khoand0000/validate_email.php

## validate_email.php
<?php

$regex = "/([a-z0-9_]+|[a-z0-9_]+\.[a-z0-9_]+)@(([a-z0-9]|[a-z0-9]+\.[a-z0-9]+)+\.([a-z]{2,4}))/i";

// usage

if(!preg_match($regex, $email)) {
    echo "Invalid email address";
}
else {
    echo "email is valid";
}
	<?php

	$regex = "/([a-z0-9_]+\|[a-z0-9_]+\.[a-z0-9_]+)@(([a-z0-9]\|[a-z0-9]+\.[a-z0-9]+)+\.([a-z]{2,4}))/i";

	// usage

	if(!preg_match($regex, $email)) {
	echo "Invalid email address";
	}
	else {
	echo "email is valid";
	}