XML

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XML (eXtensible Markup Language) 

A language for defining and representing languages. 

• HTML is a subset of XML.
• One key use is to define a data structure and data within the structure, useful for exchanging data between different applications and computer systems. 
• XML gives semantic meaning to data. For example, <Zip>47150</Zip> can mean the Zip code is 47150.
• Since XML is in text that is readable and writeable by humans, it is also computer architecture neutral, that is does not depend upon a specific bit-size representation of floating point numbers for example.
• Many database systems such as Oracle can generate entries from a database in XML form for use by other applications that understand XML without regard to the computer system word size, etc. 
• XML combined with XSL can separate implementation of the user interface from the data.

A simple example of XML use is to supply structured data to an application. Information for shipping a package from one Zip code to another can be specified in XML with a hierarchy tree as:
 

<package>    <To>47150</To>    <From>47165</From>    <Weight>17.0</Weight>    <Rate>27.50</Rate>   </package>

Basic XML Rules

• Hierarchical element structure - XML documents must have a strictly hierarchical tag structure. That is, start tags must have corresponding end tags. In XML vocabulary, a pair of start and end tags is called an element. Any element must be completely nested within another. 
• Well-formed - <To>47150</To> is well-formed because there is a <To> start tag and a </To> end tag.
• Case sensitive - XML is case sensitive. <to> 47150</To> is invalid.
• Empty - An empty element can be written as <To></To> or equivalently as <To/>.
• Text - A non-empty element can enclose other elements or text. <To>47150</To> encloses text of 47150.
• Attribute - <To Signature="Yes">47150</To> has one attribute, Signature with the value "Yes".

Parsing 

Many programming languages and applications such as the IE Web browser can parse XML to build a hierarchical tree that corresponds to the XML. For example the following defines XML for Employees. The parser reads the XML and constructs the corresponding tree. A program such as IE would use a parser to first construct the tree, then reference nodes on the tree to access each element of the XML.
 

<?xml version="1.0"?> 
 <Employees>
  <Employee>
    <First>James</First>   <Last>Borg</Last> 
  </Employee>
  <Employee>
    <First>Joyce</First>   <Last>English</Last> 
  </Employee>
 </Employees>