This example check valid email or not using regular expression if valid returns true.
PHP Code:
<?php
define("EMAIL_REG", "^[_a-zA-Z0-9-]+(\.[_a-zA-Z0-9-]+)*@[a-zA-Z0-9-]+(\.[a-zA-Z0-9-]+)*(\.[a-zA-Z]{2,3})$");
$vEmail = 'abc@xyz.com';
eregi(EMAIL_REG, $vEmail);
?>