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All you need to know about XML

There are acronyms all over the web. Most relate to instant messaging or emailing; however, that isn’t the case for the entire internet—there are some acronyms that relate to programming. One of these is XML.

What is XML?

Extensible Markup Language, or XML, has a few different functions, although it is mainly a general-purpose formal computer science language used for creating other markup languages. (Markup languages are ones that have specific tags or rules that determine how and where text is to appear on a page.) The key purposes of XML are:

• It allows the sharing of structured data across the internet and across other information systems.
• XML allows people to define elements as necessary.
• Encoding documents and serializing data are both possible through the usage of XML.
• There are two levels of XML, basically two levels of how well it was parsed or written:
• Well-formed: Documents that are well-formed follow all of the syntax rules of XML.
• Valid: Documents that are valid are well-formed and they follow some semantic rules as well.

XML Syntax

There are a few specifics to XML syntax. However, they really aren’t difficult to learn. Plus, there is only one that is an absolute must to every XML document. This is that a document has only one root element (or document element). Here is an example of a well-formed XML document with one root element:

<dog>There is a dog…. </dog>

Of course, other lines of XML text can be added to this document, such as an XML declaration of the version number. The above line, though, is the very basic, most fundamental part of XML.

XML Rules

There are some rules that well-formed documents must follow for XML:

• Elements that are not empty must have start tags and end tags to signify their boundaries.
• Elements that are empty must be marked as such—the author of the document can use an empty-element (or self-closing) tag. This can be either <IAmEmpty /> or <IAmEmpty></IAmEmpty>.
• Single quotes or double quotes need to be around all attribute values.
• It is acceptable for tags to be nested; however, it is not ok for them to overlap.
• It is important to watch and make certain that case is observed in all tags. For instance, the following would not be acceptable in a well-formed document: <Example> .… </example>

XML Semantics

By following specific semantic rules, XML documents will be not only well-formed, but they will also be recognized as valid. There are several common schemas that exist through which valid XML documents can be created:

• Document Type Definition (DTD): This is the oldest schema format for XML and lacks support for some of the newer features; however, it is still in often use because it is considered to be the easiest to read and write.
• XML Schema: This is often said to be the successor to DTD.
• There are a few other schema language types; however, these are typically more suited to advanced XML users.