create table check constraint question

I would like to add check constraints to my create table statements, but I am not sure what all can go into this field. The reference manual shows

CHECK ( dname = upper(dname) )
CHECK ( deptno between 10 and 99 )
and
CHECK ( loc in ('DALLAS', 'BOSTON', 'NEW YORK') )

and what I would like to know is what else can go in here? What if I have the field defined as a char but want char 0-9 only... is there a way I can code a CHECK for that? What things have you done with check?

-- 
Larry W. Virden			UUCP: osu-cis!chemabs!lvirden
Same Mbox: BITNET: lwv26_at_cas    INET: lvirden_at_cas.org
Personal: 674 Falls Place,   Reynoldsburg,OH 43068-1614
America Online: lvirden