The material in this section is based on the XML Specification. This is not an exhaustive list of all the constructs which appear in XML; it provides an introduction to the key constructs most often encountered in day-to-day use.
By definition, an XML document is a string of characters. Almost every legal Unicode character may appear in an XML document.
Processor and Application
Software which processes an XML document. It is expected that a processor works in the service of an application. There are certain very specific requirements about what an XML processor must do and not do, but none as to the behavior of the application. The processor (as the specification calls it) is often referred to colloquially as an XML parser.
Markup and Content
The characters which make up an XML document are divided into markup and content. Markup and content may be distinguished by the application of simple syntactic rules. All strings which constitute markup either begin with the character "<" and end with a ">", or begin with the character "&" and end with a ";". Strings of characters which are not markup are content.
A markup construct that begins with "<" and ends with ">". Tags come in three flavors: start-tags, for example
A logical component of a document which either begins with a start-tag and ends with a matching end-tag, or consists only of an empty-element tag. The characters between the start- and end-tags, if any, are the element's content, and may contain markup, including other elements, which are called child elements. An example of an element is
A markup construct consisting of a name/value pair that exists within a start-tag or empty-element tag. In this example, the name of the attribute is "number" and the value is "3":
XML documents may begin by declaring some information about themselves, as in the following example.
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