@ISALPHA[string]: Returns 1 if string is entirely composed of alphabetic (a-z, A-Z) characters; 0 otherwise.
See also: @ISALNUM, @ISASCII, @ISCNTRL, @ISDIGIT, @ISPRINT, @ISPUNCT, @ISSPACE, @ISXDIGIT.
Example:
echo %@isalpha[abc]
1
echo %@isalpha[ABC]
1
echo %@isalpha[A B C]
0