Thanks for your reply. I'm sorry I didn't explain everything in my code, because it's somewhat complicated (e.g. the parentheses for the regexp are in the XML, etc).

I assure you that I debugged the script 'till blood came out of my ears, so trust me when I say 'this works fine'.

The solution I finally found is below, but thanks for making me aware of some new unknown (to me) ways of making a localizer's life harder... ;-)