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ISALPHA(3) Library Functions Manual ISALPHA(3)

isalpha
alphabetic character test

#include <ctype.h>
int
isalpha(int c);

The isalpha() function tests for any character for which isupper(3) or islower(3) is true and for which none of iscntrl(3), isdigit(3), ispunct(3), or isspace(3) is true. In the C locale, isalpha() returns true only for the characters for which isupper(3) or islower(3) is true.

The isalpha() function returns zero if the character tests false or non-zero if the character tests true.

isalnum(3), isascii(3), isblank(3), iscntrl(3), isdigit(3), isgraph(3), islower(3), isprint(3), ispunct(3), isspace(3), isupper(3), isxdigit(3), stdio(3), toascii(3), tolower(3), toupper(3), ascii(7)

The isalpha() function conforms to ANSI X3.159-1989 (“ANSI C89”).

The isalpha() function first appeared in Version 7 AT&T UNIX.

The argument to isalpha() must be EOF or representable as an unsigned char; otherwise, the result is undefined.
July 18, 2013 OpenBSD-6.1