PARSING XML FILES

Applications that manipulate XML need to be able to move through the data structure, finding elements, tags and content. Processing data to extract meaning from it is called parsing in computing. The same term is used to describe the processing of sentences in human languages to extract their meaning. The idea, in both cases, is the same. Few developers choose to write their own XML parsers. Although the rules of the grammar are relatively simple, writing fast and accurate parsers is a difficult task. Most people use a parser written by someone else. Many XML parsers are freely available; the choice of which you use tends to depend upon your system and the language that you are developing in. Two popular choices are MSXML from Microsoft, which can be programmed using C++ or Visual Basic, and Xerces from the Apache Foundation. Xerces comes in Java, C++ and Perl versions and can be used on many different operating systems. Both these parsers can be used directly from the command line or called from within applications.

Once you have installed MSXML on your system, it is automatically available within Internet Explorer. This means that you can, for instance, open XML files in Explorer and view them as tree structures.

As you read through this book, you'll find that XML parsers can do lots of interesting things with your XML. One of the most useful is to check if the XML you have written is correct, and if it adheres to the rules set out in the DTD or schema for that particular document.

2.4.1 Valid or well-formed?
XML documents may be either valid or well-formed. The two terms relate to differing levels of conformance with the XML Recommendation, the DTD, or schema, and the basic structure of the XML. All XML documents must be well-formed. Tags should be paired, elements should be properly nested, the document should have an XML declaration. Entities should be properly formed. Any application which can handle XML will be able to cope with a well-formed document. A valid document takes conformance rather further. To be valid, a DTD or schema should be identified for the XML data. The data must meet the rules set out in that document.

All XML parsers are able to check that a document is well-formed. For some such as MSXML, this is where their capabilities end. Other parsers such as Xerces are able to validate an XML document against a DTD. At the time of writing, Schema support in Xerces is in the alpha stage of development. That means it's far from ready for the big time – but it is being implemented. XML is a new technology, it's evolving rapidly and tool support does tend to lag slightly behind. In the near future, though, the tools will be available to use XML Schema as well as DTDs. It's at that stage that we'll start to see DTDs becoming less popular with developers.

Unparsed Character Data
Most of the content in an XML file will be handled by the parser. Generally elements and entities contain text that has some meaning. The content will not include characters such as < which have special meaning to the parser, and when it does contain them, those characters are usually entered as character entities. Sometimes a document will include large numbers of these characters. In such cases using entities may be impractical. The XML standard allows for this. Your document can include sections of CDATA, unparsed character data. All characters inside a CDATA section are assumed to be content, rather than markup. A section of CDATA is started with the string as shown in Listing 2.6. You'll meet CDATA again in the discussion of DTDs in Chapter 3.

Listing 2.6: CDATA Sections





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