How to pull out a substring in Ant

I use scriptdef to create a javascript tag to substring, for exemple:

 <project>
  <scriptdef name="substring" language="javascript">
     <attribute name="text" />
     <attribute name="start" />
     <attribute name="end" />
     <attribute name="property" />
     <![CDATA[
       var text = attributes.get("text");
       var start = attributes.get("start");
       var end = attributes.get("end") || text.length();
       project.setProperty(attributes.get("property"), text.substring(start, end));
     ]]>
  </scriptdef>
  ........
  <target ...>
     <substring text="asdfasdfasdf" start="2" end="10" property="subtext" />
     <echo message="subtext = ${subtext}" />
  </target>
 </project>