Regex, short for "regular expression," is a powerful tool used in computer programming and text processing to identify and manipulate strings of text. It consists of a sequence of characters that define a search pattern, allowing users to find specific sequences, validate input, or replace text in documents and code. Commonly used in programming languages like Python, JavaScript, and Java, regex can match anything from simple text to complex patterns. For example, it can be used to check if an email address is formatted correctly or to extract phone numbers from a block of text.