. If who comes before whose in the stop word file, the result is just who. When the order is reversed in the stop word file, both lines in in.txt are found. I have the feeling that I fundamentally do not get grep.

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I cannot get inverse grep to work like I expect in order to remove lines containing a stop word from a file. The order in which the stop words are given affects the result.

Suppose I have two files. An input file in.txt:

who
whose

And a file with a list of stop words stop.txt:

who
whose

If I "filter" in.txt with an inverse grep search on the stop words in stop.txt, I get:

$ grep -vx -f stop.txt < in.txt
whose
$

Only if I change stop.txt to

whose
who

do I get:

$ grep -vx -f stop.txt < in.txt
$

I do not understand why the order of words in the file with stop words is of importance.

Answer 1

With the pattern files

$ od -bc shortlong
0000000   167 150 157 012 167 150 157 163 145 012
           w   h   o  \n   w   h   o   s   e  \n
0000012
$ od -bc longshort
0000000   167 150 157 163 145 012 167 150 157 012
           w   h   o   s   e  \n   w   h   o  \n
0000012

we can test some of the variations, here with the macOS grep (2.5.1-FreeBSD):

$ grep -x -f shortlong shortlong
who
$ grep -x -f shortlong longshort
who
$ grep -x -f longshort shortlong
who
whose
$ grep -x -f longshort longshort
whose
who

who when first hides the longer whose match. This appears to be a bug.

The problem only appears when the expressions come from a -f file, not when the equivalent (we hope) expression is given as an argument:

$ grep -x -E 'who|whose' shortlong
who
whose
$ grep -x -E 'who|whose' longshort
whose
who
$ grep -x -E 'whose|who' shortlong
who
whose
$ grep -x -E 'whose|who' longshort
whose
who

GNU grep (3.7) does not exhibit this problem (nor does the grep on current versions of OpenBSD):

$ ggrep -x -f shortlong shortlong
who
whose
$ ggrep -x -f shortlong longshort
whose
who
$ ggrep -x -f longshort shortlong
who
whose
$ ggrep -x -f longshort longshort
whose
who

Therefore to avoid this bug do not use grep verion 2.5.1-FreeBSD, or instead form the regular expressions into an argument from the file by replacing non-ultimate \n with | for grep -E.

$ paste -s -d \| shortlong
who|whose
$ grep -x -E -- "$(paste -s -d \| shortlong)" shortlong
who
whose

Concerning Alternation

Longer strings should ideally be listed before shorter strings in regular expression alternations, among other edge cases, which means one should prefer the longshort file or whose|who forms.

grep appears (when it is not buggy) to get this right, but other regular expression engines will stop on the first match, which must be listed longest first if you want those to match:

$ printf 'who whoses the whosefolk' |
grep -o -E 'who|whose|whosefolk'
who
whose
whosefolk
$ printf 'who whoses the whosefolk' |
perl -nle 'print for /who|whose|whosefolk/g'
who
who
who
$ printf 'who whoses the whosefolk' |
perl -nle 'print for /whosefolk|whose|who/g'
who
whose
whosefolk