Week 5 - Character Classes

# Test whether s[i] is a digit.
"0" <= s[i] and s[i] <= "9"

# Test whether s[i] is not a digit.
s[i] < "0" or "9" < s[i]

# Test whether s[i] is a lower-case letter.
"a" <= s[i] and s[i] <= "z"

# Test whether s[i] is an upper-case letter.
"A" <= s[i] and s[i] <= "Z"

# Test whether s[i] is a letter (upper or lower case).
("a" <= s[i] and s[i] <= "z") or ("A" <= s[i] and s[i] <= "Z")