XML - Syntax

In this chapter, we will discuss the simple syntax rules to write an XML document. Following is a complete XML document −

<?xml version = "1.0"?>
<contact-info>
   <name>Tanmay Patil</name>
   <company>Howcodex</company>
   <phone>(011) 123-4567</phone>
</contact-info>

You can notice there are two kinds of information in the above example −

• Markup, like <contact-info>

• The text, or the character data, Howcodex and (040) 123-4567.

The following diagram depicts the syntax rules to write different types of markup and text in an XML document.

Let us see each component of the above diagram in detail.

XML Declaration

The XML document can optionally have an XML declaration. It is written as follows −

<?xml version = "1.0" encoding = "UTF-8"?>

Where version is the XML version and encoding specifies the character encoding used in the document.

Syntax Rules for XML Declaration

• The XML declaration is case sensitive and must begin with "<?xml>" where "xml" is written in lower-case.

• If document contains XML declaration, then it strictly needs to be the first statement of the XML document.

• The XML declaration strictly needs be the first statement in the XML document.

• An HTTP protocol can override the value of encoding that you put in the XML declaration.

Tags and Elements

An XML file is structured by several XML-elements, also called XML-nodes or XML-tags. The names of XML-elements are enclosed in triangular brackets < > as shown below −

<element>

Syntax Rules for Tags and Elements

Element Syntax − Each XML-element needs to be closed either with start or with end elements as shown below −

<element>....</element>

or in simple-cases, just this way −

<element/>

Nesting of Elements − An XML-element can contain multiple XML-elements as its children, but the children elements must not overlap. i.e., an end tag of an element must have the same name as that of the most recent unmatched start tag.

The Following example shows incorrect nested tags −

<?xml version = "1.0"?>
<contact-info>
<company>Howcodex
</contact-info>
</company>

The Following example shows correct nested tags −

<?xml version = "1.0"?>
<contact-info>
   <company>Howcodex</company>
<contact-info>

Root Element − An XML document can have only one root element. For example, following is not a correct XML document, because both the x and y elements occur at the top level without a root element −

<x>...</x>
<y>...</y>

The Following example shows a correctly formed XML document −

<root>
   <x>...</x>
   <y>...</y>
</root>

Case Sensitivity − The names of XML-elements are case-sensitive. That means the name of the start and the end elements need to be exactly in the same case.

For example, <contact-info> is different from <Contact-Info>

XML Attributes

An attribute specifies a single property for the element, using a name/value pair. An XML-element can have one or more attributes. For example −

<a href = "http://www.howcodex.com/">Howcodex!</a>

Here href is the attribute name and http://www.howcodex.com/ is attribute value.

Syntax Rules for XML Attributes

• Attribute names in XML (unlike HTML) are case sensitive. That is, HREF and href are considered two different XML attributes.

• Same attribute cannot have two values in a syntax. The following example shows incorrect syntax because the attribute b is specified twice

  −

<a b = "x" c = "y" b = "z">....</a>

• Attribute names are defined without quotation marks, whereas attribute values must always appear in quotation marks. Following example demonstrates incorrect xml syntax

  −

<a b = x>....</a>

In the above syntax, the attribute value is not defined in quotation marks.

XML References

References usually allow you to add or include additional text or markup in an XML document. References always begin with the symbol "&" which is a reserved character and end with the symbol ";". XML has two types of references −

• Entity References − An entity reference contains a name between the start and the end delimiters. For example &amp; where amp is name. The name refers to a predefined string of text and/or markup.

• Character References − These contain references, such as &#65;, contains a hash mark (“#”) followed by a number. The number always refers to the Unicode code of a character. In this case, 65 refers to alphabet "A".

XML Text

The names of XML-elements and XML-attributes are case-sensitive, which means the name of start and end elements need to be written in the same case. To avoid character encoding problems, all XML files should be saved as Unicode UTF-8 or UTF-16 files.

Whitespace characters like blanks, tabs and line-breaks between XML-elements and between the XML-attributes will be ignored.

Some characters are reserved by the XML syntax itself. Hence, they cannot be used directly. To use them, some replacement-entities are used, which are listed below −