JSON-Pointer over canonical XML representation of JSON (just for the fun of it)

rfc6901.xml

<json:map xmlns:json="http://www.w3.org/2005/xpath-functions">
   <json:array key="foo">
      <json:string>bar</json:string>
      <json:string>baz</json:string>
   </json:array>
   <json:number key="">0</json:number>
   <json:number key="a/b">1</json:number>
   <json:number key="c%d">2</json:number>
   <json:number key="e^f">3</json:number>
   <json:number key="g|h">4</json:number>
   <json:number key="i\j">5</json:number>
   <json:number key="k&#34;l">6</json:number>
   <json:number key=" ">7</json:number>
   <json:number key="m~n">8</json:number>
</json:map>

rfc6901.xsl

<xsl:transform version="3.0"
               xmlns:json="http://www.w3.org/2005/xpath-functions"
               xmlns:jptr="tag:maus@hab.de,2018:RFC6901"
               xmlns:xs="http://www.w3.org/2001/XMLSchema"
               xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <!-- 
       Returns the selected JSON information item or the empty sequence if the pointer does not denote
       an information item in the JSON document. Expects the pointer to be a sequence of reference
       tokens, i.e. the result of applying tokenize() on the pointer's string representation.
  -->
  <xsl:function name="jptr:evaluate" as="element()?">
    <xsl:param name="context" as="element()?"/>
    <xsl:param name="pointer" as="xs:string*"/>

    <xsl:variable name="token" select="jptr:unescape($pointer[1])"/>

    <xsl:choose>
      <xsl:when test="empty($context)"/>
      <xsl:when test="empty($pointer)">
        <xsl:sequence select="$context"/>
      </xsl:when>
      <xsl:when test="$token = ''">
        <xsl:sequence select="jptr:evaluate($context, $pointer[position() gt 1])"/>
      </xsl:when>
      <xsl:when test="$context/self::json:map">
        <xsl:sequence select="jptr:evaluate($context/json:*[@key = $token], $pointer[position() gt 1])"/>
      </xsl:when>
      <xsl:when test="$context/self::json:array and matches($token, '[0-9]+')">
        <xsl:sequence select="jptr:evaluate($context/json:*[position() eq (number($token) + 1)], $pointer[position() gt 1])"/>
      </xsl:when>
    </xsl:choose>
  </xsl:function>

  <xsl:function name="jptr:unescape" as="xs:string?">
    <xsl:param name="token" as="xs:string?"/>
    <xsl:value-of select="replace(replace($token, '~1', '/'), '~0', '~')"/>
  </xsl:function>

</xsl:transform>

rfc6901.xspec

<!DOCTYPE description [<!ENTITY example SYSTEM "rfc6901.xml">]>
<description xmlns="http://www.jenitennison.com/xslt/xspec"
             xmlns:xi="http://www.w3.org/2001/XInclude"
             xmlns:jptr="tag:maus@hab.de,2018:RFC6901"
             xmlns:json="http://www.w3.org/2005/xpath-functions"
             stylesheet="rfc6901.xsl">

  <scenario label="When jptr:unescape is called">
    <scenario label="with a pointer segment containing an encoded forward slash">
      <call function="jptr:unescape">
        <param name="token" as="xs:string">foo~1bar</param>
      </call>
      <expect label="the forward slash is unescaped" select="'foo/bar'"/>
    </scenario>
    <scenario label="with a pointer segment containing an encoded tilde">
      <call function="jptr:unescape">
        <param name="token" as="xs:string">foo~0bar</param>
      </call>
      <expect label="the tilde is unescaped" select="'foo~bar'"/>
    </scenario>
    <scenario label="with a pointer segment containing an encoded tilde followed by a one">
      <call function="jptr:unescape">
        <param name="token" as="xs:string">~01</param>
      </call>
      <expect label="only the tilde is unescaped" select="'~1'"/>
    </scenario>
  </scenario>

  <scenario label="When jptr:evaluate is called">
    <scenario label="with the empty string as pointer">
      <call function="jptr:evaluate">
        <param name="context">
          &example;
        </param>
        <param name="pointer"/>
      </call>
      <expect label="the entire document is returned">
        <json:map>...</json:map>
      </expect>
    </scenario>
    <scenario label="with a string denoting the member of a map">
      <call function="jptr:evaluate">
        <param name="context">
          &example;
        </param>
        <param name="pointer" as="xs:string+" select="tokenize('/foo', '/')"/>
      </call>
      <expect label="the member with the same name is returned">
        <json:array key="foo">...</json:array>
      </expect>
    </scenario>
    <scenario label="with a string not denoting the member of a map">
      <call function="jptr:evaluate">
        <param name="context">
          &example;
        </param>
        <param name="pointer" as="xs:string+" select="tokenize('/foo/bar', '/')"/>
      </call>
      <expect label="the empty sequence is returned"/>
    </scenario>
    <scenario label="with a numeric string denoting an array index">
      <call function="jptr:evaluate">
        <param name="context">
          &example;
        </param>
        <param name="pointer" as="xs:string+" select="tokenize('/foo/0', '/')"/>
      </call>
      <expect label="the array member at this position is returned">
        <json:string>bar</json:string>
      </expect>
    </scenario>
    <scenario label="with a numeric string not denoting an array index">
      <call function="jptr:evaluate">
        <param name="context">
          &example;
        </param>
        <param name="pointer" as="xs:string+" select="tokenize('/foo/3', '/')"/>
      </call>
      <expect label="the empty sequence is returned"/>
    </scenario>
  </scenario>

</description>