Castor does not have a (good) native Regex validator, you have to specify one in a config file. See http://castor.codehaus.org/conf-lib.html
Add a config file called castor.properties to the classpath containing the following:
org.exolab.castor.regexp=org.exolab.castor.util.JakartaRegExpEvaluator
You may have to add a dependency to Jakarta Regex to Maven as well:
<dependency>
<groupId>jakarta-regexp</groupId>
<artifactId>jakarta-regexp</artifactId>
<version>1.4</version>
</dependency>