ABNF

ABNF, or Augmented Backus-Naur Form, is a metalanguage used to specify the grammar of data formats and communication protocols. It is a standardized extension of the classic Backus-Naur Form and is widely employed in IETF RFCs to describe syntax. ABNF provides a precise, machine-readable way to express sequences of characters that constitute valid messages, tokens, or values, separating structure from meaning.

A grammar in ABNF consists of a set of rules. Each rule has a nonterminal name, an equals sign, and an element that defines that nonterminal. Elements can be terminals (literal strings in quotes or numeric codes such as %x41-5A), nonterminals (names defined by other rules), or constructs for grouping, optional parts, repetition, and alternation. Repetition lets the author specify how many times an element may occur, using forms such as 1*DIGIT for one or more digits, 2DIGIT for exactly two digits, or 1*2DIGIT for one to two digits. Alternatives are expressed with a slash, and grouping uses parentheses. Optional parts use square brackets. Comments may be added after a semicolon.

Terminals may be literal text, numeric character sets, or predefined character classes. A common pattern is to define a date as: date = 4DIGIT "-" 2DIGIT "-" 2DIGIT, with DIGIT defined as %x30-39. Whitespace is generally ignored outside literals, and case-sensitivity is determined by the grammar and the protocol being defined.

ABNF grammars are descriptive and are analyzed by syntax checkers or parser generators to implement protocols and data formats in a consistent, interoperable way.