> I would like to check the that the $pattern argument to a 
> regular expression in fact follows regular expression rules, 
> since regex is not castable I guess the only way to do this 
> would be to use regular expressions and perhaps a for-each to 
> check that the $pattern argument conforms?

Short of writing a regular expression that only matches valid regular
expressions (which sounds like an interesting exercise for the reader), I
think you'll have to resort to extensions for this one.

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