Syntax rules of XML

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

Because of this, creating software that can read and manipulate XML is very easy to do.

An example XML document

XML documents use a self-describing and simple syntax.

The first line in the document - the XML declaration - defines the XML version and the character encoding used in the document. In this case the document conforms to the 1.0 specification of XML and uses the ISO-8859-1 (Latin-1/West European) character set.

The next line describes the root element of the document (like it was saying: "this document is a note"):

The next 4 lines describe 4 child elements of the root (to, from, heading, and body):

And finally the last line defines the end of the root element:

Can you detect from this example that the XML document contains a Note to John from Jany? Don't you agree that XML is pretty self-descriptive?

All XML elements must have a closing tag.

With XML, it is illegal to omit the closing tag.

In HTML some elements do not have to have a closing tag. The following code is legal in HTML:

In XML all elements must have a closing tag, like this:

Remember: 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. It is not an XML element, and it should not have a closing tag.

XML tags are case sensitive.

Unlike HTML, XML tags are case sensitive.

With XML, the tag <Letter> is different from the tag <letter>.

Opening and closing tags must therefore be written with the same case:

All XML elements must be properly nested

Improper nesting of tags makes no sense to XML.

In HTML some elements can be improperly nested within each other like this:

In XML all elements must be properly nested within each other like this:

All XML documents must have a root element.

All XML documents must contain a single tag pair to define a root element.

All other elements must be within this root element.

All elements can have sub elements (child elements). Sub elements must be correctly nested within their parent element:

Attribute values must always be quoted.

With XML, it is illegal to omit quotation marks around attribute values. 

XML elements can have attributes in name/value pairs just like in HTML. In XML the attribute value must always be quoted. See the two XML documents below. The first one is incorrect, the second is correct:

The error in the first document is that the date attribute in the note element is not quoted.

With XML, white space is preserved.

With XML, the white space in your document is not truncated.

This is unlike HTML. With HTML, a sentence like this:

will be displayed like this:

because HTML reduces multiple, consecutive white space characters to a single white space.

With XML, CR/LF is converted to LF.

With XML, a new line is always stored as LF.

Do you know what a typewriter is? Well, a typewriter is a mechanical device which was used last century to produce printed documents. :-)

After you have typed one line of text on a typewriter, you have to manually return the printing carriage to the left margin position and manually feed the paper up one line.

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.

Comments in XML

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

There is nothing special about XML.

There is nothing special about XML. It is just plain text with the addition of some XML tags enclosed in angle brackets.

Software that can handle plain text can also handle XML. In a simple text editor, the XML tags will be visible and will not be handled specially.

In an XML-aware application however, the XML tags can be handled specially. The tags may or may not be visible, or have a functional meaning, depending on the nature of the application.