.XML

Introduction

XML is a text format for documents and data made of nested, named elements. The W3C published XML 1.0 in 1998 as a simpler form of SGML. XML itself only sets the syntax; each use defines its own elements, often with a schema that a validator can check.

Many other formats are XML underneath, including SVG images, GPX tracks, RSS feeds and the parts inside a Word document. A file saved with the plain .xml extension is usually data or settings for a particular program.

Software

• Visual Studio Code (Windows, macOS, Linux)
• Notepad++ (Windows)
• Oxygen XML Editor (Windows, macOS, Linux)
• Firefox and other web browsers, which show XML as a tree
• libxml2 and its xmllint tool (Windows, macOS, Linux)
• Microsoft Excel, which can import XML data as a table

Structure

An XML file may open with a declaration giving the version and character encoding, <?xml version="1.0" encoding="UTF-8"?>. Without one the encoding is UTF-8, or UTF-16 when the file starts with a byte order mark.

The content is a single root element. An element is a start tag and an end tag around its content, <name>…</name>, or one empty tag, <name/>. Start tags carry attributes as key="value" pairs. Elements nest strictly, with no overlap. Text may use entity references such as & and <, and may include comments, processing instructions and CDATA sections. Namespaces let one file mix the elements of several vocabularies.