"Difficulties using Filter Tokens (by Region) operator"

I am using the text processing extension to extract information from patent files. If I use Tokenization and some other filters (like Stoppword - Filter) it works fine.
If I work with the Filter Tokens (by Region) operators I am getting zero results. The condition is: Contains "Klebstoff", no case sensitive. This expression appears many times in the readed documents. Interestingly, the program complains if I select the option contains that the regular expression must be specified. In my thought I need this regular expression only if I select the match condition. I am wrong here?

My idea is the automatic extraction from patentfiles content around a given subject. Any help is willcome, I am working on my master thesis.
For the test I have put in the same expression for the condition regular expression and search string. Without defining the regular expression the filter does not work.

 <process expanded="true" height="251" width="614">
      <operator activated="true" class="text:process_document_from_file" compatibility="5.2.004" expanded="true" height="76" name="Process Documents from Files" width="90" x="45" y="30">
        <list key="text_directories">
          <parameter key="B24" value="D:\Test_Information_Extraktion2\Deutsch"/>
        </list>
        <parameter key="file_pattern" value="*.pdf"/>
        <process expanded="true" height="466" width="882">
          <operator activated="true" class="text:tokenize" compatibility="5.2.004" expanded="true" height="60" name="Tokenize" width="90" x="45" y="30">
            <parameter key="language" value="German"/>
          </operator>
          <operator activated="true" class="text:transform_cases" compatibility="5.2.004" expanded="true" height="60" name="Transform Cases" width="90" x="179" y="30">
            <parameter key="transform_to" value="upper case"/>
          </operator>
          <operator activated="true" class="text:filter_tokens_by_regions" compatibility="5.2.004" expanded="true" height="60" name="Filter Tokens (by Region)" width="90" x="332" y="30">
            <parameter key="string" value="KLEBSTOFF"/>
            <parameter key="regular_expression" value="KLEBSTOFF"/>
          </operator>
          <connect from_port="document" to_op="Tokenize" to_port="document"/>
          <connect from_op="Tokenize" from_port="document" to_op="Transform Cases" to_port="document"/>
          <connect from_op="Transform Cases" from_port="document" to_op="Filter Tokens (by Region)" to_port="document"/>
          <connect from_op="Filter Tokens (by Region)" from_port="document" to_port="document 1"/>
          <portSpacing port="source_document" spacing="0"/>
          <portSpacing port="sink_document 1" spacing="90"/>
          <portSpacing port="sink_document 2" spacing="0"/>
        </process>
      </operator>
      <connect from_port="input 1" to_op="Process Documents from Files" to_port="word list"/>
      <connect from_op="Process Documents from Files" from_port="word list" to_port="result 1"/>
      <portSpacing port="source_input 1" spacing="0"/>
      <portSpacing port="source_input 2" spacing="0"/>
      <portSpacing port="sink_result 1" spacing="0"/>
      <portSpacing port="sink_result 2" spacing="0"/>
    </process>
  </operator>
</process>