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A regular expression (sometimes referred to as regex, regexp, or rational expression) is a sequence of characters that specifies a search pattern. It is usually used in the contexts of computer science and formal language theory, with patterns most often being used for string-searching algorithms such as "find" or "find and replace."
The concept of regular expressions began in the 1950s, when the American mathematician Stephen Cole Kleene formalized the description of a regular language. They came into common use with Unix text-processing utilities. Different syntaxes for writing regular expressions have existed since the 1980s, one being the POSIX standard and another, widely used, being the Perl syntax.
Regular expressions are used in search engines, search and replace dialogs of word processors and text editors. Many programming languages provide "regex" capabilities.
