I don't know if this is possible, but here's what I need to do:

Find the ancestor element named PEDiv whose ID attribute is only 2 digits and begins with the letter 'a'.

What would the syntax for this be? I know how to find the string length of a specific element's ID

string-length(//PETOC/PEDiv[1]/PEDiv[2]/@id) --this returns 2

but not how to find an element whose ID is a specific length.

ancestor::PEDiv[string-length(@ID) = 2 and substring(@ID, 1, 1) = 'a']

easy when you know how!

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference