[racket] Backreference question (regex)

I'm trying to write a small regex that would return any repeating pattern in
a string e.g. ("ababcdcdababcdcd" would return "ab" "cd" abab" "cdcd"
"ababcdcd")
I'm having trouble using the backreferencing. I see under section 3.7.2 of
the guide (and also some Perl docs I've checked out) that I should be able
to write a simple regex "(1)\1" which would return true for the data "113"
but Racket says I'm wrong. Any help would be greatly appreciated.
-Justin
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