BNF Grammar

BNF (Backus-Naur Form) is a metasyntax notation: it formally specifies the context-free grammar (syntax) of a programming language or communication protocol.

BNF, or Backus-Naur Form, is the foundational metasyntax used to define the formal grammar of a language, first introduced by John Backus and Peter Naur for the Algol 60 report in 1960. The notation employs *production rules* to specify how a non-terminal symbol (e.g., <expression>) can be expanded: the rule <name> ::= expansion states that the symbol on the left can be replaced by the sequence of symbols on the right. Terminals (literal characters like `+` or `if`) and non-terminals comprise the expansion, with the vertical bar (`|`) indicating alternatives. This precise, recursive structure enables parser generation and serves as the basis for modern extensions like EBNF and ABNF.

https://www.geeksforgeeks.org/bnf-notation-in-compiler-design/