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

It looks like you're after strings with one a and no more, regardless where in the string. While stringi can accomplish the task, a base solution would be:

s <- c("a", "aa", "aca", "", "b", "ba", "ab")

m <- gregexpr("a", s)
s[lengths(regmatches(s, m)) == 1]

[1] "a"  "ba" "ab"

Alternatively, a regex-lite approach:

s[vapply(strsplit(s, ""), function(x) sum(x == "a") == 1, logical(1))]
[1] "a"  "ba" "ab"