isalnum, isalpha, isascii, isblank, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace, isupper, isxdigit, isalnum_l, isalpha_l, isascii_l, isblank_l, iscntrl_l, isdigit_l, isgraph_l, islower_l, isprint_l, ispunct_l, isspace_l, isupper_l, isxdigit_l - character classification functions
#include <ctype.h> int isalnum(int c); int isalpha(int c); int iscntrl(int c); int isdigit(int c); int isgraph(int c); int islower(int c); int isprint(int c); int ispunct(int c); int isspace(int c); int isupper(int c); int isxdigit(int c); int isascii(int c); int isblank(int c); int isalnum_l(int c, locale_t locale); int isalpha_l(int c, locale_t locale); int isblank_l(int c, locale_t locale); int iscntrl_l(int c, locale_t locale); int isdigit_l(int c, locale_t locale); int isgraph_l(int c, locale_t locale); int islower_l(int c, locale_t locale); int isprint_l(int c, locale_t locale); int ispunct_l(int c, locale_t locale); int isspace_l(int c, locale_t locale); int isupper_l(int c, locale_t locale); int isxdigit_l(int c, locale_t locale); int isascii_l(int c, locale_t locale);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_XOPEN_SOURCE || /* Glibc since 2.19: */ _DEFAULT_SOURCE || /* Glibc versions <= 2.19: */ _SVID_SOURCE
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
- Since glibc 2.10:
_XOPEN_SOURCE >= 700
- Before glibc 2.10:
- Since glibc 2.10:
_XOPEN_SOURCE >= 700 && (_SVID_SOURCE || _BSD_SOURCE)
- Before glibc 2.10:
These functions check whether
c, which must have the value of an
unsigned char or EOF, falls into a certain character class according to the specified locale. The functions without the "_l" suffix perform the check based on the current locale.
The functions with the "_l" suffix perform the check based on the locale specified by the locale object
locale. The behavior of these functions is undefined if
locale is the special locale object LC_GLOBAL_LOCALE (see duplocale(3)) or is not a valid locale object handle.
The list below explains the operation of the functions without the "_l" suffix; the functions with the "_l" suffix differ only in using the locale object
locale instead of the current locale.
checks for an alphabetic character; in the standard "C" locale, it is equivalent to (isupper(
c) || islower(
. In some locales, there may be additional characters for which isalpha() is true—letters which are neither uppercase nor lowercase.
c is a 7-bit
unsigned char value that fits into the ASCII character set.
checks for a blank character; that is, a space or a tab.
checks for a control character.
checks for a digit (0 through 9).
checks for any printable character except space.
checks for a lowercase character.
checks for any printable character including space.
checks for any printable character which is not a space or an alphanumeric character.
checks for white-space characters. In the "C" and "POSIX" locales, these are: space, form-feed ('\f'), newline ('\n'), carriage return ('\r'), horizontal tab ('\t'), and vertical tab ('\v').
checks for an uppercase letter.
checks for hexadecimal digits, that is, one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F.
The values returned are nonzero if the character
c falls into the tested class, and zero if not.
C89 specifies isalnum(), isalpha(), iscntrl(), isdigit(), isgraph(), islower(), isprint(), ispunct(), isspace(), isupper(), and isxdigit(), but not isascii() and isblank(). POSIX.1-2001 also specifies those functions, and also isascii() (as an XSI extension) and isblank(). C99 specifies all of the preceding functions, except isascii().
POSIX.1-2008 marks isascii() as obsolete, noting that it cannot be used portably in a localized application.
isascii_l() is a GNU extension.
The standards require that the argument
c for these functions is either EOF or a value that is representable in the type
unsigned char. If the argument
c is of type
char, it must be cast to
unsigned char, as in the following example:
char c; ... res = toupper((unsigned char) c);
This is necessary because
char may be the equivalent of
signed char, in which case a byte where the top bit is set would be sign extended when converting to
int, yielding a value that is outside the range of
The details of what characters belong to which class depend on the locale. For example, isupper() will not recognize an A-umlaut (Ä) as an uppercase letter in the default C locale.
This page is part of release 4.15 of the Linux
man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.