I've tried both solutions but still have the same problem. Cannot put them in the middle in sequence.
I don't know whether it is important or not: the field I'd like to be indexed looks like this: UE00000105 - there's an extra two character in front of the numeric ones. I think the expressions Cetin suggested should work this way as well.
Now I have tried a lot - including yours - but the problem remains.
Thanks anyway

Tamas