Ant: Problem: failed to create task or type propertyregex

I'm using Ant 1.8.1. I have downloaded ant-contrib-1.0b3.jar and placed it in my $ANT_HOME/lib directory. However, when I include this in my build.xml file ...

<propertyregex property="selenium.email.success.subject"
          input="package.ABC.name"
          regexp="(.*)__ENV__(.*)"
          replace="\1${buildtarget}\2"
          override="true"
          casesensitive="false" />

I get the error "Problem: failed to create task or type propertyregex. Cause: The name is undefined." upon running my Ant build file. What else do I need to do to get this task recognized?