A regular expression, often abbreviated as regex or regexp, is a powerful sequence of characters that defines a search pattern, primarily used for string searching and manipulation. By using a specialized syntax of metacharacters (e.g., `*`, `+`, `?`) and literal characters, a regex can describe a set of strings with common properties. Theoretically grounded in formal language theory, each regular expression corresponds to a finite automaton, an abstract machine that can recognize the specified patterns, making them an indispensable tool in tasks ranging from input validation in web forms to parsing text with command-line utilities and within programming languages.