How to use PHP regular expressions to verify whether an email address actually exists

As email becomes an indispensable part of modern people's daily lives, it is increasingly important to verify the authenticity of email addresses. When developing web applications, regular expressions can be used to easily verify the validity of email addresses. However, verifying the authenticity of an email address requires more sophisticated techniques. In this article, we will introduce how to use PHP regular expressions to verify the authenticity of an email address.

Before we begin, we need to know what regular expressions are. A regular expression is a text pattern that is used to match a specific pattern of strings. In PHP, regular expressions can be matched using the preg_match() function. Here is a simple example:

$email = "example@email.com";
$pattern = "/[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}/";
if (preg_match($pattern, $email)) {
  echo "Valid email.";
} else {
  echo "Invalid email.";
}

In the above example, we use regular expressions to verify whether an email address is valid. Regular expression patterns use the RFC 2822 specification. This specification defines the format of an email address, for example: username@domainname.com. The above regular expression uses character sets, repeating metacharacters, and character classes to match qualified email addresses. If the match is successful, the email address will be considered a valid email address.

However, this verification method only determines whether the format of the email address conforms to the specifications, and cannot verify the true existence of the email address. In order to verify the real existence of the email address, we need to interact with the email server.

The method to implement this process is to send an email to the mail server and wait for the server to return a response. If the server returns a response indicating that the email address is invalid, then the email address is considered invalid. Otherwise, the email address is considered valid. This process can be accomplished through PHP's SMTP class.

First, we need to establish a connection with the mail server through the SMTP class. The following is an example of establishing a connection with the Gmail SMTP server:

$host = "smtp.gmail.com";
$port = 587;
$username = "example@gmail.com";
$password = "password";

$socket = fsockopen($host, $port, $errno, $errstr, 10);
if (!is_resource($socket)) {
  die("$errstr ($errno)");
}

if (substr(fgets($socket, 1024), 0, 3) !== "220") {
  die("Failed to connect to $host:$port");
}

fwrite($socket, "EHLO example.com
");
if (substr(fgets($socket, 1024), 0, 3) !== "250") {
  die("Failed to send EHLO command");
}

fwrite($socket, "STARTTLS
");
if (substr(fgets($socket, 1024), 0, 3) !== "220") {
  die("Failed to initiate TLS");
}

stream_socket_enable_crypto($socket, true, STREAM_CRYPTO_METHOD_TLS_CLIENT);

fwrite($socket, "AUTH LOGIN
");
if (substr(fgets($socket, 1024), 0, 3) !== "334") {
  die("Failed to send AUTH LOGIN command");
}

fwrite($socket, base64_encode($username)."
");
if (substr(fgets($socket, 1024), 0, 3) !== "334") {
  die("Failed to send username");
}

fwrite($socket, base64_encode($password)."
");
if (substr(fgets($socket, 1024), 0, 3) !== "235") {
  die("Failed to authenticate");
}

In the above example, we used the Gmail SMTP server as an example. Through the fsockopen() function, we establish a socket connection with the mail server and send an identifier to the server through the EHLO command, indicating that our starttls command can be accepted. We then use the STARTTLS command to encrypt the socket connection. Next, we send the AUTH LOGIN command to the server and use base64 encoding to send the username and password to the server for authentication.

If the authentication is successful, we can start sending emails to the server. The following is an example of checking whether the email address actually exists:

$email = "example@email.com";
$host = "mx1.example.com";

fwrite($socket, "MAIL FROM:<me@example.com>
");
if (substr(fgets($socket, 1024), 0, 3) !== "250") {
  die("Failed to send MAIL FROM command");
}

fwrite($socket, "RCPT TO:<$email>
");
$response = substr(fgets($socket, 1024), 0, 3);
if ($response === "250" || $response === "251") {
  echo "Valid email.";
} else {
  echo "Invalid email.";
}

In the above example, we use the fwrite() function to send the MAIL FROM and RCPT TO commands to the mail server. The RCPT TO command is used to verify the true existence of the recipient address. If the response code returned by the server is 250 or 251, it means that the email address is valid. Otherwise, the email address is considered invalid.

Through the above method, we can use a combination of PHP regular expressions and SMTP classes to verify the true existence of the email address. Although this method requires interaction with the mail server, its verification results are more accurate and have important practical value.

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