A Processing Instruction (PI) is an

SGML The Standard Generalized Markup Language (SGML; ISO 8879:1986) is a standard for defining generalized markup languages for documents. ISO 8879 Annex A.1 states that generalized markup is "based on two postulates": * Declarative: Markup should ...

and

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...

node type, which may occur anywhere in the document, intended to carry instructions to the application. Processing instructions are exposed in the

Document Object Model The Document Object Model (DOM) is a cross-platform and language-independent interface that treats an XML or HTML document as a tree structure wherein each node is an object representing a part of the document. The DOM represents a document wi ...

as Node.PROCESSING_INSTRUCTION_NODE, and they can be used in

XPath XPath (XML Path Language) is an expression language designed to support the query or transformation of XML documents. It was defined by the World Wide Web Consortium (W3C) and can be used to compute values (e.g., strings, numbers, or Boolean v ...

and

XQuery XQuery (XML Query) is a query and functional programming language that queries and transforms collections of structured and unstructured data, usually in the form of XML, text and with vendor-specific extensions for other data formats (JSON, b ...

with the 'processing-instruction()' command.

Syntax

An SGML processing instruction is enclosed within

 and >.

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 ?>.


The  XML Declaration at the beginning of an XML document (shown below) is not a processing instruction, however its similar syntax has often resulted in it being referred to as a processing instruction.


  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.  It can be used for both XSLT 
XSLT (Extensible Stylesheet Language Transformations) is a language originally designed for  transforming XML documents into other XML documents, or other formats such as HTML for  web pages,  plain text or  XSL Formatting Objects, which may subseq ...
 and CSS 









Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS is a cornerstone technolo ...
 stylesheets.



The DocBook 

DocBook is a semantic markup language for technical documentation. It was originally intended for writing technical documents related to computer hardware and software, but it can be used for any other sort of documentation.

As a semantic languag ...
 XSLT stylesheets understand a number of processing instructions to override the default behaviour.

A draft specification for Robots exclusion standard 




The robots exclusion standard, also known as the robots exclusion protocol or simply robots.txt, is a standard used by websites to indicate to visiting web crawlers and other  web robots which portions of the site they are allowed to visit.

Th ...
 rules inside XML documents uses processing instructions.

  References 

{{reflist

  External links 


XML specification section: Processing Instructions


 Dave Pawson

xslt:processing-instruction
 Mozilla

 XML