4. BNF example 3

Reminder:

Some of the BNF notation include:

 < > used to enclose a syntactic item 

 ::= means "is defined by" or "consists of"

 | when placed between two items means an OR choice between them

 { } curly brackets means zero or more repetitions of the content. 

Example 3 Defining an integer

Note that <integer> appears twice in the rule

This is called a 'recursive' statement. A recursive statement makes use of itself as part of its definition.

In this case the rule, stated in English becomes "An integer consists of a single digit or a single digit followed by another integer."

So you can see that this allows an integer to be any length and yet it is a very concise statement in BNF.