Institute of Information Science - University of Miskolc

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==== XML Example ====

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

- **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.

==== Special Characters in XML ====

In XML, special characters are represented as follows:

- ''&''  -  ''&amp;''

- ''<''  -  ''&lt;''

- ''>''  -  ''&gt;''

- '''''  -  ''&apos;''

- ''"''  -  ''&quot;''

==== CDATA Section ===

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

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

==== XML Namespaces ====

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: 

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

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

<sxh xml>
<?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>
</sxh>

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 ====

**XML interpreters** process XML documents by performing **syntactic validation**.

=== Definition ===
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!

=== Example ===

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

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

=== Example ===

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

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

Here’s the correct version:

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

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.