grep ... -x .. exact line match only

$cat example.txt
A for Apple.
B for Baby.
C for Cat.
D for Donkey.
$grep  A example.txt
A for Apple.
$grep -x A example.txt
$grep -x A for Apple.  example.txt
grep: for: No such file or directory
grep: Apple.: No such file or directory
$grep -x "A for Apple."  example.txt
A for Apple.
$grep -x "A for Apple"  example.txt
$

grep -x option
Select only those matches that exactly match the whole line.  (-x is  specified  by POSIX.)

source : debian grep manual.

 case 'x':
        match_lines = 1;
        break;

source : debian GNU grep.

Research Related Paper.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.85.1663&rep=rep1&type=pdf