isalnum

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< c ‎ | string ‎ | byte
C
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(C11) (C11)
Defined in header <ctype.h>
int isalnum ( int ch ) ;

Checks if the given character is an alphanumeric character as classified by the current C locale. In the default locale, the following characters are alphanumeric:

  • Digits ( 0123456789 ),
  • Uppercase letters ( ABCDEFGHIJKLMNOPQRSTUVWXYZ ),
  • Lowercase letters ( abcdefghijklmnopqrstuvwxyz ).

The behavior is undefined if the value of ch is not representable as unsigned char and is not equal to EOF .

Parameters

ch - character to classify

Return value

Non-zero value if the character is an alphanumeric character, ​ 0 ​ otherwise.

Example

Demonstrates the use of isalnum with different locales (OS-specific).

Run this code 
#include <ctype.h>
#include <locale.h>
#include <stdio.h>
 
int main(void)
{
    unsigned char c = '\xdf'; // German letter ß in ISO-8859-1
 
    printf("isalnum('\\xdf') in default C locale returned %d\n", !!isalnum(c));
 
    if (setlocale(LC_CTYPE, "de_DE.iso88591"))
        printf("isalnum('\\xdf') in ISO-8859-1 locale returned %d\n", !!isalnum(c));
}

Possible output: 

isalnum('\xdf') in default C locale returned 0
isalnum('\xdf') in ISO-8859-1 locale returned 1

References

  • C23 standard (ISO/IEC 9899:2024):
  • 7.4.1.1 The isalnum function (p: TBD)
  • C17 standard (ISO/IEC 9899:2018):
  • 7.4.1.1 The isalnum function (p: 145)
  • C11 standard (ISO/IEC 9899:2011):
  • 7.4.1.1 The isalnum function (p: 200)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.4.1.1 The isalnum function (p: 181)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 4.3.1.1 The isalnum function

See also

(C95)
checks if a wide character is alphanumeric
(function)
C++ documentation for isalnum
ASCII values characters

iscntrl
iswcntrl

isprint
iswprint

isspace
iswspace

isblank
iswblank

isgraph
iswgraph

ispunct
iswpunct

isalnum
iswalnum

isalpha
iswalpha

isupper
iswupper

islower
iswlower

isdigit
iswdigit

isxdigit
iswxdigit

decimal hexadecimal octal
0–8 \x0 – \x8 \0 – \10 control codes ( NUL , etc.) ≠0 0 0 0 0 0 0 0 0 0 0 0
9 \x9 \11 tab ( \t ) ≠0 0 ≠0 ≠0 0 0 0 0 0 0 0 0
10–13 \xA – \xD \12 – \15 whitespaces ( \n , \v , \f , \r ) ≠0 0 ≠0 0 0 0 0 0 0 0 0 0
14–31 \xE – \x1F \16 – \37 control codes ≠0 0 0 0 0 0 0 0 0 0 0 0
32 \x20 \40 space 0 ≠0 ≠0 ≠0 0 0 0 0 0 0 0 0
33–47 \x21 – \x2F \41 – \57 !"#$%&'()*+,-./ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0
48–57 \x30 – \x39 \60 – \71 0123456789 0 ≠0 0 0 ≠0 0 ≠0 0 0 0 ≠0 ≠0
58–64 \x3A – \x40 \72 – \100 :;<=>?@ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0
65–70 \x41 – \x46 \101 – \106 ABCDEF 0 ≠0 0 0 ≠0 0 ≠0 ≠0 ≠0 0 0 ≠0
71–90 \x47 – \x5A \107 – \132 GHIJKLMNOP
QRSTUVWXYZ 		0 ≠0 0 0 ≠0 0 ≠0 ≠0 ≠0 0 0 0
91–96 \x5B – \x60 \133 – \140 [\]^_` 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0
97–102 \x61 – \x66 \141 – \146 abcdef 0 ≠0 0 0 ≠0 0 ≠0 ≠0 0 ≠0 0 ≠0
103–122 \x67 – \x7A \147 – \172 ghijklmnop
qrstuvwxyz 		0 ≠0 0 0 ≠0 0 ≠0 ≠0 0 ≠0 0 0
123–126 \x7B – \x7E \173 – \176 {|}~ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0
127 \x7F \177 backspace character ( DEL ) ≠0 0 0 0 0 0 0 0 0 0 0 0
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