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Reference.org
Regular expression
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<p>A regular expression (shortened as regex or regexp), sometimes referred to as rational expression, is a sequence of <a href="/facts/Character_(computing)/X2zKLFFB">characters</a> that specifies a <a href="/facts/Pattern_matching/CEwptMs9">match pattern</a> in <a href="/facts/String_(computer_science)/4ChJu5WS">text</a>. Usually such patterns are used by <a href="/facts/String-searching_algorithm/RCvwDRle">string-searching algorithms</a> for "find" or "find and replace" operations on <a href="/facts/String_(computer_science)/4ChJu5WS">strings</a>, or for <a href="/facts/Data_validation/tFCchJTM">input validation</a>. Regular expression techniques are developed in <a href="/facts/Theoretical_computer_science/jSkGGuvy">theoretical computer science</a> and <a href="/facts/Formal_language/crDTyP8q">formal language</a> theory. </p><p>The concept of regular expressions began in the 1950s, when the American mathematician <a href="/facts/Stephen_Cole_Kleene/sKSax2se">Stephen Cole Kleene</a> formalized the concept of a <a href="/facts/Regular_language/ahxw67T1">regular language</a>. They came into common use with <a href="/facts/Unix/0pNL2seI">Unix</a> text-processing utilities. Different <a href="/facts/Syntax_(programming_languages)/zM5oInza">syntaxes</a> for writing regular expressions have existed since the 1980s, one being the <a href="/facts/POSIX/mQkSt234">POSIX</a> standard and another, widely used, being the <a href="/facts/Perl/fx31kjlT">Perl</a> syntax. </p><p>Regular expressions are used in <a href="/facts/Search_engine/SRntsbz2">search engines</a>, in search and replace dialogs of <a href="/facts/Word_processor/qAR7lagF">word processors</a> and <a href="/facts/Text_editor/jPCgtbvN">text editors</a>, in <a href="/facts/Text_processing/5UIbA5za">text processing</a> utilities such as <a href="/facts/Sed/5pXUwRdD">sed</a> and <a href="/facts/AWK/lAQ0leyj">AWK</a>, and in <a href="/facts/Lexical_analysis/T1JYWpIf">lexical analysis</a>. Regular expressions are supported in many programming languages. Library implementations are often called an "<a href="/facts/Software_engine/QhIAMMBg">engine</a>", and <a href="/facts/Comparison_of_regular_expression_engines/lMuelcvC">many of these</a> are available for reuse. </p>