> Sweet!  That seems to work...  What is the "^\d+$" doing 
> exactly?  I've done a few searches and didn't come up with 
> anything...  where would its use be documented?

^ and $ anchor the match to the start and end of the string

\d matches any Unicode digit (not just 0-9 but non-Western digits too; for
0-9 you could use [0-9])

+ means one or more.

As far as the specs are concerned, you have to read 

http://www.w3.org/TR/xpath-functions/#regex-syntax

in conjunction with

http://www.w3.org/TR/xmlschema-2/#regexs

but if you want something more digestible, Chapter 14 of my XSLT 2.0
Reference covers it all.

Michael Kay
http://www.saxonica.com/