Isalpha Isupper Islower Isdigit Isalnum Isspace Ispunct Isprint Iscntrl Isascii character classification #include <ctype.h> isalpha int c These functions use table look-up to classify characters according to their ascii value. The header file defines them as macros which means that they are implemented as fast, inline code rather than subroutines. Each results in non-zero for true or zero for false. The correct value is guaranteed for all integer values in isascii, but the result is unpredictable in the others if the argument is outside the range -1 to 127. The truth tested by each function is a follows: isalpha c is a letter isdigit c is a digit isupper c is an upper case letter islower c is a lower case letter isalnum c is a letter or a digit isspace c is a space, tab character, newline, carriage return or formfeed iscntrl c is a control character (0 to 32) or DEL (127) ispunct c is neither countrol nor alpha-numeric isprint c is printable (32 to 126) isascii c is in the range -1 to 127