1. The XML Language

2. Overview XML is a very simple language
The entire specification is only about 35 printed pages
This includes the specification for DTDs

3. The Golden Rule
XML IS CASE SENSITIVE

4. The XML Document At the heart of XML is the XML document
An XML document is a logical entity rather than a physical one
One logical document can be stored in different physical files locally and on the Web
A well-formed subset of an XML document is called a document fragment
An XML document contains markup (tags) and data

5. The XML Document (Contents)
Elements
Entity declarations and entity references
Processing instructions
Unparsed character data (CDATA)
Comments

6. Elements (Introduction)
XML tags resemble HTML tags but they are not predefined
That is, you give names to tags just as you give names to variables and other identifiers
Starting tags appear as <tag>
Ending tags appear as </tag>
Empty elements (elements without data) can appear as <tag />
XML elements MUST have an ending tag
A starting and ending tag (along with any text) makes up an element

7. Elements (Content) An element can have different types of content
If an element contains other (nested) elements, it has element content
If an element contains text and other nested elements, the element has mixed content
If an element contains only data, then the element has simple content
Elements can also have 0, 1, or many attributes

8. Elements (Simple Content)
<?xml version="1.0" encoding="utf-8" ?>
<!-- What is the answer? -->
<universe>
<question/>
<answer>
42 <!-- It's >
</answer>
</universe>
<answer> has simple content (data and no child elements)

9. Elements (Mixed Content)
<?xml version="1.0" encoding="utf-8" ?>
<!-- What is the answer? -->
<universe>
<question/>
<answer>
42 <!-- It's >
</answer>
</universe>
<universe> has mixed content (nested elements and data)

10. Elements (Nesting) Elements can be nested
These are hierarchical nodes
Elements MUST be nested correctly though
A child element must completely reside in its parent element
There must be exactly one and only one root element

11. Elements (Nesting – Example)
<?xml version="1.0" encoding="utf-8" ?>
<!-- What is the answer? -->
<universe>
<answer>
42 <!-- It's >
</answer>
</universe>
Root element is <universe>
Nested element is <answer>

12. Illegal Element Nesting (Example)
The following nesting is illegal:
<?xml version="1.0" encoding="utf-8" ?>
<!-- What is the answer? -->
<universe>
<answer>
42 <!-- It's >
</universe>
</answer>

13. (The Document Prolog – Example)
The following is an XML prolog declaration
The XML version is 1.0
The character encoding scheme is “UTF-8”
The document does not reference any external documents
While the following looks like a tag with three attributes, it is not
<?xml version="1.0" encoding=“UTF-8" standalone="yes"?>

14. XML Attributes (Example)
In the following example, id and size are attributes of the <universe> element
<?xml version="1.0" encoding="utf-8" ?>
<universe id="1" size="Infinite">
<answer>42</answer>
</universe>

15. Comparing Attributes and Elements
The following 2 documents are (roughly) equivalent
<?xml version="1.0" encoding="utf-8" ?>
<student id="12345">
<name>Bill</name>
</student>
<?xml version="1.0" encoding="utf-8" ?>
<student>
<id>12345</id>
<name>Bill</name>
</student>

16. XML Comments (Example)
The following document contains comments
<?xml version="1.0" encoding="utf-8" ?>
<!-- What is the answer? -->
<universe>
<answer>
42 <!-- It's >
</answer>
</universe>

17. The XML Document Tree
Well-formed XML documents are represented as a tree-based structure
For any given document, there exists exactly one document tree
The same genealogical terms are used to describe XML trees as other trees

18. Entities (Introduction)
Entities are part of the XML Recommendation
Think of entities as macro expanders placeholders
The XML specification uses the term storage unit to describe an entity
I think of entities as a
“macro expander” or global search and replace
way of importing files

19. Entities (Classification)
Parsed
Unparsed
General
Parameter
External
Internal
External
Internal
External
Named
Numbered
Predefined

20. Predefined Entities (Internal Parsed)
Markup characters such as “<“ or “>” must sometimes be treated as literal data
That is, these ‘special’ characters should not be parsed
For this, XML supplies the following predefined entities
& &
< <
> >
&apos; ‘
" “