Can SQL injection be done with only letters and numbers?

[Tehim]

I was just wondering if it can or not.

Otherwise wouldn't this code work great for stopping SQL injection?

$some_post = addslashes($_POST['some_post']);

if (!ctype_alnum($some_post)) {
	//error
} else {
	//all good
}

[griswolf]

So you would prevent your customers from using punctuation of any kind Wouldnt that be a little hard for them I think it would make me crazy

[Tehim]

It's for a username check on a registration form. Sorry I forgot to mention that.

striptags or htmlentities. mysql_real_escape_string as standard cleaning.

[Tehim]

Is there a way of protecting from all types of SQL injection?

IMO, you can use just mysql_real_escape_string(), but when passing the var on as an integer, there is no "" or '' around the value placeholder inside the query. This could pose a problem, therefore, you should validate (server-side) the variables for type (eg integer, float etc).