why the char comparison "if(c>='a' && c<='z')" is not portable? -

some 1 told me char comparison if(c>='a' && c<='z') not portable.

example

int main() {     char c;     scanf("%c", &c);     if(c>='a' && c<='z')         printf("lower case\n"); } 

any proof char comparison if(c>='a' && c<='z') not portable?

the c standard not guarantee lower case letters appear between 'a' , 'z' in execution character set.

in ebcdic encoding, there other characters between 'a' , 'z', , c implementations use ebcdic.

the c standard guarantee digits consecutive, '0' <= d && d <= '9' test whether d decimal digit character.