PHP: How to validate e-mail addresses using regular expressions

This function checks if an email address is valid or not. For example it is very useful if you have to validate a HTML form where someone should enter a real email address.

function ValidateEmail($email)
{
/*
(Name) Letters, Numbers, Dots, Hyphens and Underscores
(@ sign)
(Domain) (with possible subdomain(s) ).
Contains only letters, numbers, dots and hyphens (up to 255 characters)
(. sign)
(Extension) Letters only (up to 10 (can be increased in the future) characters)
*/

$regex = '/([a-z0-9_.-]+)'. # name

'@'. # at

'([a-z0-9.-]+){2,255}'. # domain & possibly subdomains

'.'. # period

'([a-z]+){2,10}/i'; # domain extension 

if($email == '') { 
	return false;
}
else {
$eregi = preg_replace($regex, '', $email);
}

return empty($eregi) ? true : false;
}

$email = '[email protected]'; 

// will return true, since it matches the regex

if(ValidateEmail($email))
{
echo ''.$email.' is a valid e-mail address.';
}
else
{
echo ''.$email.' is not a valid e-mail address.';
}