Augmented Backus–Naur form

In computer science, augmented Backus–Naur form (ABNF) is a metalanguage based on Backus–Naur form (BNF) but consisting of its own syntax and derivation rules. The motivation for ABNF is to define an easily usable tool for defining the format of communications protocol payload objects and protocol units. It is defined by Internet Standard 68 ("STD 68", type case sic), which as of December 2010 was RFC5234, and it often serves as the definition language for IETF communication protocols.

RFC5234 supersedes RFC4234, 2234 and 733. RFC7405 updates it, adding a syntax for specifying case-sensitive string literals.