ISALNUM()

this function checks whether a given character is alphanumeric (A-Z, a-z, or 0-9). this function is used for validating input, such as checking if a character belongs to a username or password.

int isalnum(int c);

 * It returns a non-zero value (true) if the character is alphanumeric, and 0 (false) 
   otherwise

#include <stdio.h>
#include <ctype.h>

int main() {
    char ch = 'A';

    if (isalnum(ch)) {
        printf("'%c' is an alphanumeric character.\n", ch);
    } else {
        printf("'%c' is not an alphanumeric character.\n", ch);
    }

    ch = '#';
    if (isalnum(ch)) {
        printf("'%c' is an alphanumeric character.\n", ch);
    } else {
        printf("'%c' is not an alphanumeric character.\n", ch);
    }

    return 0;
}