29. Syntax Diagram example

Defining a stock code

In this example, you are involved with writing a database to handle stock. The primary key for each stock item will be a stock code. Your task is to define the structure of this stock code and inform other programmers of its syntax.

A vaid stock code should look similar to this

A:A1JZ2B

Use syntax diagrams.

One answer.

The upper diagram shows that the first character has to be an 'alpha' this is always followed by a colon, the next character can be either a digit or another alpha. This can repeat as much as required.

The lower diagrams show the syntax for a digit and the item called alpha. In this case alpha is allowed to be one of four characters A, B, J or Z

Examples of invalid stock codes

• 1:1ABJZ this has a leading digit which is an invalid syntax
• AB199 the colon is missing and so is an invalid syntax
• A:333R the R character is not allowed and so it an invalid syntax