Form Validation

I have some form validation done, but it’s not very good. How do i make sure that users can only use a-z, A-Z, 0-9. Would this help make sure my site is more secure. (there for users would not be able to use special characters)

Here is the code i have so far:

[code=php]// check that username is 5 characters or more
$username = $_POST[‘username’];
if (strlen($username) > 4){

}
else {
die

//email validation – i got this from a script

$email = $_POST[’email’];

if(eregi(“^[_a-z0-9-]+(.[_a-z0-9-]+)*@[a-z0-9-]+(.[a-z0-9-]+)*(.[a-z]{2,3})$”, $email)) {

}
else {
die[/code]

how can i make sure that the $username only can have a-z, A-Z, 0-9

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