How do I validate an email address?

<?php

/*
	This function checks an email address to determine whether it is valid.
	
	It takes an email address as its first input parameter and determines whether
	the address is well-formed.  The definition of a well-formed address is based
	on RFC2821, but may vary from the strict definition.
	
		A well formed SMTP email address is made up of a username, followed by
		exactly one "@"-symbol, followed by a domain.
		
		A username begins with a digit or letter, and can be followed by any
		number of digits, letters, dots, dashes, and underscores.
		
		A domain begins with a digit or letter, and is followed by any number
		of digits, letters, dots and dashes.  A domain must contain at least one
		dot to be well-formed.
		
	
	If the function determines an address is well-formed, it can then optionally
	determine whether the domain part of the address ends in a known Top-Level
	Domain (TLD).  This option is turned on by setting the second input parameter
	to any value that does not explicitly equal FALSE.  The list of known TLDs
	are stored in the array $tld_match_array and are accurate as of 2002-09-05.
	
	Note:	This function does not take into account the inherent weirdness of
			the ".us" TLD.
	
	
	LICENSE:
	Do whatever you want with this code.
	
	WARRANTY:
	This code comes with no warranty as to the safety or accuracy of its
	operation.  Use with circumspection and trepidation.  If it breaks
	something, I recommend vituperation.

*/

function good_email ($address, $validate_tld = FALSE)
{
	$retval = TRUE;

	/*
		A list of known good TLDs.
		This data from http://www.uwhois.com/domains.html and
		http://www.icann.org/tlds .  This data accurate as of
		2003-07-08.
	*/

	$tld_matches  = '/\.(';
	$tld_matches .= 'ac|ac\.at|ac\.cn|ac\.id|ac\.jp|ac\.kr|ac\.nz|ac\.uk|ad|';
	$tld_matches .= 'ad\.jp|ae|aero|af|al|am|an|an|ao|aq|as|at|aw|ba|bb|bd|';
	$tld_matches .= 'bd|be|bf|bf|bg|bi|biz|bj|bm|bn|bo|bs|bt|bv|bw|by|bz|ca|';
	$tld_matches .= 'cc|cd|cf|cg|ch|ci|ck|cl|cm|cn|co|co\.at|co\.az|co\.hu|';
	$tld_matches .= 'co\.id|co\.il|co\.jp|co\.kr|co\.nz|co\.tz|co\.uk|co\.uz|';
	$tld_matches .= 'co\.vi|co\.za|com|com\.ag|com\.ai|com\.ar|com\.au|com\.az|';
	$tld_matches .= 'com\.bb|com\.bh|com\.br|com\.bs|com\.cn|com\.do|com\.eg|';
	$tld_matches .= 'com\.gy|com\.hm|com\.kz|com\.mt|com\.mx|com\.nf|com\.pa|';
	$tld_matches .= 'com\.pe|com\.pn|com\.pr|com\.ru|com\.sc|com\.sg|com\.ua|';
	$tld_matches .= 'coop|cr|cri\.nz|cu|cv|cv|cx|cy|cz|de|dj|dk|dm|dz|ec|edu|';
	$tld_matches .= 'edu\.au|edu\.tw|ee|er|es|et|eun\.eg|fi|fj|fk|fm|fo|fr|ga|';
	$tld_matches .= 'gd|ge|gen\.nz|gf|gg|gh|gi|gl|gm|gn|gob\.mx|gov|gov\.au|';
	$tld_matches .= 'govt\.nz|gp|gq|gr|gr\.jp|gs|gt|gu|gw|hk|hn|hr|ht|id|ie|im|';
	$tld_matches .= 'in|ind\.br|inf\.br|info|int|iq|ir|is|it|iwi\.nz|je|jm|jo|';
	$tld_matches .= 'ke|kg|kh|ki|km|kn|kw|ky|la|lb|lc|li|lk|lr|ls|lt|ltd\.uk|';
	$tld_matches .= 'lu|lv|ly|ly|ma|maori\.nz|mc|md|mg|mh|mil|mil\.nz|mk|ml|mm|';
	$tld_matches .= 'mn|mo|mp|mq|mr|ms|mu|museum|mv|mw|my|mz|na|name|nc|ne|';
	$tld_matches .= 'ne\.jp|net|net\.ae|net\.ar|net\.au|net\.bb|net\.br|';
	$tld_matches .= 'net\.bs|net\.cn|net\.eg|net\.lu|net\.mx|net\.nz|net\.ru|';
	$tld_matches .= 'net\.tw|net\.uk|ng|ni|nl|no|np|nr|nu|om|or\.jp|org|';
	$tld_matches .= 'org\.ae|org\.ar|org\.bb|org\.bs|org\.cn|org\.il|org\.lu|';
	$tld_matches .= 'org\.nz|org\.uk|pf|pg|ph|pk|pl|plc\.uk|pm|pro|pt|pw|py|qa|';
	$tld_matches .= 're|ro|ru|rw|sa|sb|school\.nz|sd|se|sh|si|si|sj|sk|sl|sm|';
	$tld_matches .= 'sn|so|sr|st|sy|sz|tc|td|tf|tg|th|tj|tk|tm|tn|to|tp|tr|tt|';
	$tld_matches .= 'tv|ug|um|us|uy|va|vc|ve|vg|vn|vu|ws|ye|yu|zm|zr|zw';
	$tld_matches .= ')$/';


	//First, split the address a the "@"-symbol into the username and domain name parts
	$address_array = preg_split ('/@/', $address);
															
	//Was exactly one "@" found?
	if (sizeof ($address_array) != 2)
	{
		$retval = FALSE;
	}
	
	if ($retval === TRUE)
	{
		//Store the user and domain parts in more readable variables.
		list($user, $domain) = $address_array;

		//Split the user-name part of the address at any dots
		$user_array = preg_split ('/\./', $user);
		
		//Check each piece for well-formedness
		for ($counter = 0;
			 $counter < sizeof($user_array) && $retval == TRUE;
			 $counter++)
		{
			if (!preg_match ('/^[a-z0-9\-_]+$/i', $user_array[$counter]))
			{
				$retval = FALSE;
			}
		}
	}
		
	if ($retval === TRUE)
	{
		//split the domain part of the address at any dots in it
		$domain_array = preg_split ('/\./', $domain);
			
		//Did the domain part contain at least one dot
		//(making two array variables)?
		if (sizeof($domain_array) > 1)
		{
			//Is each part of the domain well-formed? Stop checking if
			//any part is not
			$domain_value = TRUE;
			for ($counter = 0;
				 $counter < sizeof($domain_array) && $domain_value === TRUE;
				 $counter++)
			{
				if (!preg_match ('/^[a-z0-9]+([a-z0-9\-]*[a-z0-9]+)*$/i', $domain_array[$counter]))
				{
					$domain_value = FALSE;
				}
			}
			$retval = $domain_value;
				
			//if the domain part has passed all tests so far and the function
			//has been instructed to validate the TLD...
			if ($retval === TRUE && $validate_tld !== FALSE)
			{
				if (!preg_match ($tld_matches, $domain))
				{
					$retval = FALSE;
				}
			}
		}
	}
	
	return $retval;
}



//Below is some PHP code which will drive the function.


print '
<html>
<body>
<table border=0>
	<form method=post action="'.$PHP_SELF.'">
	<tr>
		<td align=right>Email address:</td>
		<td align=left>
			<input type=text name=address value="';

if (array_key_exists('address', $_POST))
{
	print $_POST[address];
}

print '">
		</td>
	</tr>
	<tr>
		<td align=right>Validate TLD?</td>
		<td align=left>
			<input type=checkbox name=check_tld';
print array_key_exists('check_tld', $_POST)?' checked':';
print '>
		</td>
	</tr>
	<tr>
		<td colspan=2 align=center>
			<input type=submit>
		</td>
	</tr>
	</form>
	<tr>
		<td colspan=2 align=center>';

if (array_key_exists("address", $_POST))
{
	$result = good_email ($_POST[address],
						  (array_key_exists('check_tld', $_POST)?TRUE:FALSE)
						 );

	print "'$_POST[address]' ";
		
	if ($result === TRUE)
	{
		print 'Good';
	}
	else
	{
		print 'Bad';
	}
}
else
{
	print "á";
}	

print '
		</td>
	</tr>
</table>
</body>
</html>';
?>