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BSD 2.11 - man page for ctype (bsd section 3)

CTYPE(3)						     Library Functions Manual							  CTYPE(3)

isalpha, isupper, islower, isdigit, isxdigit, isalnum, isspace, ispunct, isprint, isgraph, iscntrl, isascii, toupper, tolower, toascii - character classification macros
#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 and toascii are defined on all integer values; the rest are defined only where isascii is true and on the single non-ASCII value EOF (see stdio(3S)). isalpha c is a letter isupper c is an upper case letter islower c is a lower case letter isdigit c is a digit isxdigit c is a hex digit isalnum c is an alphanumeric character isspace c is a space, tab, carriage return, newline, vertical tab, or formfeed ispunct c is a punctuation character (neither control nor alphanumeric) isprint c is a printing character, code 040(8) (space) through 0176 (tilde) isgraph c is a printing character, similar to isprint except false for space. iscntrl c is a delete character(0177) or ordinary control character (less than 040). isascii c is an ASCII character, code less than 0200 tolower c is converted to lower case. Return value is undefined if not isupper(c). toupper c is converted to upper case. Return value is undefined if not islower(c). toascii c is converted to be a valid ascii character.
ascii(7) 7th Edition May 12, 1986 CTYPE(3)