XML Syntax Rules
The syntax rules of XML are very simple and logical. The rules are easy to learn, and easy to use.
All XML Elements Must Have a Closing Tag
In HTML, you will often see elements that don't have a closing tag:
<p>This is a paragraph
<p>This is another paragraph |
In XML, it is illegal to omit the closing tag. All elements must have a closing tag:
<p>This is a paragraph</p>
<p>This is another paragraph</p> |
Note: You might have noticed from the previous example that the XML declaration
did not have a closing tag. This is not an error. The declaration is not a part of the
XML document itself, and it has no closing tag.
XML Tags are Case Sensitive
XML elements are defined using XML tags.
XML tags are case sensitive. With XML, the tag <Letter> is different from the tag <letter>.
Opening and closing tags must be written with the same case:
<Message>This is incorrect</message>
<message>This is correct</message> |
Note: "Opening and closing tags" are often referred to as "Start and
end tags". Use whatever you prefer. It is exactly the same thing.
XML Elements Must be Properly Nested
In HTML, you might see improperly nested elements:
| <b><i>This text is bold and italic</b></i> |
In XML, all elements must be properly nested within each other:
| <b><i>This text is bold and italic</i></b> |
In the example above, "Properly nested" simply means that since the <i>
element is opened inside the <b> element, it must be closed inside the <b> element.
XML Documents Must Have a Root Element
XML documents must contain one element that is the parent of all other
elements. This element is called the root element.
<root>
<child>
<subchild>.....</subchild>
</child>
</root> |
XML Attribute Values Must be Quoted
XML elements can have attributes in name/value pairs just like in HTML.
In XML the attribute value must always be quoted. Study the two XML documents below.
The first one is incorrect, the second is correct:
<note date=12/11/2007>
<to>Tove</to>
<from>Jani</from>
</note> |
<note date="12/11/2007">
<to>Tove</to>
<from>Jani</from>
</note> |
The error in the first document is that the date attribute in the note element is not quoted.
Entity References
Some characters have a special meaning in XML.
If you place a character like "<" inside an XML element, it will generate an error because the parser interprets it as the
start of a new element.
This will generate an XML error:
| <message>if salary < 1000 then</message> |
To avoid this error, replace the "<" character with an entity reference:
| <message>if salary < 1000 then</message> |
There are 5 predefined entity references in XML:
| < |
< |
less than |
| > |
> |
greater than |
| & |
& |
ampersand |
| ' |
' |
apostrophe |
| " |
" |
quotation mark |
Note: Only the characters "<" and "&" are strictly illegal in XML. The greater than character is
legal, but it is a good habit to replace it.
Comments in XML
The syntax for writing comments in XML is similar to that of HTML.
<!-- This is a comment -->
White-space is Preserved in XML
HTML truncates multiple white-space characters to one single white-space:
|
HTML:
|
Hello my name is Tove
|
|
Output:
|
Hello my name is Tove. |
With XML, the white-space in a document is not truncated.
XML Stores New Line as LF
In Windows applications, a new line is normally stored as a pair of
characters: carriage return (CR) and line feed (LF). The character pair bears
some resemblance to the typewriter actions of setting a new line. In Unix
applications, a new line is normally stored as a LF character. Macintosh
applications also use an LF to store a new line.
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