Character handling
Clasify characters as alpha, digit, hexdigit, whitespace, printable,
lowercase, uppercase, punctuation and to map to and from the upper
and lower case alpha character.

#include <ctype.h>

int isalpha(int c);
int isalnum(int c);
int isdigit(int c);
int isxdigit(int c);

int iscntrl(int c);
int isspace(int c);

int ispunct(int c);
int isgraph(int c);
int isprint(int c);

int islower(int c);
int isupper(int c);

int tolower(int c);
int toupper(int c);

isalpha returns true if the character is in the range of A-Z or a-z.

isalnum returns true if the character is in the range of A-Z or a-z or 0-9.

isdigit returns true if the character is in the range of 0-9.

isxdigit returns true if the character is in the range of 0-9 or a-f or A-F.

iscntrl returns true if the character is in the set (FF, NL, CR, HT, VT, BEL or BS).

isspace returns true if the character is in the set (space, FF, NL, CR, HT or VT).

ispunct returns true if the character is a nonalnum, nonspace and noncntrl.

isgraph returns true if the character isalnum or ispunct.

isprint returns true if the character isspace or isgraph.

islower returns true if the character is in the range of a-z.

isupper returns true if the character is in the range of A-Z.

tolower if isupper return the lowercase character otherwise return the character.

toupper if islower return the uppercase character otherwise return the character.
 

Bibilography:
man isspace
The ANSI C Programming Language, Second Edition, Brian W. Kernighan, Dennis M. Ritchie, Printice Hall Software Series, 1988
The Standard C Library, P. J. Plauger, Printice Hall P T R, 1992
The Standard C Library, Parts 1, 2, and 3, Chuck Allison, C/C++ Users Journal, January, February, March 1995