Question: Using only stdio.h, string.h and stdlib.h libraries how would I go about implementing this?

I'm quite new to programming so please bear with me!

Answer: 
1. Allocate a new `char` array of the same length as your string. Convince yourself that this is enough space. Don't forget the NUL.
2. Loop through the string, copying to the new string only those characters that are alphanumeric. You can't do this portably without also including `<ctype.h>` and using a function/macro from that header, unless you're going to enumerate all characters that you consider alphanumeric.
3. Again, don't forget the NUL.
Every char you read in your string is a byte (you can think it as a number between 0 and 255, and that's how the computers handle them) so you just need to check the ascii table to see what letter refers to.

Every alphanumerical char is in this range: [48, 58] (for numbers), or [65, 90] (upper case), or [97, 122] (lower case).

Look at this:

```
#include <stdio.h>
#include <stdlib.h>

#define SIZE    64

int isalphanum(char);    /*states if a char is alphanumerical or not*/
char *getalphanum(char *, char*);    /*modifies the second string to get the result*/

int main(void) {
    char in[SIZE] = "Hello, W@#@#@#@#@#orl...,.,d!";    /*just a random to try out*/
    char out[SIZE];
    getalphanum(in, out);
    printf("%s", out);
    return 0;
}

int isalphanum(char a) {
    if ((a >= 48) && (a <= 58))
        return 1;
    if ((a >= 65) && (a <= 90))
        return 1;
    if ((a >= 97) && (a <= 122))
        return 1;
    return 0;
}

char *getalphanum(char *s, char *t) {
    if ((s == NULL) || (t == NULL))    /*tests if the strings are "handble"*/
        return NULL;
    int i = 0;
    int j = 0;
    char c;
    while ((c = *(s + i)) != '\0') {
    if (isalphanum(c)){
        *(t + j) = c;
        j++;
    }  
    i++;
    }
    *(t + j) = '\0';
    return t;
}

```

This code works and is very simple and can be improved, but there is evertything you need.