BNF - Syntax Diagrams

Backus Naur Form is a way of describing the syntax of a value to form a new value.
For example:
<digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
This declares that <digit> can only be the numbers 0-9, specifically 0 OR 1 OR 2 OR 3 etc.

This notation can also be recursive:
<integer> ::= <digit> | <digit><integer>
This states that an integer can be a digit or a digit followed by another integer.