XML is very similar to HTML. It has tags, which identify elements. These tags also contain
attributes about these elements. In XML a Tag is what is written between angled brackets,
i.e. XML tags open with the < symbol and end with the > symbol. They always come in
and matched pairsm, with the defined element between the open and close tag. <composer> is an
example for an opening tag. In XML all opening Tags must have closing tags, in this
case the closing tag would look like this: </composer>. Most of you will
already be familiar with tags from HTML and SGML.
Element
<composer>Mozart</composer>
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Start Tag
The beginning of every non-empty XML element is marked by a start-tag.
An example of a start-tag:
End Tag
The end of every non-empty XML element is marked by a end-tag.
An example of an end-tag:
Element Content
The text between the start-tag and end-tag is called the element's content. The element
content in this case would be:
Empty Element Tag
If an element is empty, it must be represented either by a start-tag immediately followed
by an end-tag or by an empty-element tag. An empty-element tag takes a special form:
<BR/>...empty
element tag in XML, or
<BR></BR>
As opposed to HTML where a line break is declared with:
<BR>...start
tag without an end tag
Empty-element tags may be used for any element which has no
content, whether or not it is declared using the keyword EMPTY (see chapter on Element Type Declaration). For interoperability, the empty-element
tag must be used, and can only be used, for elements which are declared EMPTY. Examples of
empty elements:
<IMG align="center"
scr="logo.gif"/>
<BR></BR>
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Element Type
The Name in the start- and end-tags gives the element's type. In our example it would be composer.
The Name-AttValue pairs are referred to as the attribute
specifications of the element, with the Name in each pair referred to as the attribute
name and the content of the AttValue as the attribute value. In the following example we
have an empty element 'IMG' and two attributes 'align' and 'src'.
<IMG align="center"
src="logo.gif"/>
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As you will see later on in the tutorial,
XML and HTML markup will be used in the same document. By convention put HTML tags in
upper case and XML tags in lower case.
Furthermore, XML is case sensitive. Always remember that <composer>, <composer> and <composer> are different kinds of tags in XML.
Tags should begin with either a letter, an underscore (_)
or a colon (:) followed by some combination of letters, numbers, periods (.), colons,
underscores, or hyphens (-) but no white space, with the exception that no tags should
begin with any form of "xml". It is also a good idea to not use colons as the
first character in a tag name even if it is legal. Using a colon first could be confusing.
The XML 1.0 standard does not limit the length of tag
names, but XML processors may limit the length of markup names.
In the next chapter we will have a look at an example XML
file, which will then be analyzed in detail. |
