When i write below command, it selects "aapp". but it should be "aap". Because '*' means zero or more 'a'.'p' means one p.
$ echo "aapple" | grep "a*p"
can somebody please explain the reason, why the grep not select as expected?
Grep is line oriented, so it reads lines from input, and prints the entire line when any part of it matches the specified RE. Thus any input line containing a p will be printed with a*p.
$ echo "aapple" | grep "a*p"
aapple
$ echo "nothing special" | grep "a*p"
nothing special
If you only want to print the part that matches the RE, you can use the -o option, but then it will print multiple lines if the RE matches in multiple non-overlapping places:
$ echo "aapple" | grep -o "a*p"
aap
p
If you use the --color option, it will print these two parts of the line in red, giving what you see in your image.