Defining a language using itself

Hi Folks,

I think it is mind-blowing when a language is defined using the language itself.

This document defines the XML Schema language using XML Schema:

http://www.w3.org/TR/xmlschema11-1/#normative-schemaSchema

That is the XML schema-for-schemas!

This document defines the JSON Schema language using JSON Schema:

http://json-schema.org/draft-04/schema# 

That is the JSON schema-for-schemas!

These two documents define the ANTLR grammar language using the ANTLR grammar:

https://github.com/antlr/grammars-v4/blob/master/antlr4/ANTLRv4Parser.g4
https://github.com/antlr/grammars-v4/blob/master/antlr4/ANTLRv4Lexer.g4

That is the ANTLR grammar-for-grammars!

Neat!

Do you know of other languages that are defined using the language itself?

/Roger