Part 3 - Syntax | XML

The syntax rules of XML are very simple and logical. The rules are easy to learn, and easy to use.

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  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.

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  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.

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  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.

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  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>

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  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.

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  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.

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  Comments in XML

The syntax for writing comments in XML is similar to that of HTML.
<!-- This is a comment -->

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  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.

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  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 use only a CR character to store a new line.