Struts Validation framework to compare two dates

Question

I have a page where I enter Start Date and End Date. I need to validate that End Date should not be less than Start date for that I am trying to implement the same using Xwork Validator, but not Working. Below is the code

<?xml version="1.0" encoding="UTF-8"?>
        <!DOCTYPE validators PUBLIC "-//OpenSymphony Group//XWork Validator 1.0.2//EN"
        "http://www.opensymphony.com/xwork/xwork-validator-1.0.2.dtd"> 
           <validators>     
                 <field name="endDateString">
                    <field-validator type="requiredstring">
                       <param name="trim">true</param>
                       <message>Please enter ABC end date</message>
                     </field-validator>
                  </field>
                <field name="endDateString">
               <field-validator type="regex">
                   <param name="expression">
                           ^(0[1-9]|[12][0-9]|3[01])[- /.](0[1-9]|1[012])[- /.]  
                                (19|20)\d\d$</param>
                       <message>Please enter a valid end date (dd/mm/yyyy)</message>
                  </field-validator>
            </field>
                       <field name="endDateString">         
                                    <field-validator type="fieldexpression">
                           <param name="expression">
                                         <![CDATA[#endDateString > #startDateString]]>
                                        </param>            
                                           <message>ABC end date is before start date </message>             
                                     </field-validator>     
                         </field> 
           </validators>

First Two validation are working but not "fieldexpression" Please help me to resolve this

Using Struts : 2.0.11