backreferences in gregexpr

Hi Folks,

I'm trying to extract just the backreferences from a regex.

temp = "abcd1234abcd1234"
regmatches(temp, gregexpr("(?:abcd)(1234)", temp))

[[1]]
[1] "abcd1234" "abcd1234"

What I would like is:
[1] "1234" "1234"

Note: I know I can just match 1234 here, but the actual example is
complicated enough that I have to match a larger string, and just want
to pass out the backreferenced portion.

Any help greatly appreciated!

library(gsubfn)
strapplyc(temp, "abcd(1234)")