If you have advanced, you can use a custom function to validate to make things easier. Otherwise, you would have to write a lengthy calc to try and validate.
Another option since you are comfortable with RegEx is http://jensteich.de/regex-plugin/
Thanks for the plugin, I will try it next week. Of course we have FileMaker Advanced! I don't leave home without it :-)
(as soon as you write something interesting for FM, you need the Advanced)