> fields[field[contains(@id,'Metadata')] and
field[contains(@id,'.subject.')]]

Or if you're one of those people with an aversion to nested square brackets,
you can write this as

fields[contains(field/@id, 'Metadata') and contains(field/@id, 'subject.')]

I wonder also if you're using contains() where you really want to test for
"=" - it's a common mistake.

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