Partitioning by letter question

Hi, I was wondering if this was possible. I'm trying to partition a
table, which is straightforward enough thanks to the great
documentation, but i have a question:

If I partition using something like a product_id for example and have
check constraints such as (id>=1000 and id<2000) then everything is fine
and the planner correctly uses the right subset of the tables. However I
would like to partition by the first letter and using something like
this substr(word,1,1)='a' is ignored by the planner. From reading the
docs I understand that complicated check constraints are ignored, but
this doesn't seem overly complicated.

Am i doing something wrong or is there another better way to do this

Thanks

John