One way is to use a criterion like:

(>="A*" And <="Z*") Or (>="a*" And <="z*")
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/information-technology/13350-filter-based-on-first-character-field.html

if you want both upper and lower case, or just the first part if you
only want upper case.

HTH,
Nikos