Regex to match substrings containing n non-repeated characters - regex

Question

I am facing a (naive) problem with regular expression.
I need to find any substrings composed of a fixed number (n) of different characters.
So, for "aaabcddd", if n=3 the substrings that I expect to find are: "abc" and "bcd".
My idea is to use n-1 capture groups and '[^' to exclude characters already matched. Thus, I wrote the following Perl regex (in Julia):
r"(([[:alpha:]])[^\2])[^\1]"
But, it is not working.
Do you have any tips?

Answer 1

You can not use a backreference to a capture group using a negated character class [^\1]
What you can do is use a negative lookahead to assert what is directly to the right of the current position is not what you have already captured in a previous group.
If that is the case, capture a single alpha in a new group.
The matches abc and bcd are in capture group 1
(?=(([[:alpha:]])(?!\2)([[:alpha:]])(?!\3|\2)[[:alpha:]]))
(?= Positive lookahead
( Capture group 1
([[:alpha:]]) Capture the first char in group 2
(?!\1)([[:alpha:]]) If not looking at what is captured by group 2 to the right, capture the second char in group 3
(?!\2|\1) If not looking to the right at what is captured by group 2 or 3
[[:alpha:]] Mach the 3rd char
) Close group 1
) Close the lookahead
Regex demo
Or a bit shorter using a case insensitive match:
(?=(([a-z])(?!\2)([a-z])(?!\3|\2)[a-z]))

Answer 2

Here is a solution to an arbitrary value of n characters:
#!/usr/local/bin/perl
use strict; use warnings; use feature ':5.10';
my $s="aaabcded";
my $n=3;
while ($s=~/(?=([[:alpha:]]{$n}))/g){
my $hit=$1;
my #chars = split //, $hit;
my %uniq;
#uniq{#chars} = ();
say "$hit" if (scalar keys %uniq) == $n;
}
Running with $n=3 prints:
abc
bcd
cde
Running with $n=4 prints:
abcd
bcde
And $n=5:
abcde