#! perl -slw
use strict;

my $s = join '', map {
    ('a'..'z')[ rand 26 ]
} 1 .. 1000;

our $N //= 4;

for my $key ( 'aa' .. 'zz' ) {
    print "$-[1] : $1"
        while $s =~ m[(?:.{$N})*(?=($key\G..))]g;
}