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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.

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.

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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. 978-0974152134 ↩

  2. Comparison of SGML and XML; World Wide Web Consortium Note, 15 December 1997 http://www.w3.org/TR/NOTE-sgml-xml-971215 ↩

  3. Bryan, Martin (1997). SGML and HTML Explained. Addison Wesley Longman. ISBN 0-201-40394-3. Retrieved 2010-08-18. 0-201-40394-3 ↩

  4. Hossein Bidgoli (2004). The Internet encyclopedia, Volume 3. John Wiley and Sons. p. 877. ISBN 0-471-22203-8. 0-471-22203-8 ↩

  5. "XML 1.0 - slide "Processing Instructions (PIs)"". www.w3.org. Retrieved 2023-02-08. https://www.w3.org/Consortium/Offices/Presentations/XML1.0/slide30-0.htm ↩

  6. Elliotte Rusty Harold, W. Scott Means (2004). XML in a nutshell. p. 23. ISBN 978-0-596-00764-5. 978-0-596-00764-5 ↩

  7. "Associating Style Sheets with XML documents 1.0 (Second Edition)". http://www.w3.org/TR/xml-stylesheet/ ↩

  8. "Part 2. FO Processing Instruction Reference". http://docbook.sourceforge.net/release/xsl/current/doc/pi/pi-fo.html ↩

  9. "Robots Processing Instruction Homepage". Archived from the original on 2010-09-21. Retrieved 2010-08-18. https://web.archive.org/web/20100921021126/http://www.atrus.org/writings/technical/robots_pi/ ↩