Including a mask within the field tag isn't enough, you'll also need to specify that you want the mask procedure to be run by adding 'mask' to the depends attribute. Struts Validator Reference (makes a great bookmark)
Make sure the mask to start with "^" sign and end with "$" sign. Its new requirement since Struts 1.1. Also, "*" before "$" sign (at the end) tells that the lower case letters can occur any number of times