What's The Frequency, Bobby? - XML

A number of special symbols can be added to an element declaration in order to define its frequency and order, or the frequency and order of its child elements. Here's a quick list:

symbol  description
---------------------------------------------------------
+ 
one 
or more occurrence(s)
*  zero or more occurrence(s)
?  zero or one occurrence(s)
| 
choice

If you're familiar with regular expressions, you'll feel right at home with these symbols - they're almost identical to the symbols used to build regular expression patterns.

Let's take this for a quick spin. Consider the following revised XML document

<?xml version="1.0"?>
<!DOCTYPE weather SYSTEM "weather.dtd">
<weather>
<city>New 
York</city>
<high>26</high>
<low>18</low>
<forecast>rain</forecast>
<city>Boston</city>
<forecast>snow</forecast>
<city>London</city>
<high>32</high>
<forecast>sun</forecast>
</weather>

and then take a look at its associated DTD

<!ELEMENT weather (city, high*, low*, forecast)+>
<!ELEMENT city (#PCDATA)>
<!ELEMENT 
forecast (#PCDATA)>
<!ELEMENT high (#PCDATA)>
<!ELEMENT low (#PCDATA)>

How did I come up with this? It's simple - you just have to take it step by step.

The first thing to do is allow for more than one "city" block within the "weather" element.

<!ELEMENT weather (city, high, low, forecast)+>

Next, the "high" and "low" elements must be made optional.

<!ELEMENT weather (city, high*, low*, forecast)+>

And Bob's your uncle!

The | operator sets up a list of alternatives, and comes in handy when an element must contain any one of a finite list of alternatives. Consider the following XML document,

<?xml version="1.0"?>
<addressbook>
<record>
<name>John Smith</name>
<street>24, 
Main Street</street>
<city>Poodle Springs</city>
<zip>16628</zip>
<country>USA</country>
<tel>
<home>947 
3838</home>
</tel>
</record>
<record>
<name>Sherlock Holmes</name>
<tel>
<home>827 
3483</home>
</tel>
<fax>
<home>364 
2929</home>
</fax>
<email>
<home>holmes@greatdetectives.org</home>
</email>
</record>
<record>
<name>Jane 
Doe</name>
<email>
<work>jane@somedomain.com</work>
</email>
</record>
</addressbook>

and take a look at the corresponding DTD, specifically at the declarations for the "tel", "fax" and "email" elements, which may contain either "home" or "work" nested child elements.

<!ELEMENT addressbook (record+)>
<!ELEMENT record (name, street?, city?, 
zip?, country?, tel?, fax?, email?)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT 
street (#PCDATA)>
<!ELEMENT city (#PCDATA)>
<!ELEMENT country (#PCDATA)>
<!ELEMENT 
zip (#PCDATA)>
<!ELEMENT tel (home | work)>
<!ELEMENT fax (home | work)>
<!ELEMENT 
email (home | work)>
<!ELEMENT home (#PCDATA)>
<!ELEMENT work (#PCDATA)>

The | operator also comes in handy when defining elements which are of so-called "mixed" type - they can contain either data or other elements. Here's an example:

<?xml version="1.0"?>
<surrealism>
The elongated <color>blue</color> 
<animal>fox</animal> jumped over the
<color>green</color> <vegetable>pumpkin</vegetable> 
and morphed into
<personality>Richard VIII</personality>
</surrealism>

Pay close attention to the "surrealism" element, which can contain either character data or any one of the listed elements:

<!ELEMENT animal (#PCDATA)>
<!ELEMENT color (#PCDATA)>
<!ELEMENT personality 
(#PCDATA)>
<!ELEMENT surrealism (#PCDATA | animal | color | personality | 
vegetable)*>
<!ELEMENT 
vegetable (#PCDATA)>

Obviously, all these symbols can also be combined to create weird and wonderful rules for the document to follow. An example awaits you at the end of the article...but first, attributes.

>>> More XML Articles          >>> More By Vikram Vaswani, (c) Melonfire

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