Augmented BNF for Syntax Specifications: ABNF
RFC 4234, “Augmented BNF for Syntax Specifications: ABNF”, is a Draft Standard document published in October 2005 by D. Crocker, P. Overell. It obsoletes RFC 2234. It has been obsoleted by RFC 5234 — refer to the newer document for the authoritative version. The canonical text is published by the RFC Editor.
Abstract
Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity, with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK]
What “Draft Standard” means
A historical maturity level (retired in 2011) that sat between Proposed Standard and Internet Standard and required multiple interoperable implementations.
The canonical text of RFC 4234 is hosted at rfc-editor.org. Available in TXT,HTML.
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