How to prevent SQL injection of $_POST e-mail field using PHP 4

This would be a bit easier if I was able to use PHP 5 unfortunately this is not a viable option?

I am already using RFC 2822 from this stackoverflow thread to validate the e-mail format is valid, granted this is using JS on the form page which is not the best practice.

I will again verify it conforms to this format before saving it but I was wondering if there were any methods that should be used to help prevent SQL injection?

Solution

Wrong way - "Never trust user input"!

First be sure the data is in the format you want, then query database. So first, check if $_POST[email_address] is in a valid email format, e.g. with regex. Only if it is in a valid email format, you query the database.

Code for email regex (PHP):

<?php
$email = "test@test.com";

if(eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$", $email)) {
  echo "Email is valid.";
}
else {

  echo "Email in invalid.";
}
?>