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The eXtensible Markup Language ( XML ) is a meta-markup language defined by the World Wide Web Consortium (W3C) . It is not strictly a markup language itself, but rather a set of rules for creating markup languages. Forour purposes a markup language is any language (HTML, for example) that uses tags surrounding text to conveyinformation such as content or format. CNXML , the markup language used by the Connexions Project is an example of a language written in XML. There are many otherexamples at the W3C site. Here is an example of some markup in CNXML.
<para>This is a paragraph in<term>CNXML</term>. Notice that the markup
contains tags that express the meaning of the text.</para>
<para> and
</para> are the tags that
enclose the text. In XML, tags are always marked by anglebrackets (also known as
< and
> ).
Tags generally come
in pairs. An opening tag will look like
<tagname> . A closing tag will look
like
</tagname> , with a
/ preceding the tag name.
XML allows the separation of presentation from content. For example, HTML
has tags such as
<u> and
<i> , which underline and italicize
text respectively. This does not express content information,only formatting. XML allows you to define your own language
of tags to represent content. You could create a tag called
<book> to represent book titles,
and create a stylesheet (a separate formatting document), thatsays that every
<book> tag should
be italicized or underlined. Then when you want to change thepresentation of that type of content, you just change one
small part of the stylesheet. Also, if you make tags thatconvey the content of the document, you can enable better
searching. For example, you might look for the author of a
document by looking at the author tag.
XML has a few rules that apply to all of its languages, including CNXML. If a document satisfies theserules, then it is well-formed . XML documents are required to be well-formed.
<module> and a
closing tag looks like
</module> . There is a
shortcut. If your tag contains no other tags (referred toas an
empty tag ), then you can can type a /
before the end of the opening tag and delete the closingtag. For example,
<media></media> can be abbreviated
<media/> .<b>red<i>and</i>blue</b> is fine, but
<b>red<i>and</b>blue</i> is incorrect because the
<b> and
<i> tags have overlapping
content.<module id="m0001"> and
<module id='m0001'> are fine, but
<module id=m0001> is incorrect.<?xml version="1.0"?> You can also include other information such as the
encoding of the document or whether the document dependson other files or not.<html> and
</html> must surround all of
the other tags. There are some things that are includedat the top of the document that are not tags and that are
not included with the tags. The XML declaration is anexample of this.It is possible to define a set of rules that apply to all of the tags in a particular XML language. These rules can bedefined in a couple of different ways. The most common way is to use a DTD (Document Type Definition). Any document which follows all of the rules for that language iscalled valid . A document is not required to be valid in order to be XML. However, it is generally a goodidea.
XML uses several characters in special ways as part of its markup, in particular the less-than symbol ( < ), the greater-than symbol ( > ), the double quotation mark ( " ), the apostrophe ( ' ), and the ampersand ( & ). You've already seen examples of markup using the first four of those previously in this module.But what if you need to these characters in your content, and you don't want them to be treated as part of the markup by XML processors? You can use XML entity references for this purpose. The XML Specification defines the following five entity references for use in any well-formed XML document:
& refers to an ampersand (&)< refers to a less-than symbol (<)> refers to a greater-than symbol (>)" refers to a double-quote mark (")' refers to an apostrophe (')Suppose you have a document with the following:
<para id="p1">The firm was known as Scrooge and Marley.</para> you could replace 'and' with the entity reference
& :
<para id="p1">The firm was known as Scrooge& Marley.</para>
All entity references outside the above five must be defined in a document type declaration, and they may only be used in documents that conform to that DTD. Note that an entity reference always begins with
& and ends with
; .
You can also use any character defined in Unicode in an XML document by means of character references . Unicode is a project to define a unique code for everycharacter in any human language. Unicode is very useful any time that you need to use a symbol that is not a part ofASCII.
Character references in XML either begin with
&# , or they begin with
&#x , and they end with a semicolon
; . A character reference contains a representation of a Unicode code point: if it begins with
&# , then it contains a decimal representation of a Unicode code point; if it begins with
&#x , then it contains a hexidecimal representation of a Unicode code point.
The hexidecimal representation of the Unicode code point for the small 'o' with a stroke is
00F8 , and the decimal representation for the same is
248 . Therefore, the character references for the small 'o' with a stroke are
ø and
ø So you could write
<emphasis>The majestik møøse</emphasis> or
<emphasis>The majestik møøse</emphasis> or even
<emphasis>The majestik møøse</emphasis> to get
The majestik møøse
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