Augmented BNF for Syntax Specifications: ABNF
RFC 4234
Document | Type |
RFC - Draft Standard
(October 2005; Errata)
Obsoleted by RFC 5234
Obsoletes RFC 2234
Was draft-crocker-abnf-rfc2234bis (individual in app area)
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Authors | Paul Overell , Dave Crocker | ||
Last updated | 2020-01-21 | ||
Stream | IETF | ||
Formats | plain text html pdf htmlized with errata bibtex | ||
Stream | WG state | (None) | |
Document shepherd | No shepherd assigned | ||
This information refers to IESG processing after the RFC was initially published: | |||
IESG | IESG state | RFC 4234 (Draft Standard) | |
Consensus Boilerplate | Unknown | ||
Telechat date | |||
Responsible AD | Lisa Dusseault | ||
IESG note | As an RFC this has been published; as a draft to track, this has been replaced with draft-crocker-rfc4234bis. | ||
Send notices to | (None) |
Network Working Group D. Crocker, Ed. Request for Comments: 4234 Brandenburg InternetWorking Obsoletes: 2234 P. Overell Category: Standards Track THUS plc. October 2005 Augmented BNF for Syntax Specifications: ABNF Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2005). 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. Crocker & Overell Standards Track [Page 1] RFC 4234 ABNF October 2005 Table of Contents 1. INTRODUCTION ....................................................2 2. RULE DEFINITION .................................................3 2.1. Rule Naming ................................................3 2.2. Rule Form ..................................................3 2.3. Terminal Values ............................................4 2.4. External Encodings .........................................5 3. OPERATORS .......................................................6 3.1. Concatenation: Rule1 Rule2 ................................6 3.2. Alternatives: Rule1 / Rule2 ...............................6 3.3. Incremental Alternatives: Rule1 =/ Rule2 ...................7 3.4. Value Range Alternatives: %c##-## .........................7 3.5. Sequence Group: (Rule1 Rule2) .............................8 3.6. Variable Repetition: *Rule ................................8 3.7. Specific Repetition: nRule ................................9 3.8. Optional Sequence: [RULE] .................................9 3.9. Comment: ; Comment ........................................9 3.10. Operator Precedence .......................................9 4. ABNF DEFINITION OF ABNF ........................................10 5. SECURITY CONSIDERATIONS ........................................11 6. References .....................................................11 6.1. Normative References ......................................11 6.2. Informative References ....................................11 Appendix A. ACKNOWLEDGEMENTS .....................................13 Appendix B. APPENDIX - CORE ABNF OF ABNF .........................13 B.1. Core Rules ...............................................13 B.2. Common Encoding ..........................................14 1. INTRODUCTION Internet technical specifications often need to define a formal syntax and are free to employ whatever notation their authors deem useful. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. It balances compactness and simplicity, with reasonable representational power. In the early days of the Arpanet, each specification contained its own definition of ABNF. This included the email specifications, [RFC733] and then [RFC822], which came to be the common citations for defining ABNF. The current document separates those definitions to permit selective reference. Predictably, it also provides some modifications and enhancements. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. Appendix B supplies rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. It is provided as a convenience and is otherwise separate from the meta Crocker & Overell Standards Track [Page 2] RFC 4234 ABNF October 2005 language defined in the body of this document, and separate from its formal status.Show full document text