How about using a couple of passes through BETWEEN() ?
off the top of my head,
i=1
nLen = len(cString)+1
cBadChars = ""
do while i < nLen
cChar = substr(cString,i,1)
nAsc = asc( cChar )
if !(between(nAsc,65,90)
cBadChars = cBadChars+cChar
else
if !(between(nAsc,97,122)
cBadChars = cBadChars+cChar
endif
endif
i = i+1
enddo
if !empty(cBadChars)
cNewString=strtran(cBadChars,cString,"")
endif