XSLT deepening content structure

Question

given the following structure:

<TITEL1>...</TITEL1>  
<p>..</p>
<TITEL2>...</TITEL2>  
<TITEL3>...</TITEL3>
<TITEL3>...</TITEL3>  
<P>...<P>  

is there a way to get to this:

<TITEL1>
    <TITEL>...</TITEL>  
    <p>...</p>
    <TITEL2>
        <TITEL>...</TITEL>  
        <TITEL3>
            <TITEL>...</TITEL>
            <P>...</P>
        </TITEL3>
        <TITEL3>
            <TITEL>...</TITEL>
            <P>...</P>
        </TITEL3>
    </TITEL2>
</TITEL1>

or in other words,is there a way to have higher level titels inclose lower level titels and all content that follows them, thus creating a nested structure. The content of each TITEL1,2 and 3 tag should go into a new <TITEL>-element

Answer 1

With XSLT 2.0 (as implemented by Saxon 9 or AltovaXML tools) you can use xsl:for-each-group group-starting-with and a recursive function:

<xsl:stylesheet
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="2.0"
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns:mf="http://example.com/2010/mf"
  exclude-result-prefixes="xsd mf">

  <xsl:output indent="yes"/>

  <xsl:function name="mf:nest" as="element()*">
    <xsl:param name="elements" as="element()*"/>
    <xsl:param name="level" as="xsd:integer"/>
    <xsl:for-each-group select="$elements" group-starting-with="*[starts-with(local-name(), concat('TITEL', $level))]">
      <xsl:choose>
        <xsl:when test="self::*[starts-with(local-name(), concat('TITEL', $level))]">
          <xsl:element name="TITEL{$level}">
            <xsl:apply-templates select="."/>
            <xsl:sequence select="mf:nest(current-group() except ., $level + 1)"/>
          </xsl:element>
        </xsl:when>
        <xsl:otherwise>
          <xsl:apply-templates select="current-group()"/>
        </xsl:otherwise>
      </xsl:choose>
    </xsl:for-each-group>
  </xsl:function>

  <xsl:template match="ROOT">
    <xsl:sequence select="mf:nest(*, 1)"/>
  </xsl:template>

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

  <xsl:template match="*[starts-with(local-name(), 'TITEL')]">
    <TITEL>
      <xsl:apply-templates select="@* | node()"/>
    </TITEL>
  </xsl:template>

</xsl:stylesheet>

With that stylesheet the input

<ROOT>
<TITEL1>Titel 1, 1</TITEL1>  
<p>..</p>
<TITEL2>Titel 2, 1</TITEL2>  
<TITEL3>Titel 3, 1</TITEL3>
<TITEL3>Titel 3, 2</TITEL3>  
<P>...</P>
</ROOT>

is transformed to the output

<TITEL1>
   <TITEL>Titel 1, 1</TITEL>
   <p>..</p>
   <TITEL2>
      <TITEL>Titel 2, 1</TITEL>
      <TITEL3>
         <TITEL>Titel 3, 1</TITEL>
      </TITEL3>
      <TITEL3>
         <TITEL>Titel 3, 2</TITEL>
         <P>...</P>
      </TITEL3>
   </TITEL2>
</TITEL1>