use strict;
use warnings;
#use re 'debug';

$_ = "17341234173412341734123417341234";

my $i;
pos() = $i
   while s<\G
           (?{print pos($_) . "\n"})
           (.*?)(.)(...)\2
           (?{$i=pos()-4})
          >
          <defined $1?$1."A".$3."B":"A".$3."B">ex;
print;
__END__
0
1
3
4
9
11
12
17
19
20
25
27
28
A7AAB2BBA7AAB2BBA7AAB2BBA7AAB2BB