We are going to use preg_match to validate an email. The pattern is:

<?php 
    $pattern = "#^([a-z0-9-_]+)(.[a-z0-9-_]+)*@([a-z0-9-]+)(.[a-z0-9-]+)*.[a-z]{2,4}$#i""; 
?> 

Here is how you validate it:

<?php 
    $pattern = "#^([a-z0-9-_]+)(.[a-z0-9-_]+)*@([a-z0-9-]+)(.[a-z0-9-]+)*.[a-z]{2,4}$#i"; 
    $email = "someone@someplace.com"; 
        if(preg_match($pattern, $email)){ 
            echo "Email is valid."; 
        }else{ echo "Email is invalid."; } 
?>

PHP will print out "Email is valid" if the email is valid.

Here is a useful function to validate emails:

<?php 
    function is_valid_email($email){ 
        if(preg_match("#^([a-z0-9-_]+)(.[a-z0-9-_]+)*@([a-z0-9-]+)(.[a-z0-9-]+)*.[a-z]{2,4}$#i",$email)){ 
            return true; 
        }else{ 
            return false; }
    } 
?> 

Here is how you can use the function:

<?php 
    if(is_valid_email("someone@someplace.com")){ 
        echo "email is valid!"; 
        }else{ 
            echo "not valid"; } 
?>