Character Class Testing and Conversion

isalpha(c)

non-zero if c is alphabetic, 0 if not.
isdigit(c)

non-zero if c is a digit (0...9), 0 if not.
isspace(c)

non-zero if c is '','\t', or '\n', 0 if not
islower(c)

non-zero if c is lower case, 0 if not
isupper(c)

non-zero if c is upper case, 0 if not
tolower(c)

convert upper case c to lower case.
toupper(c)

convert lower case c to upper case