regex - How to prevent regmatches drop non matches?

Question

I would like to capture the first match, and return NA if there is no match.

regexpr("a+", c("abc", "def", "cba a", "aa"), perl=TRUE)
# [1]  1 -1  3  1
# attr(,"match.length")
# [1]  1 -1  1  2

x <- c("abc", "def", "cba a", "aa")
m <- regexpr("a+", x, perl=TRUE)
regmatches(x, m)
# [1]  "a"  "a"  "aa"

So I expected "a", NA, "a", "aa"

Answer 1

Staying with regexpr:

r <- regexpr("a+", x)
out <- rep(NA,length(x))
out[r!=-1] <- regmatches(x, r)
out
#[1] "a"  NA   "a"  "aa"