$email = "test@ansoncheung.com";
if (preg_match('/^[^0-9][a-zA-Z0-9_]+([.][a-zA-Z0-9_]+)*[@][a-zA-Z0-9_]+([.][a-zA-Z0-9_]+)*[.][a-zA-Z]{2,4}$/',$email)) {
    echo "Your email is ok.";
} else {
    echo "Wrong email address format";
}