$string =~/\w{4,8,16}/\/[0-9a-fA-F]/;

There are two errors here:

1. The quantifier syntax X{y,z} means: at least y and no more than z occurrences of X. You want to say: either exactly 4 occurrences, or exactly 8 occurrences, or exactly 16 occurrences; but you can’t do that with this quantifier. See “Quantifiers” in perlre#Regular-Expressions.
2. The construct /.../\/.../ is a syntax error: the regex ends at the second /.

Now for the bigger picture.

You can probably do what you want with regexes, but it quickly becomes complicated. Here is some code I came up with to identify repeated 4-character sequences:

#! perl
use strict;
use warnings;
use List::MoreUtils 'uniq';

my $string = '0a0a0a0a0b0b0a0a0c0c0c0c0c0c0c0c' .
             '1f1f2b2b2b2b3e3e7b7b7b7b7b7b7b7b' .
             '8f8f8f8f8f8f8f8f6c6c4b4b4b4b3f3f' .
             '9d9d0f0f0f0f0f0f0f0f3a3a2e2e2e2e';

my @seqs = $string =~ /(([0-9a-fA-F]{2})\2)/g;
   @seqs = uniq grep { length == 4 } @seqs;

for my $seq (@seqs)
{
    my $matches = () = $string =~ /$seq/g;
    printf "%s: %d\n", $seq, $matches;
}
[download]

Output:

17:30 >perl 914_SoPW.pl
0a0a: 3
0b0b: 1
0c0c: 4
1f1f: 1
2b2b: 2
3e3e: 1
7b7b: 4
8f8f: 4
6c6c: 1
4b4b: 2
3f3f: 1
9d9d: 1
0f0f: 4
3a3a: 1
2e2e: 2

17:30 >
[download]

What concerns me here is the alignment problem: you presumably do not want to match a non-aligned sequence like the following:

0a0axxx0a0a0yyyy
^^^^   ^^^^
[download]

See, for example, the discussion of the \G anchor in the “Global matching” section of perlretut#Using-regular-expressions-in-Perl.

I’m not sure that regexes are the best tool for this job. I would look at converting your string into an array of integers, then building a hash of integer sequences (of the desired lengths) mapped to their number of occurrences in the original string.

Hope that helps,

Update (June 1): Corrected alignment example.

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