<form action="chkEmail.php" method="post">
    Email: <input type="text" name="email">
    <input type="submit" value="Check Email">
</form>
chkEmail.php

<?php
    $email = $_POST['email'];
    if(preg_match("~([a-zA-Z0-9!#$%&'*+-/=?^_`{|}~])@([a-zA-Z0-9-]).([a-zA-Z0-9]{2,4})~",$email)){
        echo 'This is a valid email.';
    }else{
        echo 'This is an invalid email.';
    }
?>