How to create Custom Exception Class?

<?php
/* Creating Custom Exception Class */
class customException extends Exception {
    public function errorMessage() {
        //error message
        $errorMsg = 'Error on line '.$this->getLine().' in '.$this->getFile()
        .': <b>'.$this->getMessage().'</b> is not a valid E-Mail address';
        return $errorMsg;
    }
}
/* Creating Custom Exception Class */


$email = "[email protected]";
try {
    //check if
    if(!eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$", $email)) {
        //throw exception if email is not valid
        throw new customException($email);
    }
    echo "Email Id is Valid";
} catch (customException $e) {
  //display custom message
  echo $e->errorMessage();
}
?>