r grep by regex - finding a string that contains a sub string exactly one once

Question

I am using R in Ubuntu, and trying to go over list of files, some of them i need and some of them i don\'t need,

I try to get the one\'s i need by finding a sub string

Answer 1

we can try with ^ and $ to make sure that there is only a single 'a' in the string

grep("^a$", a)
#[1] 1

It is not clear what the OP wanted.