The document introduces regular expressions from both a theoretical and practical perspective. Theoretically, regular expressions are formal expressions that define search patterns to match or recognize strings. They are made up of base elements like characters, alternation, concatenation, and Kleene star. Practically, regular expressions are widely used to match and extract data in text editors, programming languages, and applications like data cleaning and extraction. They allow flexible matching of patterns like dates, numbers, or text in various formats.