Identity transform

Using XSLT

The most frequently cited example of the identity transform (for XSLT version 1.0) is the "copy.xsl" transform as expressed in XSLT. This transformation uses the xsl:copy command^[1] to perform the identity transformation:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="@*|node()">
    <xsl:copy>
      <xsl:apply-templates select="@*|node()"/>
    </xsl:copy>
  </xsl:template>
</xsl:stylesheet>

This template works by matching all attributes (@*) and other nodes (node()), copying each node matched, then applying the identity transformation to all attributes and child nodes of the context node. This recursively descends the element tree and outputs all structures in the same structure they were found in the original file, within the limitations of what information is considered significant in the XPath data model. Since node() matches text, processing instructions, root, and comments, as well as elements, all XML nodes are copied.

A more explicit version of the identity transform is:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="@*|*|processing-instruction()|comment()">
    <xsl:copy>
      <xsl:apply-templates select="*|@*|text()|processing-instruction()|comment()"/>
    </xsl:copy>
  </xsl:template>
</xsl:stylesheet>

This version is equivalent to the first, but explicitly enumerates the types of XML nodes that it will copy. Both versions copy data that is unnecessary for most XML usage (e.g., comments).

<xsl:stylesheet version="3.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:mode on-no-match="shallow-copy" />
</xsl:stylesheet>

<xsl:output method="xml" encoding="utf-8" cdata-section-elements="element-name-1 element-name-2"/>

Using XQuery

XQuery can define recursive functions. The following example XQuery function copies the input directly to the output without modification.

Wikibooks has a book on the topic of: XQuery/Filtering_Nodes

declare function local:copy($element as element()) {
  element {node-name($element)}
    {$element/@*,
     for $child in $element/node()
        return if ($child instance of element())
          then local:copy($child)
          else $child
    }
};

Wikibooks has a book on the topic of: XQuery/Typeswitch_Transformations

The same function can also be achieved using a typeswitch-style transform.

xquery version "1.0";

(: copy the input to the output without modification :)
declare function local:copy($input as item()*) as item()* {
for $node in $input
   return 
      typeswitch($node)
        case document-node()
           return
              document {
                local:copy($node/node())
              }
        case element()
           return
              element {name($node)} {

                (: output each attribute in this element :)
                for $att in $node/@*
                   return
                      attribute {name($att)} {$att}
                ,
                (: output all the sub-elements of this element recursively :)
                for $child in $node
                   return local:copy($child/node())

              }
        (: otherwise pass it through.  Used for text(), comments, and PIs :)
        default return $node
};

The typeswitch transform is sometime preferable since it can easily be modified by simply adding a case statement for any element that needs special processing.

Using XSLT

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <xsl:copy-of select="."/>
  </xsl:template>
</xsl:stylesheet>

Using XProc

<p:pipeline name="pipeline" xmlns:p="http://www.w3.org/ns/xproc">
  <p:identity/>
</p:pipeline>

Here one important note about the XProc identity, is that it can take either one document like this example or a sequence of document as input.

Remove named element transform

  <xsl:template match="@*|node()">
    <xsl:copy>
      <xsl:apply-templates select="@*|node()"/>
    </xsl:copy>
  </xsl:template>

  <!-- remove all social security numbers -->
  <xsl:template match="PersonSSNID"/>

 declare function local:copy-filter-elements($element as element(), 
   $element-name as xs:string*) as element() {
   element {node-name($element) }
             { $element/@*,
               for $child in $element/node()[not(name(.)=$element-name)]
                  return if ($child instance of element())
                    then local:copy-filter-elements($child,$element-name)
                    else $child
           }
 };

$filtered-output := local:copy-filter-elements($input, 'PersonSSNID')

<p:pipeline name="pipeline" xmlns:p="http://www.w3.org/ns/xproc">
  <p:identity/>
  <p:delete match="PersonSSNID"/>
</p:pipeline>