WSO2 ESB - How to check whether a string is encoded or not?

The following sample demonstrate how to encode a string in WSO2 ESB and how to check whether a string is encoded or not.

<property name="string" value="abcd"/>
<property name="encodedString" expression="base64Encode('abcd')"/>
<log level="custom">
  <property name="encodedString" expression="get-property('encodedString')"/>
</log>
<filter source="get-property('encodedString')"
     regex="^([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{4}|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)$">
  <then>
     <log level="custom">
        <property name="encoded?" value="yes"/>
     </log>
  </then>
  <else>
     <log level="custom">
        <property name="encoded?" value="no"/>
     </log>
  </else>
</filter>

This is based on the regular expression given here to check whether a string is encoded or not.