ProGrammar Grammar Definition Language (GDL) is a high-level language that expresses the structure of data.

GDL is based on Backus-Naur Form (BNF), a popular notation for describing the syntaxes of many programming languages. Because BNF is intended to describe syntax in a way that is easily understood, it tends to be informal and many variations of the notation are in use. Also, because BNF postpones the "implementation details" until development time, the task of building a production-quality parser from its BNF description can be tedious and error-prone. To address these issues, ProGrammar GDL makes a number of extensions to BNF, which result in a language that is formal enough and practical enough to generate executable parsers, yet one that remains intuitive and easy to use. These extensions are summarized below:

	GDL is object-oriented, introducing the keywords "grammar" and "extends" to the notation. A grammar encapsulates a group of related symbols as a single named entity, which can then be re-used by other grammars. The extends keyword allows grammars to inherit symbols from other grammars.
	GDL supports many high-level constructs that can significantly reduce the size and complexity of large grammars. These include repeaters, wildcards, length quantifiers, and a negation operator.
	GDL allows regular expressions and other lexical expressions to be declared directly within the grammar production rules. ProGrammar makes no hard distinction between "lexical analysis" and "parsing", but rather uses a common notation for both.
	GDL provides predefined terminal types for commonly used tokens; including "alpha", "alphanumeric" and "identifier".
	GDL uses symbol attributes to support advanced whitespace handling, application-defined (external) symbols, and to control how the parse tree is built during the parse.
	GDL is an executable language, in that it doesn't have to be converted to some other programming language before it can be built and tested. GDL is fully supported by the ProGrammar IDE, which allows grammar definitions to be run, tested and debugged within a feature-rich development environment.