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How to select a node referenced by an other node with XPath?
<root>
<purchact hhid="xxx">
  <purchase pgnr="41">
   <purchvalues field_name="Number" field_value="1"/>
  </purchase>
  <purchase pgnr="KEY9802">
   <purchvalues field_name="Number" field_value="2"/>
  </purchase>
 <purchase pgnr="9802c">
   <purchvalues field_name="Number" field_value="3"/>
  </purchase>
 </purchact>
</root>

root/purchact/purchase[@pgnr=concat(substring-after(@pgnr, "KEY"), "c")]

root/purchact/purchase[
    @pgnr = concat(
        substring-after(
            ../purchase[
                contains(@pgnr, 'KEY')
                ]/@pgnr,
            'KEY'
            )
        , 'c')
    ]

root/purchact/purchase[
   @pgnr[substring(.,string-length()) = 'c']
][
   concat(
      'KEY',
      substring-before(
         @pgnr,
         'c'
      )
   ) = ../purchase/@pgnr
]