In the above mentioned way I have mapped the password field to required rule and mask rule. The required rule is applied to the password field but the mask rule is not applied when i click the submit button.
All regular expressions in the Struts mask rule must start with a ^ and end with a $. <var> <var-name>mask</var-name> <var-value>^[a-zA-Z0-9]*$</var-value> </var>
And make sure that you define a "errors.password.maskMsg" in your MessageResources.properties file e.g