This is a language to define the formal grammar of a language, whether that be a ProgrammingLanguage, a MarkupLanguage or simply the syntax of a configuration file.

An example grammar would be

<sentance> ::= <verb-part> <noun-part>

<verb-part> ::= <noun-part> <verb>

<noun-part> ::= <proper-noun>

| THE <noun>

<verb> ::= throws

| kicks | hides | fetches

<proper-noun> ::= John

| Jane

<noun> ::= ball

::= dog

This grammar accepts sentances such as

• Jane throws the ball
• The dog fetches the ball
• John kicks the dog
• John hides Jane
• The ball fetches Jane

but would not accept sentences such as

• John deflates the ball
• Jane hides in the basement

because deflates is not a verb known in this grammer and in the basement is not a noun. The grammar would have to be extended to cover such cases.

Because some common cases are inconvenient to define using pure BNF, some extensions have been added in EBNF:

• ... means 1 or more
• [? means optional