Introduction to XML

Attributes

Some elements have attributes that specify extra metadata for the content. An abbreviation might for example have its meaning specified:

<abbr title="eXtensible Markup Language">XML</abbr>

An attribute is allways specified as a name followed by an equals sign and a quoted value (the plain ASCII double quote #34 (") or single quote #39 ('), no fancy quotes).

The document tree

An xml document must consist of exactly one root element and its contents. Since elements can’t overlap partially, each element (except the root node) has exactly one parent element. Hence, the entire document can be seen as a tree.

Attributes and text nodes are considered separate nodes, and the attributes of an element allways appears before the contents of the element. Thus the following simple document:

<text lang="en">Hello, World!  I am <name>Rasmus</name>.</text>

… has the following document tree:

• element node text
  • attribute node lang = en
  • text node Hello, World! I am
  • element node name
    • text node Rasmus
  • text node .

The document tree is often a good way to think of a document, and should map directly to document semantics, even if some vocabularies (notably HTML) doesn’t enforce such a mapping.

Whitespace

There is two different modes for how whitespace is handled, default or preserve.

When preserving whitespace, each whitespace (plain space, tab, new line, carriage return) is handled as just that character.

When in default mode, each sequence of consecutive whitespace, including space, horizontal tab, newline and carriage return, is equivalent to a single whitespace.

There is a special attribute, xml:space, that may be used to specify the white space mode for the content of an element.

Characters and Entities

An xml file can be written using any character encoding, as long as it tells the reader which one. That is done on the first line of the file, which looks like this:

<?xml version="1.0" encoding="utf-8"?>

Both the version and the encoding has default values (for version, only 1.0 and 1.1 is specied and 1.0 is the default, for encoding, utf-8 is the default, but any encoding can be used) so the above is equivalent to:

<?xml?>

Some characters have special meaning in xml. What if I want to express the mathematical relation x < 2 ? If I write it just like that, the < will mark the beginning of a tag, and not the less than character. Luckily, there is something called a character entity for this. Using that, I can write x < 2 and it will mean x < 2 (there is also a specific vocabulary for mathematical statements, called MathML, but that is way beoynd the scope of this introduction).

The character “&” always marks the start of an entity reference (except in a comment). It is followed by a name and a semicolon. XML defines a few character entities by default.

There is also numerical character entities, which refers to Unicode characters (regardless of the document encoding). They can be expressed as &#NNN; in decimal or as &#xNNN; in hexadecimal. For example A is a the letter A and α is the greek letter α.

It is also possible to declare arbitrary named entities, but that requires using a DTD (see below) and isn’t used much today, except in legacy html documents.

Comments

A comment in xml starts with <!-- and ends with -->. It can contain anything except two sequential dashes, including newlines and commented out markup. Entitys is not recognized within comments.

<!-- This is a comment (which may contain < & >) -->

Comments can’t be nested, since a comment must not contain the string --. A common workaround for that is to put a space between the dashes in the inner comment:

<!-- a comment
<!- - with a "nested" comment - ->
end of comment: -->

Processing instructions

A processing instruction is a special tag that tells a program reading the file what to do with it. It might for example tell it to use a specific xslt transform when presenting the document to a user.

<?name and stuff?>

A processing instruction, just like a tag, starts with a name. But after that the syntax is freer, allowing anything but the string ?> (which terminates the processing instruction). Detailed syntax of processing instructions can be specific to the toolchain that processes it.

Doctype and cdata

These are SGML legacy. A DOCTYPE was used to tell the parser which SGML vocabulary to expect, by linking to a DTD. A CDATA block was used for a block of verbatim text without the normal SGML parsing. Both is possible to use in the same way in XML, but neither is used much.

Specifying a name space

Each namespace is identified by an URI. In the simplest case, a tag identifies the name space for itself and everything it contains with the xmlns attribute.

In the following example, both the tags foo and bar are inside the name space http://my.domain/name.

<foo xmlns="http://my.domain/name">
  <bar>hello</bar>
</foo>

Often, a document contains a mix of a few name spaces. It is of course possible to use the above syntax, with a xmlns attribute for each element that is from a different namespace than its parent element. However, that soon becomes extremely verbose. Then it is more practical to declare a shortcut for each namespace and use that on the elements.

<x:foo xmlns:x="http://my.domain/name"
       xmlns:y="http://some.other/markup">
  <y:exclamation>
    Hello <x:location>world</x:location>;
  </y:exclamation>
</x:foo>

Note that when using this syntax, an unqualified name does not belong to the same namespace as that of its parent, but instead to the current default namespace (i.e. the namespace that is defined without a shortcut). The following example is equivalent to the previous one, the location element belongs to the http://my.domain/name namespace.

<foo xmlns="http://my.domain/name"
     xmlns:y="http://some.other/markup">
  <y:exclamation>
    Hello <location>world</location>
  </y:exclamation>
</x:foo>

The name space URI

A namespace is identified by an URI, not by a resource found by that URI. On the web, for example http://www.kth.se/, https://www.kth.se and http://www.kth.se/index.html may all identify the same resource (either simply returning the same content, or redirecting to each other). But if used as a namespace identityfiers, they would refer to three separate namespaces.

A program processing the XML data has no need to fetch the actual resource identyfied by a name space URI, so useing a web uri does not require that the processor has network access. The name space URI does not even need to be specified useing a known protocol.

If you do point your web reader at a name space URI anyway, you will usually find a document specifying the semantics and syntax of the vocabulary.

xml:id

A unique identifier for the element the attribute belongs to. The value is required to be unique in the file. Note: Some vocabularies use id without the xml prefix for this.

xml:lang

The (natural) language of the content of the element the attribute belongs to. The value should be a registered language code.

xml:base

An URI to use as the base when resolving relative URIs in the content of the element the attribute belongs to (regardless of link semantics). The value should be an URI.

xml:space

Determins which kind of white-space handling should be done for the content of the element the attribute belongs to. The value should be either default or preserve. See the section on whitespace.

xmlns or xx:xmlns

See the section about name spaces.