regex - Why does grep show me different output depending on my input file size? -

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i'm little puzzled output of grep command, seems truncating results based on size of -f file. instance, consider 1000-line file of strings, patterns.txt, e.g.:

adkgjwofjdjglkadjglkjasdfahdg dsklfjsldkfjaghwioeghsdlkjfld sdkljfsdkljghsdlfhkwhfklshdfo ... sdklfjsdklfjsdklfjslkjghdfkjj

and 1gb queryfile.txt search patterns. when run

grep -f -o -f patterns.txt queryfile.txt | grep -c adkgjwofjdjglkadjglkjasdfahdg

in case, command reports 0 matches 1st line, (adkgjwofjdjglkadjglkjasdfahdg) of patterns.txt, though there 35 occurrences in queryfile.txt. verified reducing patterns.txt file first 10 lines. rerunning

grep -f -o -f patterns_reduced-list.txt queryfile.txt | grep -c adkgjwofjdjglkadjglkjasdfahdg

properly reports 35 occurrences of adkgjwofjdjglkadjglkjasdfahdg.

what's happening?

this shouldn't happen unless... patterns overlap.

check example:

echo "xyxx" | grep -o -f yx$'\n'xy   # output: xy

this finds second pattern (xy), , because of won't find first pattern (yx).

echo "xyxx" | grep -o -f yx          # output: yx

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