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What is XML (Extensible Markup Language)? – Complete Guide

XML document structure showing nested elements and attributes
Visual representation of an XML document with structured tags and elements

The Extensible Markup Language (XML) is a universal format for structured documents and data on the web. Developed by the World Wide Web Consortium (W3C), XML allows developers to describe, store, and transport data in a platform-independent and flexible way. Unlike HTML, XML focuses on what data is rather than how it looks.

What is XML?

XML, or Extensible Markup Language, is a markup language that provides a standard way to structure, store, and transport data. It allows developers to create their own custom tags to describe the data meaningfully. While HTML is used for displaying web pages, XML is used to carry data in a structured and organized manner.

In XML:

  • Tags are user-defined to describe the content.
  • It does not affect the display of content but defines its structure.
  • It ensures easy data exchange between applications and systems.

XML vs HTML

Feature HTML XML
Purpose Display data Store and transport data
Tags Predefined tags like <p>, <h1> User-defined tags
Case Sensitivity Not case-sensitive Case-sensitive
Structure Flexible, not strictly nested Strictly nested and well-formed
Validation Limited Validated using DTD or XML Schema

Markup Language and Its Role in XML

A markup language is a set of rules that defines how text and data should be structured and interpreted. XML is a markup language that allows developers to assign meaning to data using tags. Unlike programming languages, markup languages do not perform actions; they provide structure and meaning.

Examples of markup languages:

XML Elements and Tags

XML elements are defined by tags that have a clear structure. Here are the main parts:

  • <tag> – Start tag
  • </tag> – End tag
  • <tag /> – Self-closing tag
  • Element name – Defines the type of data

Example:

<book>

    <title>Introduction to XML</title>

    <author>John Doe</author>

</book>

Evolution of XML

XML was developed to improve on the limitations of HTML and SGML. HTML focuses on presentation, which often loses important data information. XML preserves data meaning and ensures interoperability between different systems. Over time, XML has become the backbone of data exchange for web services, RSS feeds, and modern applications.

Some XML-based standards in use today:

  • Channel Definition Format (CDF) – Syndication of web content
  • Open Trading Protocol (OTP) – Electronic commerce
  • Mathematical Markup Language (MathML) – Representing mathematical equations

Rules for Writing XML

XML has strict rules to ensure documents are well-formed and valid:

  • XML tags are case-sensitive. <Book> is different from <book>.
  • Every start tag must have a corresponding end tag.
  • Elements must be properly nested.
  • Attributes must be quoted, e.g., <book id="101">.
  • There must be a single root element in the document.

Advantages of XML

  • Platform-independent and can run on any operating system.
  • Customizable tags allow developers to define data meaning.
  • Improves interoperability between systems.
  • Supports data validation using DTD or XML Schema.
  • Separates data from presentation, making it flexible for applications.

XML Parsers

XML documents are processed using parsers. There are two main types:

  • Non-validating parser: Checks only if the document is well-formed.
  • Validating parser: Checks both well-formedness and validity against a DTD or XML Schema.

Conclusion

XML is a flexible, powerful markup language designed to structure and transport data on the web. By creating custom tags and following simple rules, developers can preserve information, ensure interoperability, and improve data exchange. XML remains a critical technology for web services, applications, and data-driven platforms.

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