How to use Rex to find letter which following by

I have the following code

def append_letter():  string = 'ACAABAACAAABACDBADDDFSDDDFFSSSASDAFAAACBAAAFASD'  result = []  # compete the pattern below  pattern = r'(?<=AAA)\w+'  for item in re.finditer(pattern, string):    # identify the group number below.    result.append(item.group(1))  return result

How to append the letter not included A letter

from script above I want to append any letter follow by triple A included A to a list.

How to achieve this by modifies only this line of code:

pattern = r'(?<=AAA)\w+'