The idea of describing the structure of language using rewriting rules can be traced back to at least the work of Pāṇini, an ancient Indian Sanskrit grammarian and a revered scholar in Hinduism who lived sometime between

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A BNF specification is a set of derivation rules, written as
where <symbol> is a nonterminal, and the __expression__ consists of one or more sequences of symbols; more sequences are separated by the vertical bar "|", indicating a

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As an example, consider this possible BNF for a U.S. postal address:
This translates into English as:
• A postal address consists of a name-part, followed by a

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BNF's syntax itself may be represented with a BNF like the following:
Note that "" is the empty string.
The original BNF did not use quotes as shown in <literal> rule. This assumes that no whitespace is necessary for proper interpretation of the rule.

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• Compiler Description Language (CDL)
• Syntax diagram – railroad diagram
• Translational Backus–Naur form (TBNF)

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• Garshol, Lars Marius, BNF and EBNF: What are they and how do they work?, NO: Priv.
• RFC 5234 — Augmented BNF for Syntax Specifications: ABNF.
• RFC 5511 — Routing BNF: A Syntax Used in Various Protocol Specifications.

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There are many variants and extensions of BNF, generally either for the sake of simplicity and succinctness, or to adapt it to a specific application. One common feature of many variants is the use of regular expression repetition operators such as * and +. The

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