Processing Instruction

Short description: Node type in SGML and XML

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.

## Contents

* 1 Syntax
* 2 Examples
* 3 References
* 4 External links

## 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. http://www.sagehill.net/docbookxsl/ProcessingInstructions.html.
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. https://archive.org/details/sgmlhtmlexplaine0000brya. Retrieved 2010-08-18.
4. ↑ Hossein Bidgoli (2004). The Internet encyclopedia, Volume 3. John Wiley and Sons. p. 877. ISBN 0-471-22203-8. https://archive.org/details/internetencyclop00bidg_359.
5. ↑ "XML 1.0 - slide "Processing Instructions (PIs)"". 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.
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". https://docbook.sourceforge.net/release/xsl/current/doc/pi/pi-fo.html.
9. ↑ "Robots Processing Instruction Homepage". Archived from the original on 2010-09-21. https://web.archive.org/web/20100921021126/http://www.atrus.org/writings/technical/robots_pi/. Retrieved 2010-08-18.

## External links

* XML specification section: Processing Instructions
* XSLT FAQ: Processing Instructions, Dave Pawson
* xsl:processing-instruction, Mozilla

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