PHP implements RFC compliant email address verification

/*
Copyright 2009 Dominic Sayers
(dominic_sayers@hotmail.com)
(http://www.dominicsayers.com)
This source file is subject to the Common Public Attribution License Version 1.0 (CPAL) license.
The license terms are available through the world-wide-web at http://www.opensource.org/licenses/cpal_1.0
*/
function is_email ($email, $checkDNS = false) {
    //    Check that $email is a valid address
    //        (http://tools.ietf.org/html/rfc3696)
    //        (http://tools.ietf.org/html/rfc5322#section-3.4.1)
    //        (http://tools.ietf.org/html/rfc5321#section-4.1.3)
    //        (http://tools.ietf.org/html/rfc4291#section-2.2)
    //        (http://tools.ietf.org/html/rfc1123#section-2.1)

    //    Contemporary email addresses consist of a "local part" separated from
    //    a "domain part" (a fully-qualified domain name) by an at-sign ("@").
    //        (http://tools.ietf.org/html/rfc3696#section-3)
    $index = strrpos($email,'@');
    if ($index === false)        return false;    //    No at-sign
    if ($index === 0)            return false;    //    No local part
    if ($index > 64)            return false;    //    Local part too long
    $localPart        = substr($email, 0, $index);
    $domain            = substr($email, $index + 1);
    $domainLength    = strlen($domain);

    if ($domainLength === 0)    return false;    //    No domain part
    if ($domainLength > 255)    return false;    //    Domain part too long
    //    Let's check the local part for RFC compliance...
    //
    //    Period (".") may...appear, but may not be used to start or end the
    //    local part, nor may two or more consecutive periods appear.
    //        (http://tools.ietf.org/html/rfc3696#section-3)
    if (preg_match('/^\.|\.\.|\.$/', $localPart) > 0)                return false;    //    Dots in wrong place
    //    Any ASCII graphic (printing) character other than the
    //    at-sign ("@"), backslash, double quote, comma, or square brackets may
    //    appear without quoting.  If any of that list of excluded characters
    //    are to appear, they must be quoted
    //        (http://tools.ietf.org/html/rfc3696#section-3)
    if (preg_match('/^"(?:.)*"$/', $localPart) > 0) {
        //    Local part is a quoted string
        if (preg_match('/(?:.)+[^\\]"(?:.)+/', $localPart) > 0)    return false;    //    Unescaped quote character inside quoted string
    } else {
        if (preg_match('/[ @\[\]\\",]/', $localPart) > 0)
            //    Check all excluded characters are escaped
            $stripped = preg_replace('/\\[ @\[\]\\",]/', '', $localPart);
            if (preg_match('/[ @\[\]\\",]/', $stripped) > 0)        return false;    //    Unquoted excluded characters
    }
    //    Now let's check the domain part...
    //    The domain name can also be replaced by an IP address in square brackets
    //        (http://tools.ietf.org/html/rfc3696#section-3)
    //        (http://tools.ietf.org/html/rfc5321#section-4.1.3)
    //        (http://tools.ietf.org/html/rfc4291#section-2.2)
    if (preg_match('/^\[(.)+]$/', $domain) === 1) {
        //    It's an address-literal
        $addressLiteral = substr($domain, 1, $domainLength - 2);
        $matchesIP        = array();

        //    Extract IPv4 part from the end of the address-literal (if there is one)
        if (preg_match('/\b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$/', $addressLiteral, $matchesIP) > 0) {
            $index = strrpos($addressLiteral, $matchesIP[0]);

            if ($index === 0) {
                //    Nothing there except a valid IPv4 address, so...
                return true;
            }else {
                //    Assume it's an attempt at a mixed address (IPv6 + IPv4)
                if ($addressLiteral[$index - 1] !== ':')            return false;    //    Character preceding IPv4 address must be ':'
                if (substr($addressLiteral, 0, 5) !== 'IPv6:')        return false;    //    RFC5321 section 4.1.3
                $IPv6 = substr($addressLiteral, 5, ($index ===7) ? 2 : $index - 6);
                $groupMax = 6;
            }
        } else {
            //    It must be an attempt at pure IPv6
            if (substr($addressLiteral, 0, 5) !== 'IPv6:')            return false;    //    RFC5321 section 4.1.3
            $IPv6 = substr($addressLiteral, 5);
            $groupMax = 8;
        }
        $groupCount    = preg_match_all('/^[0-9a-fA-F]{0,4}|\:[0-9a-fA-F]{0,4}|(.)/', $IPv6, $matchesIP);
        $index        = strpos($IPv6,'::');
        if ($index === false) {
            //    We need exactly the right number of groups
            if ($groupCount !== $groupMax)                            return false;    //    RFC5321 section 4.1.3
        } else {
            if ($index !== strrpos($IPv6,'::'))                        return false;    //    More than one '::'
            $groupMax = ($index === 0 || $index === (strlen($IPv6) - 2)) ? $groupMax : $groupMax - 1;
            if ($groupCount > $groupMax)                            return false;    //    Too many IPv6 groups in address
        }
        //    Check for unmatched characters
        array_multisort($matchesIP[1], SORT_DESC);
        if ($matchesIP[1][0] !== '')                                    return false;    //    Illegal characters in address
        //    It's a valid IPv6 address, so...
        return true;
    } else {
        //    It's a domain name...
        //    The syntax of a legal Internet host name was specified in RFC-952
        //    One aspect of host name syntax is hereby changed: the
        //    restriction on the first character is relaxed to allow either a
        //    letter or a digit.
        //        (http://tools.ietf.org/html/rfc1123#section-2.1)
        //
        //    NB RFC 1123 updates RFC 1035, but this is not currently apparent from reading RFC 1035.
        //
        //    Most common applications, including email and the Web, will generally not permit...escaped strings
        //        (http://tools.ietf.org/html/rfc3696#section-2)
        //
        //    Characters outside the set of alphabetic characters, digits, and hyphen MUST NOT appear in domain name
        //    labels for SMTP clients or servers
        //        (http://tools.ietf.org/html/rfc5321#section-4.1.2)
        //
        //    RFC5321 precludes the use of a trailing dot in a domain name for SMTP purposes
        //        (http://tools.ietf.org/html/rfc5321#section-4.1.2)
        $matches    = array();
        $groupCount    = preg_match_all('/(?:[0-9a-zA-Z][0-9a-zA-Z-]{0,61}[0-9a-zA-Z]|[a-zA-Z])(?:\.|$)|(.)/', $domain, $matches);
        $level        = count($matches[0]);
        if ($level == 1)                                            return false;    //    Mail host can't be a TLD
        $TLD = $matches[0][$level - 1];
        if (substr($TLD, strlen($TLD) - 1, 1) === '.')                return false;    //    TLD can't end in a dot
        if (preg_match('/^[0-9]+$/', $TLD) > 0)                        return false;    //    TLD can't be all-numeric
        //    Check for unmatched characters
        array_multisort($matches[1], SORT_DESC);
        if ($matches[1][0] !== '')                            return false;    //    Illegal characters in domain, or label longer than 63 characters
        //    Check DNS?
        if ($checkDNS && function_exists('checkdnsrr')) {
            if (!(checkdnsrr($domain, 'A') || checkdnsrr($domain, 'MX'))) {
                                                                    return false;    //    Domain doesn't actually exist
            }
        }
        //    Eliminate all other factors, and the one which remains must be the truth.
        //        (Sherlock Holmes, The Sign of Four)
        return true;
    }
}
function unitTest ($email, $reason = '') {
    $expected    = ($reason === '') ? true : false;
    $valid        = is_email($email);
    $not        = ($valid) ? '' : ' not';
    $unexpected    = ($valid !== $expected) ? ' This was unexpected!' : '';
    $reason        = ($reason === '') ? "" : " Reason: $reason";

    return "The address $email is$not valid.$unexpected$reasonn";
}
//    Email validator test cases (Dominic Sayers, January 2009)
//    Valid addresses
echo unitTest('first.last@example.com');
echo unitTest('1234567890123456789012345678901234567890123456789012345678901234@example.com');
echo unitTest('"first last"@example.com');
echo unitTest('"first\"last"@example.com');    //    Not totally sure whether this is valid or not
echo unitTest('first\@last@example.com');
echo unitTest('"first@last"@example.com');
echo unitTest('first\\last@example.com');    //    Note that  is escaped even in single-quote strings, so this is testing "first\last"@example.com
echo unitTest('first.last@x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.
x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x2345');
echo unitTest('first.last@[12.34.56.78]');
echo unitTest('first.last@[IPv6:::12.34.56.78]');
echo unitTest('first.last@[IPv6:1111:2222:3333::4444:12.34.56.78]');
echo unitTest('first.last@[IPv6:1111:2222:3333:4444:5555:6666:12.34.56.78]');
echo unitTest('first.last@[IPv6:::1111:2222:3333:4444:5555:6666]');
echo unitTest('first.last@[IPv6:1111:2222:3333::4444:5555:6666]');
echo unitTest('first.last@[IPv6:1111:2222:3333:4444:5555:6666::]');
echo unitTest('first.last@[IPv6:1111:2222:3333:4444:5555:6666:7777:8888]');
echo unitTest('first.last@x23456789012345678901234567890123456789012345678901234567890123.example.com');
echo unitTest('first.last@1xample.com');
echo unitTest('first.last@123.example.com');
//    Invalid addresses
echo unitTest('first.last', "No @");
echo unitTest('@example.com', "No local part");
echo unitTest('12345678901234567890123456789012345678901234567890123456789012345@example.com', "Local part more than 64 characters");
echo unitTest('.first.last@example.com', "Local part starts with a dot");
echo unitTest('first.last.@example.com', "Local part ends with a dot");
echo unitTest('first..last@example.com', "Local part has consecutive dots");
echo unitTest('"first"last"@example.com', "Local part contains unescaped excluded characters");
echo unitTest('first\\@last@example.com', "Local part contains unescaped excluded characters");
echo unitTest('first.last@', "No domain");
echo unitTest('first.last@x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.
x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456789.x23456', "Domain exceeds 255 chars");
echo unitTest('first.last@[.12.34.56.78]', "Only char that can precede IPv4 address is ':'");
echo unitTest('first.last@[12.34.56.789]', "Can't be interpreted as IPv4 so IPv6 tag is missing");
echo unitTest('first.last@[::12.34.56.78]', "IPv6 tag is missing");
echo unitTest('first.last@[IPv5:::12.34.56.78]', "IPv6 tag is wrong");
echo unitTest('first.last@[IPv6:1111:2222:3333::4444:5555:12.34.56.78]', "Too many IPv6 groups (4 max)");
echo unitTest('first.last@[IPv6:1111:2222:3333:4444:5555:12.34.56.78]', "Not enough IPv6 groups");
echo unitTest('first.last@[IPv6:1111:2222:3333:4444:5555:6666:7777:12.34.56.78]', "Too many IPv6 groups (6 max)");
echo unitTest('first.last@[IPv6:1111:2222:3333:4444:5555:6666:7777]', "Not enough IPv6 groups");
echo unitTest('first.last@[IPv6:1111:2222:3333:4444:5555:6666:7777:8888:9999]', "Too many IPv6 groups (8 max)");
echo unitTest('first.last@[IPv6:1111:2222::3333::4444:5555:6666]', "Too many '::' (can be none or one)");
echo unitTest('first.last@[IPv6:1111:2222:3333::4444:5555:6666:7777]', "Too many IPv6 groups (6 max)");
echo unitTest('first.last@[IPv6:1111:2222:333x::4444:5555]', "x is not valid in an IPv6 address");
echo unitTest('first.last@[IPv6:1111:2222:33333::4444:5555]', "33333 is not a valid group in an IPv6 address");
echo unitTest('first.last@example.123', "TLD can't be all digits");
echo unitTest('first.last@com', "Mail host must be second- or lower level");
echo unitTest('first.last@-xample.com', "Label can't begin with a hyphen");
echo unitTest('first.last@exampl-.com', "Label can't end with a hyphen");
echo unitTest('first.last@x234567890123456789012345678901234567890123456789012345678901234.example.com', "Label can't be longer than 63 octets");
//    Test cases from RFC3696 (February 2004, http://tools.ietf.org/html/rfc3696#section-3)
echo unitTest('Abc\@def@example.com');
echo unitTest('Fred\ Bloggs@example.com');
echo unitTest('Joe.\\Blow@example.com');
echo unitTest('"Abc@def"@example.com');
echo unitTest('"Fred Bloggs"@example.com');
echo unitTest('user+mailbox@example.com');
echo unitTest('customer/department=shipping@example.com');
echo unitTest('$A12345@example.com');
echo unitTest('!def!xyz%abc@example.com');
echo unitTest('_somename@example.com');
//    Test cases from Doug Lovell (LinuxJournal, June 2007, http://www.linuxjournal.com/article/9585)
echo unitTest("dclo@us.ibm.com");
echo unitTest("abc\@def@example.com");
echo unitTest("abc\\@example.com");
echo unitTest("Fred\ Bloggs@example.com");
echo unitTest("Joe.\\Blow@example.com");
echo unitTest(""Abc@def"@example.com");
echo unitTest(""Fred Bloggs"@example.com");
echo unitTest("customer/department=shipping@example.com");
echo unitTest("$A12345@example.com");
echo unitTest("!def!xyz%abc@example.com");
echo unitTest("_somename@example.com");
echo unitTest("user+mailbox@example.com");
echo unitTest("peter.piper@example.com");
echo unitTest("Doug\ \"Ace\"\ Lovell@example.com");
echo unitTest(""Doug \"Ace\" L."@example.com");
echo unitTest("abc@def@example.com", "Doug Lovell says this should fail");
echo unitTest("abc\\@def@example.com", "Doug Lovell says this should fail");
echo unitTest("abc\@example.com", "Doug Lovell says this should fail");
echo unitTest("@example.com", "Doug Lovell says this should fail");
echo unitTest("doug@", "Doug Lovell says this should fail");
echo unitTest(""qu@example.com", "Doug Lovell says this should fail");
echo unitTest("ote"@example.com", "Doug Lovell says this should fail");
echo unitTest(".dot@example.com", "Doug Lovell says this should fail");
echo unitTest("dot.@example.com", "Doug Lovell says this should fail");
echo unitTest("two..dot@example.com", "Doug Lovell says this should fail");
echo unitTest(""Doug "Ace" L."@example.com", "Doug Lovell says this should fail");
echo unitTest("Doug\ \"Ace\"\ L\.@example.com", "Doug Lovell says this should fail");
echo unitTest("hello world@example.com", "Doug Lovell says this should fail");
echo unitTest("gatsby@f.sc.ot.t.f.i.tzg.era.l.d.", "Doug Lovell says this should fail");
?>