import re
def iterative(s):
    stack = ['']
    for token in re.findall(r'[,\[\]]|.+?', s):
        if token == '[':
            stack.append('')
        elif token == ']':
            if stack[-1]:
                yield '.'.join(stack)
            stack.pop()
            stack[-1] = ''
        elif token == ',':
            if stack[-1]:
                yield '.'.join(stack)
        else:
            stack[-1] = token
    if stack and stack[0]:
        yield ''.join(stack)
 
 
s = 'a,b,c[a,b,c[a]],d'
print(list(iterative(s)))

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