Regular expressions, often abbreviated as regex or regexp, are a powerful tool used for pattern matching within text. They allow developers to search, manipulate, and analyze strings based on specific patterns defined by a sequence of characters and metacharacters. Regular expressions are implemented in various programming languages, including Python, Java, and JavaScript, making them versatile for tasks such as data validation, searching, and text processing. By utilizing regex, users can efficiently identify and extract information from large datasets, automate repetitive tasks, and enhance the functionality of applications that handle textual data.