[Zope] Field index problem

Hi all, I'm using a Field Index to index the "lastname" attribute of some content types. I would like to be able to search on those content types for lastnames which start with 'b' for instance. But I can't do a truncated search (B*) for a field index. Is there any way I can do that. TextIndex doesn't suit my needs as a lastname can contain several words. So B* would also match "Van Bil" . Any ideas on how I can achieve this ? -- Jean