如何从python中一个标记和另一个标记第n次出现之间的字符串中提取子字符串

def extract_substring(string, start_marker, end_marker, n):
    start_index = -1
    end_index = -1
    count = 0

    # 寻找第n次出现的start_marker的位置
    for i in range(n):
        start_index = string.find(start_marker, start_index + 1)
        if start_index == -1:
            return None

    # 寻找第n次出现的end_marker的位置
    for i in range(n):
        end_index = string.find(end_marker, end_index + 1)
        if end_index == -1:
            return None

    # 提取子字符串
    substring = string[start_index + len(start_marker):end_index]

    return substring

string = "This is a sample string. Start marker is [ and end marker is ]. This is the first occurrence. [This is the second occurrence.] This is the third occurrence. [This is the fourth occurrence.]"
start_marker = "["
end_marker = "]"
n = 2

substring = extract_substring(string, start_marker, end_marker, n)
print(substring)

This is the second occurrence.