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CTYPE(3) Library Functions Manual CTYPE(3)

isalpha, isupper, islower, isdigit, isalnum, isspace, ispunct, isprint, iscntrl, isascii - character classification

#include <ctype.h>

isalpha(c)

. . .

These macros classify ASCII-coded integer values by table lookup. Each is a predicate returning nonzero for true, zero for false. Isascii is defined on all integer values; the rest are defined only where isascii is true and on the single non-ASCII value EOF (see stdio(3)).

c is a letter
c is an upper case letter
c is a lower case letter
c is a digit
c is an alphanumeric character
c is a space, tab, carriage return, newline, or formfeed
c is a punctuation character (neither control nor alphanumeric)
c is a printing character, code 040(8) (space) through 0176 (tilde)
c is a delete character (0177) or ordinary control character (less than 040).
c is an ASCII character, code less than 0200

ascii(7)

UNIX-7