The isalnum() in C programming language is useful for checking whether a given character is either an alphabet (lowercase and uppercase letters) or a numeric (digit) value. We can use the isalnum() function for validating usernames, filtering user inputs for special characters, etc.
C isalnum() function syntax
The syntax of the isalnum in this language is as shown below.
int isalnum(int char)
Parameter: The above C isalnum() function accepts a single character of integer type as the parameter and checks whether the given character is either a number or an alphabet.
Return Value
- If the given character is an alphanumeric value, it returns a non-zero integer value.
- If it is not alphanumeric (neither an alphabet nor a digit), it returns 0 as the output.
NOTE: To use the isalnum() function, we must include the <ctype.h> header file.
How does isalnum() in C programming work internally?
The isalnum() function checks whether the given character is alphanumeric (Alphabet + Digit). ASCII values represent the characters, so it checks whether the character is in the following bracket. If it does not fit in this range, isalnum() returns 0 as the output.
- Digits (0 -9): ASCII values between 48 and 75.
- Uppercase Alphabets (A to Z): Values between 65 and 90.
- Lowercase Alphabets (a to z): Values between 97 and 122.
TIP: The isalnum() function only works with standard alphabets. If you pass non-ASCII or Unicode characters, it considers them as not alphanumeric.
C isalnum function with Alphabets, Digits, and Special Characters
The isalnum method (alphanumeric) is used to determine if a given character is either an alphabet or a number. This C program allows the user to enter any character. Next, it checks whether the character is between A to Z, or a to z, or a numeric value using the isalnum() function.
#include <stdio.h>
#include <ctype.h>
int main()
{
char ch;
printf("Please Enter Either an Alphabet, or a Number: \n");
scanf("%c", &ch);
if(isalnum(ch))
{
printf("\n You have entered an Alphanumeric Character");
}
else
{
printf("\n %c is not an Alphanumeric Character", ch);
printf("\n I request you to enter Valid Number, or an Alphabet");
}
}
Checking Uppercase alphabets
Let me enter the Uppercase letter.
Please Enter Either an Alphabet, or a Number:
K
You have entered an Alphanumeric CharacterChecking Digits or Numbers
Let me enter the Numeric value.
Please Enter Either an Alphabet, or a Number:
9
You have entered an Alphanumeric CharacterIn this isalnum in C program, we first declared a character variable called ch. The following C Programming statement will ask the user to enter any character. Then we use the scanf to assign the user-entered character to the ch variable
printf("Please Enter Either an Alphabet, or a Number: \n");
scanf("%c", &ch);
In this next line, we added the If Statement to check whether the character is between ‘A’ to ‘Z’, or ‘a’ to ‘z’, or a number using the isalnum function. If the condition is True, the following statement will print
printf("\n You have entered an Alphanumeric Character");
If the above condition is FALSE, then the given character is not an Alphabet or a number. So, this C program will print the below statements.
printf("\n %c is not an Alphanumeric Character", ch);
printf("\n I request you to enter Valid Number, or an Alphabet");
Checking Special Characters
The above isalnum code perfectly checks whether the given character is either an alphabet or a number. But what if we enter the symbols or special characters?
Please Enter Either an Alphabet, or a Number:
*
* is not an Alphanumeric Character
I request you to enter Valid Number, or an AlphabetLet me try with the space character.
Please Enter Either an Alphabet, or a Number:
is not an Alphanumeric Character
I request you to enter Valid Number, or an AlphabetNOTE: As we all know, special characters like @, #, $, etc are not alphanumeric, so the isalnum() function returns 0 as the output.
Real-time examples of isalnum in C programming
Username Validation
In the example below, the while loop iterates over the string characters. The logical not and isalnum() checks whether a character is not alphanumeric. If true, return 0. Otherwise, complete the string. The fi else statement prints the message based on the result.
#include <stdio.h>
#include <ctype.h>
int usernameValidation(const char *name)
{
while (*name)
{
if (!isalnum((unsigned char)*name))
{
return 0;
}
name++;
}
return 1;
}
int main()
{
char username[] = "sample1234";
if (usernameValidation(username))
{
printf("Valid.");
}
else
{
printf("Invalid Username");
}
return 0;
}
Valid.Validate a string by removing special characters
#include <stdio.h>
#include <ctype.h>
int main(void)
{
const char str[] = "Happy@$20#$2*6!";
for (int i = 0; str[i] != '\0'; i++)
{
if (isalnum((unsigned char)str[i]))
{
putchar(str[i]);
}
}
putchar('\n');
return 0;
}
Happy2026Can I directly use isalnum() on strings?
It accepts only a character as the parameter, so you can’t use a string directly as the isalnum() parameter.
isalnum("hi") = Does not work.
If you want, use the for loop to iterate over the string characters and apply the isalnum() function on every character.
isalnum(str[i])
Use Negative Values as the C isalnum() parameter
The isalnum() function accepts a positive (unsigned) value as the parameter. When you pass a negative value, its behaviour is undefined.
char ch = -1 or
char ch = 150
Character above 127 becomes negative, so 150 eventually becomes (150 – 256 = -106), which is wrong.
Always cast the character to avoid undefined behaviour.
isalnum((unsigned char)ch)
Difference between isalnum(), isalpha(), and isdigit()?
| Function | Checks | Description |
|---|---|---|
| isalnum() | Alphabets + Digits | It checks whether the character is a letter (alphabet) and digit. |
| isdigit() | Digits Only (0-9) | It checks whether the character is a digit. |
| isalpha() | Alphabets Only(a-z and A-Z) | It checks whether the character is an alphabet. |