Institute of Information Science - University of Miskolc

<?xml version='1.0' encoding='UTF-8' ?>
<!-- first XML example -->
<message id="123456">
  <text>Hello World!</text>
</message>

- version: The version number of the XML document.

- encoding: Character encoding - 'ISO-8859-2' refers to Latin-2 encoding.

- comments: Comments can be placed between <!– –>, and they can span multiple lines.

- tagging data: Data is labeled using tags. For example, <name></name>, where <name> is the opening tag and </name> is the closing tag. If the tag doesn't contain data, the closing can be simplified like <name />.

- In this example, “Hello World!” is stored in the <text> tag.

- An XML file contains a root element (or root tag) to which all other elements belong, in this case <message>. Elements that contain other elements are called parent elements, and the elements within them are called child elements.

- Elements can contain an unlimited number of attributes (properties), such as the id attribute = “123456”. Attribute names are not limited in length, but every attribute name must start with a letter.

In XML, special characters are represented as follows:

- & - &

- < - <

- > - >

- ' - '

- “ - "

A CDATA section can contain embedded data, such as the following example:

<script language="JavaScript" type="text/javascript">
  <![CDATA[
    function sayHello() {
      document.write("Hello World!");
    }
  ]]>
</script>

Since creators of XML documents use their own vocabulary to build XML, name conflicts are possible. For example, the <student> tag might be too generic. Using namespace prefixes, the element can be specialized:

<miskolc:student>John Smith</miskolc:student> 

Here, the qualified element is student and the namespace prefix is miskolc.

<?xml version="1.0" encoding="UTF-8"?>
<data xmlns:unimiskolc="www.uni-miskolc.hu">
  <unimiskolc:file filename="aula.jpg">
    <unimiskolc:description>Photo of the university hall</unimiskolc:description>
    <unimiskolc:size width="200" height="100" />
  </unimiskolc:file>
</adatok>

In this example, the namespace prefix unimiskolc is defined to avoid name conflicts, and it links the data to the www.uni-miskolc.hu namespace.

XML interpreters process XML documents by performing syntactic validation.

An XML document is considered “well-formed” if it is syntactically flawless.

This means: - It has a single root element.

- Every opening tag has a corresponding closing tag.

- The attributes are properly specified.

Therefore, the term “well-formed” does not refer to the visual formatting of the XML file with tabs, spaces, or its human-readable structure!

<root><element>abc</element><a>this worng<b></root>

This XML is syntactically incorrect because the `<a>` tag is not closed.

The following example is neatly formatted but still not well-formed:

<CONTACT>
   <NAME>Frank Lee</NAME>
   <EMAIL>flee@flee.com
</CONTACT>

Here’s the correct version:

<CONTACT>
   <NAME>Frank Lee</NAME>
   <EMAIL>flee@flee.com</EMAIL>
</CONTACT>

What’s the difference? In the incorrect version, the `<EMAIL>` tag is not closed, causing the document to be invalid, even though it is visually well-structured.