Wikipedia article.

A Wikipedia article that defines context free grammars and uses them to generate context free languages.

A Wikipedia article which defines context sensitive languages in terms of context sensitive grammars which are defined in a linked article.

Wikipedia offers 3 equivalent definitions of recursively enumerable languages and states closure properties for certain operations.

A Wikipedia article on regular expressions with an informal discussion, a formal definition and examples.

Basic definitions of regular languages, how they are generated, closure properties, and comparison with context free languages.

Encyclopedia article about the archive file format.

Encylopedia article about the forum, including background, standardization efforts, and controversies.

Encyclopedia article explaining the format, its features and implications.

Encylopedia article about the language and document format.