Processing Instruction

A processing instruction (PI) is an SGML and XML node type, which may occur anywhere in a document, intended to carry instructions to the application.[1][2]

Processing instructions are exposed in the Document Object Model as Node.PROCESSING_INSTRUCTION_NODE, and they can be used in XPath and XQuery with the 'processing-instruction()' command.

Syntax

An SGML processing instruction is enclosed within <? and >.[3]

An XML processing instruction is enclosed within <? and ?>, and contains a target and optionally some content, which is the node value, that cannot contain the sequence ?>.[4]

<?PITarget PIContent?>

The XML Declaration at the beginning of an XML document (shown below) is another example of a processing instruction,[5] however it may not technically be considered one.[6]

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

Examples

The most common use of a processing instruction is to request the XML document be rendered using a stylesheet using the 'xml-stylesheet' target, which was standardized in 1999.[7] It can be used for both XSLT and CSS stylesheets.

<?xml-stylesheet type="text/xsl" href="style.xsl"?>

<?xml-stylesheet type="text/css" href="style.css"?>

The DocBook XSLT stylesheets understand a number of processing instructions to override the default behaviour.[8]

A draft specification for Robots exclusion standard rules inside XML documents uses processing instructions.[9]

References
  1. Stayton, Bob (September 2007). "Chapter 9. Customization methods §Processing instructions". DocBook XSL: The Complete Guide. Sagehill Enterprises. ISBN 978-0974152134.
  2. Comparison of SGML and XML; World Wide Web Consortium Note, 15 December 1997
  3. Bryan, Martin (1997). SGML and HTML Explained. Addison Wesley Longman. ISBN 0-201-40394-3. Retrieved 2010-08-18.
  4. Hossein Bidgoli (2004). The Internet encyclopedia, Volume 3. John Wiley and Sons. p. 877. ISBN 0-471-22203-8.
  5. "XML 1.0 - slide "Processing Instructions (PIs)"". www.w3.org. Retrieved 2023-02-08.
  6. Elliotte Rusty Harold, W. Scott Means (2004). XML in a nutshell. p. 23. ISBN 978-0-596-00764-5.
  7. "Associating Style Sheets with XML documents 1.0 (Second Edition)".
  8. "Part 2. FO Processing Instruction Reference".
  9. "Robots Processing Instruction Homepage". Archived from the original on 2010-09-21. Retrieved 2010-08-18.
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