How to split delimited string to XML node Tree
heading1_sub1_element1 = data1
heading1_sub1_element2 = data2
heading1_sub1_element3 = data3
heading1_sub2_element1 = data4
heading1_sub2_element2 = data5
heading1_sub2_element3 = data6

<table>
    <heading1_sub1_element1>data1</heading1_sub1_element1>
    <heading1_sub1_element2>data2</heading1_sub1_element2>
    <heading1_sub1_element3>data3</heading1_sub1_element3>
    <heading1_sub2_element1>data4</heading1_sub2_element1>
    <heading1_sub2_element2>data5</heading1_sub2_element2>
    <heading1_sub2_element3>data6</heading1_sub2_element3>
</table>

<heading1>
    <sub1>
        <element1>Data1</element1>
        <element2>Data2</element2>
        <element3>Data3</element3>
    </sub1>
    <sub2>
        <element1>Data4</element1>
        <element2>Data5</element2>
        <element3>Data6</element3>
    </sub2>
</heading1>

<xsl:stylesheet version="2.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 xmlns:xs="http://www.w3.org/2001/XMLSchema"
 xmlns:my="my:my" exclude-result-prefixes="my xs">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>

 <xsl:variable name="vLines" select="tokenize(/*, 'r?n')[.]"/>

 <xsl:variable name="vPass1">
  <t>
   <xsl:apply-templates mode="pass1"/>
  </t>
 </xsl:variable>


 <xsl:template match="/*" mode="pass1">
  <xsl:for-each select="$vLines">
   <xsl:sequence select="my:makeTree(normalize-space(.))"/>
  </xsl:for-each>
 </xsl:template>

 <xsl:template match="/">
  <xsl:apply-templates select="$vPass1" mode="pass2"/>
 </xsl:template>

 <xsl:function name="my:makeTree">
  <xsl:param name="pLine"/>

  <xsl:variable name="vName" select="substring-before($pLine, '_')"/>

  <xsl:choose>
    <xsl:when test="$vName">
      <xsl:element name="{$vName}">
        <xsl:sequence select="my:makeTree(substring-after($pLine, '_'))"/>
      </xsl:element>
    </xsl:when>
    <xsl:otherwise>
     <xsl:element name=
       "{normalize-space(substring-before($pLine, '='))}">
       <xsl:sequence select="substring-after($pLine, '=')"/>
     </xsl:element>
    </xsl:otherwise>
  </xsl:choose>
 </xsl:function>

 <xsl:function name="my:group">
  <xsl:param name="pNodes" as="node()*"/>

  <xsl:for-each-group select="$pNodes[self::*]" group-by="name()">
    <xsl:element name="{name()}">
      <xsl:for-each select="current-group()">
         <xsl:sequence select="my:group(node())"/>
      </xsl:for-each>
    </xsl:element>
  </xsl:for-each-group>
  <xsl:copy-of select="$pNodes[not(self::*)]"/>
 </xsl:function>

  <xsl:template match="*[not(my:path(.) = preceding::*/my:path(.))]" mode="pass2">
  <xsl:copy>
   <xsl:apply-templates select="//*[my:path(.) = my:path((current()))]/node()"
        mode="pass2"/>
  </xsl:copy>
 </xsl:template>

 <xsl:template match="*" mode="pass2"/>

 <xsl:template match="/*" mode="pass2" priority="3">
   <xsl:apply-templates mode="pass2"/>
 </xsl:template>

 <xsl:function name="my:path" as="xs:string">
  <xsl:param name="pElement" as="element()"/>

  <xsl:sequence select=
   "string-join($pElement/ancestor-or-self::*/name(.), '/')"/>
 </xsl:function>

</xsl:stylesheet>

<t>
    heading1_sub1_element1 = data1
    heading1_sub1_element2 = data2
    heading1_sub1_element3 = data3
    heading1_sub2_element1 = data4
    heading1_sub2_element2 = data5
    heading1_sub2_element3 = data6
</t>

<heading1>
   <sub1>
      <element1> data1</element1>
      <element2> data2</element2>
      <element3> data3</element3>
   </sub1>
   <sub2>
      <element1> data4</element1>
      <element2> data5</element2>
      <element3> data6</element3>
   </sub2>
</heading1>

<t>
    heading1_sub1_element1 = data1
    heading1_sub1_element2 = data2
    heading1_sub1_element3 = data3
    heading1_sub2_element1 = data4
    heading1_sub2_element2 = data5
    heading1_sub2_element3 = data6
    heading2_sub1_sub2_sub3 = data7
    heading2_sub1_sub2_sub3_sub4 = data8
    heading2_sub1_sub2 = data9
    heading2_sub1 = data10
    heading2_sub1_sub2_sub3 = data11
</t>

<heading1>
   <sub1>
      <element1> data1</element1>
      <element2> data2</element2>
      <element3> data3</element3>
   </sub1>
   <sub2>
      <element1> data4</element1>
      <element2> data5</element2>
      <element3> data6</element3>
   </sub2>
</heading1>
<heading2>
   <sub1>
      <sub2>
         <sub3>
            data7
            <sub4> data8</sub4>
            data11
         </sub3>
         data9
      </sub2>
      data10
  </sub1>
</heading2>

<t>
   <heading1>
      <sub1>
         <element1> data1</element1>
      </sub1>
   </heading1>
   <heading1>
      <sub1>
         <element2> data2</element2>
      </sub1>
   </heading1>
   <heading1>
      <sub1>
         <element3> data3</element3>
      </sub1>
   </heading1>
   <heading1>
      <sub2>
         <element1> data4</element1>
      </sub2>
   </heading1>
   <heading1>
      <sub2>
         <element2> data5</element2>
      </sub2>
   </heading1>
   <heading1>
      <sub2>
         <element3> data6</element3>
      </sub2>
   </heading1>
</t>

$vals = explode("_", $string_input);

$vals[0] = "heading1";
$vals[1] = "sub1";
$vals[2] = "element1";