XML Syntax Rules

Guest · Post by **Guest** » Fri Oct 27, 2023 9:35 am

XML Syntax Rules

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

XML Documents Must Have a Root Element
XML documents must contain one root element that is the
parent
of all other elements:

<root>
<child>
<subchild>.....</subchild>
</child>
</root>
In this example <note> is the root element:

<?xml version="1.0" encoding="UTF-8"?>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

The XML Prolog
This line is called the XML prolog:

<?xml version="1.0" encoding="UTF-8"?>
The XML prolog is optional. If it exists, it must come first in the document.
XML documents can contain international characters, like Norwegian øæå or
French êèé.
To avoid errors, you should specify the encoding used, or save your XML files
as UTF-8.
UTF-8 is the default character encoding for XML documents.
Character encoding can be studied in our
Character Set Tutorial.

UTF-8 is also the default encoding for HTML5, CSS, JavaScript, PHP, and SQL.

All XML Elements Must Have a Closing Tag
In XML, it is illegal to omit the closing tag. All elements must have
a closing tag:

This is a paragraph.
 

Note: The XML prolog does not have a closing tag!
This is not an error. The prolog is not a part of the XML document.

XML Tags are Case Sensitive
XML tags are case sensitive. The tag <Letter> is different from the tag
<letter>.
Opening and closing tags must be written with the same case:

<message>This is correct</message>
"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:

This text is bold and italic
In XML, all elements must be properly nested within each other:

This text is bold and italic
In the example above, "Properly nested" simply means that since the 
element is opened inside the element, it must be closed inside the 
element.

XML Attribute Values Must Always be Quoted
XML elements can have attributes in name/value pairs just like in HTML.
In XML, the attribute values must always be quoted:

<note date="12/11/2007">
<to>Tove</to>
<from>Jani</from>
</note>

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>salary < 1000</message>
To avoid this error, replace the "<" character with an entity reference:

<message>salary < 1000</message>
There are 5 pre-defined entity references in XML:

<
<
less than

>
>
greater than

&
&
ampersand

'
'
apostrophe

"
"
quotation mark

Only < and & are strictly illegal in XML, but it is a good habit to replace >
with > as well.

Comments in XML
The syntax for writing comments in XML is similar to that of HTML:



Two dashes in the middle of a comment are not allowed:



White-space is Preserved in XML
XML does not truncate multiple white-spaces (HTML truncates multiple
white-spaces to one single white-space):

XML:

Hello Tove

HTML:

Hello Tove

XML Stores New Line as LF
Windows applications store a new line as: carriage return and line feed
(CR+LF).
Unix and Mac OSX use LF.
Old Mac systems use CR.
XML stores a new line as LF.

Well Formed XML
XML documents that conform to the syntax rules above are said to be "Well
Formed" XML documents.

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Reference: https://www.w3schools.com/xml/xml_syntax.asp