Problem Solving: Backus-Naur Form

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 <Address> ::= <House Number> <Street Name> <Town Name> <City Name> <Country> <Postcode> | <House Number> <Street Name> <City Name> <Country> <Post Code> <Postcode> ::= <Area Code> <Street Code> <Area Code> ::= <City Prefix> <digit> | <City Prefix> <digit> <digit> <Street Name> ::= <Name> <Street Type> <Flat Number> ::= <character> | <digit> <House number>::= <digit> | <digit> <House number> <Street Type> ::= <string> <City Prefix> ::= <string> <Street Code> ::= <string> <Town Name> ::= <string> <City Name> ::= <string> <Country> ::= <string> <Name>::= <string> 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