Editing Context-free grammar (section)

== Extensions ==
An obvious way to extend the context-free grammar formalism is to allow nonterminals to have arguments, the values of which are passed along within the rules. This allows natural language features such as [[Agreement (linguistics)|agreement]] and [[reference]], and programming language analogs such as the correct use and definition of identifiers, to be expressed in a natural way. E.g. we can now easily express that in English sentences, the subject and verb must agree in number. In computer science, examples of this approach include [[affix grammar]]s, [[attribute grammar]]s, [[indexed grammar]]s, and Van Wijngaarden [[two-level grammar]]s. Similar extensions exist in linguistics.

An '''extended context-free grammar''' (or '''regular right part grammar''') is one in which the right-hand side of the production rules is allowed to be a [[regular expression]] over the grammar's terminals and nonterminals. Extended context-free grammars describe exactly the context-free languages.<ref>{{cite web | url=http://www.engr.mun.ca/~theo/Courses/fm/pub/context-free.pdf |archive-url=https://web.archive.org/web/20050324113326/http://www.engr.mun.ca/~theo/Courses/fm/pub/context-free.pdf |archive-date=2005-03-24 |url-status=live | title=A Short Introduction to Regular Expressions and Context-Free Grammars | access-date=August 24, 2012 | author=Norvell, Theodore | pages=4}}</ref>

Another extension is to allow additional terminal symbols to appear at the left-hand side of rules, constraining their application. This produces the formalism of [[context-sensitive grammar]]s.