For more information, check also the following links!

▎ENGLISH
Regular Expression Definition ▸  https://en.wikipedia.org/wiki/Regular_expression
Regular Expression Extensive Tutorial ▸ https://www.regular-expressions.info/tutorial.html

▎日 本 語
正規表現の定義 ▸ https://ja.wikipedia.org/wiki/%E6%AD%A3%E8%A6%8F%E8%A1%A8%E7%8F%BE
正規表現とは? ▸ https://qiita.com/soarflat/items/2b50c1efe2ea54a762d7
分かりやすいpythonの正規表現の例 ▸ https://qiita.com/luohao0404/items/7135b2b96f9b0b196bf3