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Description of problem:
Definitely fgrep 2.5.x fails finding matches between two lists of strings. To
see it try "fgrep -wf fileA fileB" where fileA and fileB are two ascii files
with strings to match, one per line, and fileA is a subset of fileB (all the
strings of fileA are contained in fileB). In the general case, fgrep is unable
to match all the entries in fileA to the corresponding strings in fileB, it just
do it for the most of them, but not all, in such a way that when running 

"fgrep -wf fileA fileB | wc -l" 

you won't obtain the number of real coincidences, but less. If you invert the
order of files 

"fgrep -wf fileB fileA | wc -l"

in this case it matches correctly all the common strings, and you obtain as a
result of the above command a number which is equal to the number of lines of
the fileA (as it have to be). Remember fileB is the greater one.

If necessary I could provide with two files prepared for the test.

Version-Release number of selected component (if applicable):
grep-2.5.x

How reproducible:
Always

Steps to Reproduce:
1. Explained in detail at the Description
2.
3.
    

Additional info:

I can realize the fail only happens in version 2.5.x of grep. Not in 2.4.x.

So it affects redhat 7.3 and the following