$ perl -le '$x = "abcdefgxxabcdefgzzabcdsjfhkdfab"; $len=int(length($x)/2); $restring = join"|", map {"(?:.*?\\1){$_}"} reverse(1..$len); print $restring; print $1 if $x =~ /(\w{2,})($restring)/;'

(?:.*?\1){15}|(?:.*?\1){14}|(?:.*?\1){13}|(?:.*?\1){12}|(?:.*?\1){11}|(?:.*?\1){10}|(?:.*?\1){9}|(?:.*?\1){8}|(?:.*?\1){7}|(?:.*?\1){6}|(?:.*?\1){5}|(?:.*?\1){4}|(?:.*?\1){3}|(?:.*?\1){2}|(?:.*?\1){1}
abcdefg