3.1 What is XML?

XML is a framework for creating new languages. An XML document looks a lot like HTML not by coincidence, since the design of XML was heavily influenced by experience with HTML. It might be more correct to call XML a meta-language rather than a language because XML allows you to invent your own tags and attributes and give them any meaning you choose.

As an illustration, we'll invent an XML language which describes information about people. XML is often used to exchange data between different software. An older technology to do this is the comma-separated file. Here is a sample comma-separated file we will translate into XML:

James Roberts,47,(415) 555-1212Alice Hacker,19,(408) 555-1213Edwin Eavesdropper,,(212) 555-1214

The fields are name, age and phone number. Notice that we don't know Edwin's age. Here is an XML file containing the same data.

    James Roberts (415) 555-1212   (415) 555-2212 Alice Hacker   Edwin Eavesdropper (415) 555-3212  

An XML document always starts with a declaration like this:

This is a comment:

The meat of the document is enclosed between a pair of tags .

The information relating to a given person is found between and .

We chose to represent the name and phone number fields by using and respectively. Age, on the other hand, we represented as an attribute of the tag. Since we're inventing our own language here, we're free to use an attribute or a tag as we please. A key difference is that a given...