Your logic is incorrect, you should be using &&, not ||
Also you are comparing against strings of the character values, whereas you should either compare against the characters themselves or against the character numeric values. It makes no sense to compare against strings of the character values.
So you should be doing either one of these:
a>=65 && a<=90
a>='A' && a<='Z'
By the way, here is a much simpler vectorized method to detect upper/lower case letters:
>> v = 'aBCde$';
>> isup = lower(v)~=v
isup =
0 1 1 0 0 0
>> islo = upper(v)~=v
islo =
1 0 0 1 1 0
>> isstrprop(v,'upper')
ans =
0 1 1 0 0 0
>> isstrprop(v,'lower')
ans =
1 0 0 1 1 0
