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2.2. Elements, Tags, and Character Data

The document in Example 2-1 is composed of a single element named person. The element is delimited by the start-tag <person> and the end-tag </person>. Everything between the start-tag and the end-tag of the element (exclusive) is called the element's content. The content of this element is the text string:

Alan Turing

The whitespace is part of the content, though many applications will choose to ignore it. <person> and </person> are markup. The string "Alan Turing" and its surrounding whitespace are character data. The tag is the most common form of markup in an XML document, but there are other kinds we'll discuss later.

2.2.1. Tag Syntax

XML tags look superficially like HTML tags. Start-tags begin with < and end-tags begin with </. Both of these are followed by the name of the element and are closed by >. However, unlike HTML tags, you are allowed to make up new XML tags as you go along. To describe a person, use <person> and </person> tags. To describe a calendar, use <calendar> and </calendar> tags. The names of the tags generally reflect the type of content inside the element, not how that content will be formatted.

2.2.2. XML Trees

Let's look at a slightly more complicated XML document. Example 2-2 is a person element that contains more information suitably marked up to show its meaning.

Example 2-2. A more complex XML document describing a person

  <profession>computer scientist</profession>

2.2.3. Mixed Content

In Example 2-2, the contents of the first_name, last_name, and profession elements were character data, that is, text that does not contain any tags. The contents of the person and name elements were child elements and some whitespace that most applications will ignore. This dichotomy between elements that contain only character data and elements that contain only child elements (and possibly a little whitespace) is common in documents that are data oriented. However, XML can also be used for more free-form, narrative documents such as business reports, magazine articles, student essays, short stories, web pages, and so forth, as shown by Example 2-3.

Example 2-3. A narrative-organized XML document

  <name><first_name>Alan</first_name> <last_name>Turing</last_name>
  </name> was one of the first people to truly deserve the name 
  <emphasize>computer scientist</emphasize>. Although his contributions 
  to the field are too numerous to list, his best-known are the 
  eponymous <emphasize>Turing Test</emphasize> and 
  <emphasize>Turing Machine</emphasize>.

  <definition>The <term>Turing Test</term> is to this day the standard
  test for determining whether a computer is truly intelligent. This 
  test has yet to be passed. </definition>

  <definition>The <term>Turing Machine</term> is an abstract finite 
  state automaton with infinite memory that can be proven equivalent 
  to any any other finite state automaton with arbitrarily large memory. 
  Thus what is true for a Turing machine is true for all equivalent 
  machines no matter how implemented.

  <name><last_name>Turing</last_name></name> was also an accomplished   
  <profession>mathematician</profession> and
  <profession>cryptographer</profession>. His assistance 
  was crucial in helping the Allies decode the German Enigma
  machine. He committed suicide on <date><month>June</month> 
  <day>7</day>, <year>1954</year></date> after being 
  convicted of homosexuality and forced to take female 
  hormone injections.

The root element of this document is biography. The biography contains name, definition, profession, and emphasize child elements. It also contains a lot of raw character data. Some of these elements such as last_name and profession only contain character data. Others such as name contain only child elements. Still others such as definition contain both character data and child elements. These elements are said to contain mixed content. Mixed content is common in XML documents containing articles, essays, stories, books, novels, reports, web pages, and anything else that's organized as a written narrative. Mixed content is less common and harder to work with in computer-generated and processed XML documents used for purposes such as database exchange, object serialization, persistent file formats, and so on. One of the strengths of XML is the ease with which it can be adapted to the very different requirements of human-authored and computer-generated documents.

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