bool input_is_valid(char str[WANTED_STRING_LENGTH + 1]) {
    if(string_length(str) > WANTED_STRING_LENGTH) {
        return false;
    }

    for(int i = 0; str[i] != '\0'; i++) {
        if(str[i] < 'a' || str[i] > 'z') {
            return false;
        }
    }

    return true;
}

int main() {
    char str[WANTED_STRING_LENGTH + 1];

    do{
        cout << "Enter a string up to " << WANTED_STRING_LENGTH << " characters long"
             << " and composed only of small letters from the english alphabet:" << endl;
        cin.getline(str, WANTED_STRING_LENGTH + 2);
    } while(!input_is_valid(str));

    return 0;
}