I cannot get the following code to capture what is in a lookahead (when the lookahead is within capturing parens. .

What I'm trying to achieve is to capture substrings of length 'x' beginning with the first character in the fasta sequence (and the lookahead portion) progressing from the first character, the second, and so on, until the substrings run up against the end of the characters.

Thanks in advance for any help.

Chris

#!/usr/bin/perl
use strict;
use warnings;

my $str = do {local $/; <DATA>};
$str =~ s/\s+//g;
my $len = length $str;

while (--$len > 780) {
    while ($str =~ /(.(?=.{$len}))/g) {
        print pos, "$1\n";
    }
    pos = 0;
}

__DATA__
CAATATGCGAGGGACCTACATGTTGAGCATGACAATGAATTCTATTGAAAAAAAGAGTTG
GAAGTATATACGAATATAAATAATGTGAAACAAAAGAAGAAAAGTGAATAAAAGGCACTT
AAGACGCTATCCAATTGTGTATGAGAAGTGCAAACTCAATTTTTTTGCAAAAGACTTTTC
TCAAACCTTTAATTGATGCGTTCCGTTCATACATCAACCGCGTCCCATACGTTTCGAAGA
AAATTGCCTATATGTGTTATTTACATACTAGAGATATTTTAATATTGAAACAGTTGTATT
TCTATTGTAATTACTATCTAAACGTCTCGTCCCACCGCTGTTGATAAAGCGGTGCCAATA
TAATTTATAATCACGCCTGGGTAAATGACTTTTAATTTCTTAAATCATCGAAGTATGCGA
AAACAAGAAGTCTTTATTCATAATAAAAAACAAATTCGGTTACTACGACTTTTATATGTC
ATTCAATATTTGGTAATAATTTTACAGTATTAACCCTATCCTCATCTGATTCACTCTCTT
CTAATTGCATATATTTTCAAATTCGCTTTCAGCGCTGTACAAAACCAGTAAGCAGATCTC
GTACGAAGACTCAAATAAGTTGCATTGTTCGTATTCAAGGAAACCGGGGGGCAAAATTTC
CAACCATATTTAAGTATGACAATATTTCCAAGTCAAGGATGCATGCTGTTCTTCTCTTCA
TTAACTAGCTAACCAATTAGCTGAACGGCTTTGTATTTTACTTAACATATTGTCTATTGC
ATAAAAAACCACTATTCAGC
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