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:
Alan Turing
The whitespace is part of the content, although 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.
Superficially, XML tags look 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.
There's also a special syntax for empty elements,
elements that have no content. Such an element can be represented
by a single empty-element tag that begins
with < but ends with
/>. For instance, in
XHTML, an XMLized reformulation of standard HTML,
the line-break and horizontal-rule elements are written as
<br /> and <hr /> instead of <br> and <hr>. These are exactly equivalent
to <br></br> and
<hr></hr>, however.
Which form you use for empty elements is completely up to you.
However, what you cannot do in XML and XHTML (unlike HTML) is use
only the start-tag—for instance <br> or <hr>—without using the matching
end-tag. That would be a well-formedness error.
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
<person>
<name>
<first_name>Alan</first_name>
<last_name>Turing</last_name>
</name>
<profession>computer scientist</profession>
<profession>mathematician</profession>
<profession>cryptographer</profession>
</person>The XML document in Example 2-2 is still composed
of one person element. However,
now this element doesn't merely contain undifferentiated character
data. It contains four child elements: a
name element and three profession elements. The name element contains two child elements
of its own, first_name and
last_name.
The person element
is called the parent of the name element and the three profession elements. The name element is the parent of the
first_name and last_name elements. The name element and the three profession elements are sometimes called
each other's siblings . The first_name
and last_name elements are also
siblings.
As in human society, any one parent may have multiple
children. However, unlike human society, XML gives each child
exactly one parent, not two or more. Each element (with one
exception we'll note shortly) has exactly one parent element. That
is, it is completely enclosed by another element. If an element's
start-tag is inside some element, then its end-tag must also be
inside that element. Overlapping tags, as in <strong><em>this common example from HTML</strong></em>, are
prohibited in XML. Since the em
element begins inside the strong element, it must also finish
inside the strong
element.
Every XML document has one element that does not have a
parent. This is the first element in the document and the element
that contains all other elements. In Examples Example 2-1 and Example 2-2, the person element filled this role. It is
called the root element of the
document . It is also sometimes called the document
element. Every well-formed XML document has exactly one
root element. Since elements may not overlap, and since all
elements except the root have exactly one parent, XML documents
form a data structure programmers call a
tree. Figure 2-1 diagrams this
relationship for Example
2-2. Each gray box represents an element. Each black box
represents character data. Each arrow represents a containment
relationship.
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 record-like documents. 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
<biography> <paragraph> <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>. </paragraph> <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>A <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 one Turing machine is true for all Turing machines no matter how implemented. </definition> <paragraph> <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 cipher. 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. </paragraph> </biography>
The root element of this document is biography. The biography contains paragraph and definition child elements. It also
contains some whitespace. The paragraph and definition elements contain still other
elements, including term,
emphasize, name, and profession. They also contain some
unmarked-up character data. Elements like paragraph and definition that contain child elements and
non-whitespace character data are said to have 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.