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Regex support is part of the standard library of many programming languages, including Java and Python, and is built into the syntax of others, including Perl and ECMAScript.Implementations of regex functionality is often called a regex engine, and a number of libraries are available for reuse.Since the 1980s, different syntaxes for writing regular expressions exist, 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, in text processing utilities such as sed and AWK and in lexical analysis.Together, metacharacters and literal characters can be used to identify textual material of a given pattern, or process a number of instances of it.Pattern-matches can vary from a precise equality to a very general similarity (controlled by the metacharacters).In the 1980s the more complicated regexes arose in Perl, which originally derived from a regex library written by Henry Spencer (1986), who later wrote an implementation of Advanced Regular Expressions for Tcl.The result is a mini-language called Perl 6 rules, which are used to define Perl 6 grammar as well as provide a tool to programmers in the language.
These arose in theoretical computer science, in the subfields of automata theory (models of computation) and the description and classification of formal languages.
These rules maintain existing features of Perl 5.x regexes, but also allow BNF-style definition of a recursive descent parser via sub-rules.
The use of regexes in structured information standards for document and database modeling started in the 1960s and expanded in the 1980s when industry standards like ISO SGML (precursored by ANSI "GCA 101-1983") consolidated.
programs at Bell Labs in the 1970s, including vi, lex, sed, AWK, and expr, and in other programs such as Emacs.
Regexes were subsequently adopted by a wide range of programs, with these early forms standardized in the POSIX.2 standard in 1992.
Each character in a regular expression (that is, each character in the string describing its pattern) is understood to be a metacharacter (with its special meaning), or a regular character (with its literal meaning).