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What are XML Syntax Rules?
Ishola Wasiu    Jun 26, 2017 at 08:05 PM    1    750

Hello house!

Please I have made a lot of research of how to code in XML and found series of results. I need certainty of the rules that guide XML syntax. This will serve as a guide for me in my subsequent projects in which there will be need for XML code.

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1 Answer
Ishola Ayinla says...
Jun 26, 2017 at 09:00 PM

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

XML documents must contain one root element that is the parent of all other elements:

<root>
  <child>
    <subchild>.....</subchild>
  </child>
</root>

In the example below, <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. It does not have a closing tag. This is not an error. The prolog is not a part of the XML 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.

In HTML, some elements might work well, even with a missing closing tag:

<p>This is a paragraph.
<br>

In XML, it is illegal to omit the closing tag. All elements must have a closing tag:

<p>This is a paragraph.</p>
<br />

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 incorrect</message>
<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.

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 elements can have attributes in name/value pairs just like in HTML.

In XML, the attribute values must always be quoted.

INCORRECT:

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

CORRECT:

<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.

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 &lt; 1000</message>

There are 5 pre-defined entity references in XML:

&lt; < less than
&gt; > greater than
&amp; & ampersand 
&apos; ' apostrophe
&quot; " quotation mark

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

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

<!-- This is a comment -->

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

Not allowed:

<!-- This is a -- comment -->

Strange, but allowed:

<!-- This is a - - comment -->

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

XML: Hello           Tove
HTML: Hello Tove


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