I'm trying to construct a regex that will match the pattern 'XXY', where X and Y can be any word character, but X and Y must be different characters. In English, I want to find all occurrences where a given character is duplicated and is followed by a different character.

Attempt 1: At first I thought this would be relatively straightforward by using a backreference in a negated character class, but I seem to be missing something. Could someone please explain why the code below doesn't DWIM?

use strict;
use warnings;

my $string = 'ABCDEEFGHIJJJKLMNOOOOPQRSTUVWXXYZ';
print "matching: $string\n";

while( $string =~ m/((\w)\2[^\2])/g )
{
    print $1, "\n";
}

=pod
matching: ABCDEEFGHIJJJKLMNOOOOPQRSTUVWXXYZ
EEF
JJJ
OOO
XXY
=cut
[download]

The regex is matching 'EEF' and 'XXY', which are correct, but it is also matching 'JJJ' and 'OOO'. The negated character class isn't acting how I expected.

Attempt 2: I also tried using a negative lookahead assertion, but also without success:

while( $string =~ m/((\w)\2($!\2)\w)/g )
{
    print $1, "\n";
}
=pod
matching: ABCDEEFGHIJJJKLMNOOOOPQRSTUVWXXYZ
JJJK
OOOO
=cut
[download]

This regex matches four characters rather than the 3 I expected (since the lookahead is zero-width), and it also lacks specificity at the last position (matching 'OOOO').

The whole story: Understanding this problem is only part of my goal. I'm actually trying to match 'AABCCCCAD'. My first attempt was this:

$string = 'WXYYZAABCCCCADWWXYYYZ';

while( $string =~ m/((\w)\2([^\2])([^\2\3]){4}\1[^\2\3\4])/g )
{
    print $1, "\n";
}
[download]

but, given my first question, this obviously doesn't work.

Educate me in the ways of thine regexen, that I might faithfully wield their power.

Many thanks in advance.

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