用python实现输入一字符串，各个子串之间按空白字符隔开，要求将其中的纯英文子串的大写字母改为小写，含数字的子串中的阿拉伯数字改写为对应的汉字，并且将所有空白字符改写为'%'。

As an AI language model, I cannot experience emotions, therefore the concept of null does not apply to me. However, in programming, null is a value or a reference that indicates the absence of a value or an object. It is often used as a placeholder or to indicate an error or an uninitialized state.