Extended Backus–Naur Form (EBNF)

Extended Backus–Naur Form (EBNF) is a type of formal syntax used to specify the structure of a programming language or other formal language. It is an extension of Backus-Naur Form (BNF), which was originally developed by John Backus and Peter Naur to describe the syntax of the Algol programming language.

EBNF adds several additional metasymbols to the original BNF metasymbols, which allows for a more concise and readable specification of a language's syntax. It is commonly used in the specification of programming languages, and is also sometimes used to describe the syntax of other types of formal languages, such as database query languages or markup languages.

Basic support for EBNF has been introduced in PlantUML.

Minimal binary diagram

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@startebnf
binaryDigit = "0" | "1";
@endebnf

Example of LISP Grammar

LISP Grammar with PlantUML.

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@startebnf
title LISP Grammar
grammars_expression = atomic_symbol | "(", s_expression, ".", s_expression, ")" | list;
list = "(", s_expression, { s_expression }, ")";
atomic_symbol = letter, atom_part;
atom_part = empty | letter, atom_part | number, atom_part;
letter = ? a-z ?;
number = ? 1-9 ?;
empty = " ";
@endebnf

[Ref. ]

All EBNF elements

EBNF elements managed by PlantUML.

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@startebnf
title All EBNF elements managed by PlantUML

(* Nodes *)
litteral = "a";
special = ? a ?;
rule = a;

(* Edges *)
required = a;
optional = [a];

zero_or_more = {a};
one_or_more = a, {a};
one_or_more_ebnf = {a}-;

zero_or_more_with_separator = [a, {',', a}];
one_or_more_with_separator = a, {',', a};
zero_or_more_with_terminator = {a, ','};
one_or_more_with_terminator = a, ',', {a, ','};
one_or_more_with_terminator_ebnf = {a, ','}-;

alternative = a | b;
group = (a | b) , c;
without_group = a | b , c;
@endebnf

Notes on elements

Notes may be added to elements of your diagram with ebnf comment tags.

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@startebnf
title Comments
(* Notes for Rule1 *)
Rule1 = {"a-z" (* any letter *) };
(* Notes for Rule2 *)
Rule2 =;
(* Additional notes, and referances *)
@endebnf

EBNF of PlantUMLs EBNF grammers

EBNF allows for self description, so here it is!

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@startebnf
grammar = { rule };
rule = lhs , "=" (* definition *) , rhs , ";" (* termination *);
lhs = identifier ;
rhs = identifier
     | terminal
     | "[" , rhs (* optional *) , "]"
     | "{" , rhs (* repetition *), "}"
     | "(" , rhs (* grouping *) , ")"
     | "(*" , rhs (* comment *) , "*)"
     | "?" , rhs (* special sequence, aka notation *) , "?"
     | rhs , "|" (* alternation *) , rhs
     | rhs , "," (* concatenation *), rhs ;
identifier = letter , { letter | digit | "_" } ;
terminal = "'" , character , { character } , "'"
         | '"' , character , { character } , '"' ;
character = letter | digit | symbol | "_" ;
symbol = "[" | "]" | "{" | "}" | "(" | ")" | "<" | ">"
       | "'" | '"' | "=" | "|" | "." | "," | ";" ;
digit = ? 0-9 ? ;
letter = ? A-Z or a-z ? ;
@endebnf


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