I am so sorry. You've done the same flip that tye did, capturing the partial rep rather than the complete rep. Threw me completely.

With a couple of enhancements that works for the 1 rep/1 partial case:

$s = 'aaaabaaaabaaaaabaaaab';;

$s =~ /^((.*).*)\2$/ and print "$1/$2";;
aaaabaaaaba/aaaabaaaab
[download]

And still works for that when enhanced to allow for more than 1 complete rep:

$s =~ /^((.*).*)\1*\2$/ and print "$1/$2";;
aaaabaaaaba/aaaabaaaab
[download]

But then fails when given more than one complete rep:

$s = 'aaaabaaaabaaaaabaaaabaaaaabaaaab';;

$s =~ /^((.*).*)\1*\2$/ and print "$1/$2";;
aaaabaaaabaaaaabaaaaba/aaaabaaaab
[download]