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Extended Backus–Naur Form (EBNF)

Extended Backus-Naur Form (EBNF) is a notation used to describe the syntax of programming languages, data formats, and communication protocols. It is an extension of the original Backus-Naur Form (BNF) notation, which was developed by John Backus and Peter Naur in the late 1950s and early 1960s as a means of formally specifying the syntax of the ALGOL 60 programming language. EBNF extends BNF by introducing additional syntax rules and features, making it more expressive and easier to read and write.

EBNF is a meta-language, which means it is a language used to describe other languages. It consists of a set of production rules that define the structure and syntax of a language. Each production rule consists of a non-terminal symbol (the left-hand side of the rule) and a sequence of terminal and non-terminal symbols (the right-hand side of the rule) that can be used to derive the non-terminal symbol. Terminal symbols represent the basic elements of a language (e.g., keywords, operators, or identifiers), while non-terminal symbols represent higher-level syntactic constructs that can be further expanded using other production rules.

EBNF introduces several enhancements over the original BNF notation:

  1. Optional elements: EBNF allows for the specification of optional elements using square brackets ([ ]). For example, the rule "A ::= B [C]" indicates that the non-terminal symbol A can be derived from the symbols B followed by an optional C.
  2. Repetition: EBNF supports the specification of repeated elements using curly braces ({ }). For example, the rule "A ::= B {C}" indicates that the non-terminal symbol A can be derived from the symbol B followed by zero or more occurrences of the symbol C.
  3. Alternatives: EBNF uses the vertical bar (|) to represent alternatives between different syntax options. For example, the rule "A ::= B | C" indicates that the non-terminal symbol A can be derived from either the symbol B or the symbol C.
  4. Grouping: EBNF allows for the grouping of elements using parentheses (()). This is useful for specifying the precedence or order of operations within a rule. For example, the rule "A ::= B (C | D)" indicates that the non-terminal symbol A can be derived from the symbol B followed by either the symbol C or the symbol D.
  5. Terminal symbols: EBNF supports the specification of terminal symbols using single or double quotes ( or ""). This makes it easier to distinguish between terminal and non-terminal symbols in the production rules.

EBNF is widely used for documenting the syntax of programming languages and data formats, as well as for developing parsers and compilers. Its standardized, human-readable notation allows for the clear and unambiguous specification of complex language constructs, making it easier to understand, implement, and maintain language specifications.


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