grep with -f option returns different results based on the order of the patterns file

Question

I am trying to match keywords listed in one file from text in another file. The keywords file is new line separated.

grep returns different results based on the order of the keywords (not the input file).

Here is an example.

With this input

# input.txt
TRITONS, sea-deities; they sung the marriage chorus of ACHILLES.

Case 1

# terms.txt
ACHILLES
TRITONS

# grep returns just one match
grep -o -f terms.txt input.txt  
ACHILLES

Case 2: I just changed the order of the keywords file

# terms.txt
TRITONS
ACHILLES

# grep returns both matches ! 
grep -o -f terms.txt input.txt 
TRITONS
ACHILLES

Trying to understand the behavior.

user_6106864 · Answer

I found this issue in grep (BSD grep) 2.5.1-FreeBSD (this is the default version that ships on Mac High Sierra).

Fixed by using Gnu Grep ggrep (GNU grep) 3.4. instead.

Installed via brew install grep.

Note that this version of grep is now called ggrep.