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

As I said in comments, grep looks for a pattern inside your string and there is indeed "a" (or "a{1}", which is the same for grep) in "aa". You need to add to the pattern that the "a" is followed by not a : "a[^a]":

grep("a[^a]", c("aa", "ab"), value=TRUE)
#[1] "ab"

EDIT

Considering your specific problem, it seems you can try by the "opposite" : filter out the strings that contains more than one occurence of the pattern, using a "capture" of the pattern:

!grepl("(ab).+\\1", c("ab.t", "ab-ab.t"))
#[1]  TRUE FALSE

!grepl("(ab).*\\1", c("ab", "ab-ab","ab-cc-ab", "abab"))
#[1]  TRUE FALSE FALSE FALSE

The brackets permit to capture the pattern (here ab but it can be any regex), the .* is for "anything" zero or more times and the \\1 asks for a repeat of the captured pattern