# Problem Solving: Backus-Naur Form

 ← Finite State Machines Backus-Naur Form (BNF) Reverse Polish Notation →

Backus-Naur Form (also known as Backus Normal Form (BNF) or BNF for short) is a notation technique to express syntax of languages in computing. The expression is put in lists and can be used to see if syntax that is written is valid. Backus-Naur means "not in normal form compared to convention".

## Structure and Layout

### Symbols

BNF is represented using the following symbols:

 ::=   'is defined as'
|     'or'
<>    category names


The way that these symbols are laid out are as such:

 <Parent Expression> ::= <Child Expression 1> | <Child Expression 2>


In plain English, the expression above means "The parent expression is defined as the child expression 1 or the child expression 2". This means that to make up the parent expression, it must have a child expression and a child expression is made up of other things.

## Example

In this example, The BNF structure is breaking down the syntax to create

 Backus-Naur Form Breakdown of a Home Address 
::= |   ::= ::= | ::= ::= | ::= | ::= | ::= ::= ::= ::= ::= ::=  An Image to show how the address BNF would be laid out.

## Practice Questions

 Using the BNF from the example above about addresses, state whether each input is a valid input. 15 Jubilee Lane Blackpool England FY98 5ER Answer : YES 15 Jubilee Lane Blackpool England FY423 5ER Answer : NO 32 Parkstone Road Syston England Leicester LE7 3ZY Answer : NO