根据值从嵌套字典中检索键，其中键名称未知

def find_key(dictionary, value):
    for key, val in dictionary.items():
        if val == value:
            return key
        elif isinstance(val, dict):
            result = find_key(val, value)
            if result is not None:
                return key + "." + result
    return None

example_dict = {
    "key1": "value1",
    "key2": {
        "key3": "value2",
        "key4": "value3"
    },
    "key5": "value4"
}

result = find_key(example_dict, "value2")
print(result)
# 输出：key2.key3