https://www.rfc-editor.org/info/rfc5234/
RFC 5234: Augmented BNF for Syntax Specifications: ABNF | RFC Editor
Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF...
augmented bnfrfcsyntaxspecificationsabnf