In theoretical computer science and formal language theory, a regular expression (sometimes called a rational expression) is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings, or string matching, i.e. "find and replace"-like operations. The concept arose in the 1950s, when the American mathematician Stephen Kleene formalized the description of a regular language, and came into common use with the Unix text processing utilities ed, an editor, and grep, a filter. In modern usage, "regular expressions" are often distinguished from the derived, but fundamentally distinct concepts of regex( pronunciation: /ˈrɛɡɛks/; hard g as in gaggle; rhymes with FedEx) or regexp, which no longer describe a regular language. See for details. Regexes are so useful in computing that the various systems to specify regexes have evolved to provide both a basic and extended standard for the grammar and syntax; modern regexes heavily augment the standard. Regex processors are found in several search engines, search and replace dialogs of several word processors and text editors, and in the command lines of text processing utilities, such as sed and AWK. Many programming languages provide regex capabilities, some built-in (for example Perl, JavaScript, Ruby, AWK, and Tcl) and others via a standard library (for example .NET languages, Java, Python, POSIX C, and C++ since C++11).Most other languages offer regexes via a library.

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