EBNF
2019-11-20


BNF stands for Backus-Naur Form which is a notation (or as I like to call it: metasyntax language) designed for description of <i>context-free grammars</i>. Many extensions and augmentations exist like EBNF (Extended BNF), ABNF (Augmented BNF) or TBNF (Translational BNF).
EBNF is the easiest one to read for humans. Another interesting one is a more complex TBNF, I'd say it's an Extended BNF Extended - it lets you define a structure of the Abstract Syntax Tree and the output Intermediate Code. Seems powerful, but personally I didn't delve into it; for now EBNF will make do.
It generally is a useful tool for language and protocol specification description. You can even use *BNF flavours to generate lexers and parsers automatically (see: <a href="https://en.wikipedia.org/wiki/Yacc">Yacc</a>).

EBNF is more or less defined like this:
<pre>
Notation   Description
    =     - definition
    ,     - concatenation
    ;     - termination of an expression
    |     - alternation (OR)
 [ ... ]  - option, element may be present once or not at all
 { ... }  - repetition, element omited or repeated
 ( ... )  - grouping
 " ... "  - literal strings
 (*   *)  - comments
 ? ... ?  - special sequence, describes something outside of the EBNF grammar
    -     - exception, can be used to exclude something from a rule
    *     - literal repetition (assert 4*"A" == "AAAA")
</pre>

Here's a wikipedia article as an example: <a href="https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form#Examples">https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form#Examples</a>
Another great practical example is the Go Programming Language Specification: <a href="https://golang.org/ref/spec">https://golang.org/ref/spec</a>
Go's formal specification played a huge part in it's adoption and even before that - it was crucial for the first implementation.
*BNF is quite intuitive. Writing it, however, is a tedious task.