This program is a search tool by using a regular expression(字元序列通則表示式，常見中譯：正規表示式) to express a common pattern of names of those files and folders which you want to search under a specific path of a file system.
To learn more about the term 'regular expression' in computer science, one can find some basic resource on Internet, or read book for its basic usage.
Regex tutorial — A quick cheatsheet by examples:
https://medium.com/factory-mind/regex-tutorial-a-simple-cheatsheet-by-examples-649dc1c3f285