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  m{
    (\w) \1
    (?! \1) (\w)
  }x;

  m{
    (\w) \1
...
    (?! \1 | \2) (\w) \3 \3 \3 \1
    (?! \1 | \2 | \3) (\w)
  }x;

my $s = 'AABCCCCAD';
our $DEBUG = 1;
...
  warn "$s: $re\n" if $DEBUG;
  qr/$re/x;
}