There are an overwhelming number of regex sequences out there to validate an email address against, and frankly I get tired of running through them all trying to find one that fits my situation. I went looking for one that most closely follows the IETF rules and lo and behold I find a document describing exactly what I was after. It is a great article that I encourage all web developers to read regardless of if you are a PHP developer or not. The article steps through some of the most common techiques and shows how they are flawed, then describes in detail how to build a perfect email validation function in PHP. You can find it on the Linux Journal website.
Just in case their site should ever go down I am providing the entire function for reference here:
/**
* Validate an email address.
* Provide email address (raw input)
* Returns true if the email address has the email
* address format and the domain exists.
*
* From: http://www.linuxjournal.com/article/9585
*
* @param String $email
* @return Boolean
*/
function validateEmail($email)
{
$isValid = true;
$atIndex = strrpos($email, "@");
if (is_bool($atIndex) && !$atIndex)
{
$isValid = false;
}
else
{
$domain = substr($email, $atIndex+1);
$local = substr($email, 0, $atIndex);
$localLen = strlen($local);
$domainLen = strlen($domain);
if ($localLen < 1 || $localLen > 64)
{
// local part length exceeded
$isValid = false;
}
else if ($domainLen < 1 || $domainLen > 255)
{
// domain part length exceeded
$isValid = false;
}
else if ($local[0] == '.' || $local[$localLen-1] == '.')
{
// local part starts or ends with '.'
$isValid = false;
}
else if (preg_match('/\\.\\./', $local))
{
// local part has two consecutive dots
$isValid = false;
}
else if (!preg_match('/^[A-Za-z0-9\\-\\.]+$/', $domain))
{
// character not valid in domain part
$isValid = false;
}
else if (preg_match('/\\.\\./', $domain))
{
// domain part has two consecutive dots
$isValid = false;
}
else if
(!preg_match('/^(\\\\.|[A-Za-z0-9!#%&`_=\\/$\'*+?^{}|~.-])+$/',
str_replace("\\\\","",$local)))
{
// character not valid in local part unless
// local part is quoted
if (!preg_match('/^"(\\\\"|[^"])+"$/',
str_replace("\\\\","",$local)))
{
$isValid = false;
}
}
// **** NOTE: If you are getting failures you may need to comment out this section
if ($isValid && !(checkdnsrr($domain,"MX") || checkdnsrr($domain,"A")))
{
// domain not found in DNS
$isValid = false;
}
}
return $isValid;
}