BNF is a notation used to describe the syntax of programming languages or other formal languages.

BNF can be described as a metasyntax notation for context-free grammars.

A BNF specification is a set of derivation rules, written as

1<symbol> ::= __expression__

where:

1. <symbol> is a nonterminal variable, always enclosed in <>
2. ::= means that the symbol on the left must be replaced with the expression on the right.
3. __expression__ consists of one or more sequences of either terminal or nonterminal symbols where each sequence is separated by a vertical bar | indicating a choice, the whole being a possible substitution for the symbol on the left.
4. terminal symbols are characters enclosed in "" and are meant to be used literally

References

• https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form
• https://realpython.com/preview/python-bnf-notation/