Program to check if a string contains any special character in C

In C programming, checking if a string contains special characters is a common validation task. Special characters are those that are neither alphabetic nor numeric, such as symbols like !@#$%^&*() etc.

Syntax

int checkSpecialCharacter(char str[], int length);

Method 1: Using Character Comparison

This method iterates through each character and compares it against a predefined set of special characters −

#include <stdio.h>
#include <string.h>

int checkSpecialCharacter(char str[], int n) {
    int i;
    for (i = 0; i < n; i++) {
        if (str[i] == '!' || str[i] == '@' || str[i] == '#' || str[i] == '$'
        || str[i] == '%' || str[i] == '^' || str[i] == '&' || str[i] == '*'
        || str[i] == '(' || str[i] == ')' || str[i] == '-' || str[i] == '+'
        || str[i] == '=' || str[i] == '{' || str[i] == '}' || str[i] == '['
        || str[i] == ']' || str[i] == ':' || str[i] == ';' || str[i] == '"'
        || str[i] == ''' || str[i] == '<' || str[i] == '>' || str[i] == '.'
        || str[i] == '/' || str[i] == '?' || str[i] == '~' || str[i] == '`') {
            return 1; /* Special character found */
        }
    }
    return 0; /* No special character */
}

int main() {
    char str1[] = "tutorialspoint";
    char str2[] = "tutorials-point";
    
    printf("String 1: %s<br>", str1);
    if (checkSpecialCharacter(str1, strlen(str1))) {
        printf("The string is not accepted<br><br>");
    } else {
        printf("The string is accepted<br><br>");
    }
    
    printf("String 2: %s<br>", str2);
    if (checkSpecialCharacter(str2, strlen(str2))) {
        printf("The string is not accepted<br>");
    } else {
        printf("The string is accepted<br>");
    }
    
    return 0;
}

String 1: tutorialspoint
The string is accepted

String 2: tutorials-point
The string is not accepted

Method 2: Using ASCII Values

This approach checks if characters fall outside the alphanumeric range using ASCII values −

#include <stdio.h>
#include <string.h>

int isAlphanumeric(char c) {
    return ((c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || (c >= '0' && c <= '9'));
}

int checkSpecialCharacterASCII(char str[], int n) {
    int i;
    for (i = 0; i < n; i++) {
        if (!isAlphanumeric(str[i])) {
            return 1; /* Special character found */
        }
    }
    return 0; /* No special character */
}

int main() {
    char str1[] = "Hello123";
    char str2[] = "Hello@123";
    
    printf("String 1: %s<br>", str1);
    if (checkSpecialCharacterASCII(str1, strlen(str1))) {
        printf("The string is not accepted<br><br>");
    } else {
        printf("The string is accepted<br><br>");
    }
    
    printf("String 2: %s<br>", str2);
    if (checkSpecialCharacterASCII(str2, strlen(str2))) {
        printf("The string is not accepted<br>");
    } else {
        printf("The string is accepted<br>");
    }
    
    return 0;
}

String 1: Hello123
The string is accepted

String 2: Hello@123
The string is not accepted

Comparison

Conclusion

Both methods effectively detect special characters in strings. The ASCII approach is more concise and automatically handles all non-alphanumeric characters, while character comparison gives precise control over which characters to consider special.