How to verify email subject using PHP regular expression

In our daily work and life, we often need to use emails, and the writing of email subjects is also very important. In order to ensure the legitimacy of the email subject input, we can use PHP regular expressions to verify the email subject. This article will introduce the method of verifying email subjects with PHP regular expressions to help you accurately write email subjects that comply with specifications.

1. What is PHP regular expression?
PHP regular expression is a special syntax that is used to express some complex string matching rules. Generally speaking, regular expressions have many metacharacters that represent different meanings. These metacharacters can be used to match and replace strings.

2. PHP regular expression rules for verifying email subject
In order to verify the normativeness of the email subject, we need to understand some basic rules of the email subject so that we can write verification rules that comply with the specifications. The following are common email subject rules:

1. The email subject length should be between 1 and 78 characters, otherwise display problems will occur.
2. The email subject should avoid using special symbols, such as: ()[],,;:@". Because these symbols have special meaning in the email system and may cause errors.
3. The email subject can contain: letters, numbers, spaces and some common special characters (such as: $ % & - / = ? ^ _ ` { | } ~).

3. PHP Method of verifying email subject with regular expression
After understanding the rules of email subject, you can then write PHP regular expression verification code to check the legitimacy of email subject. The following is a simple PHP function for verifying email Whether the subject complies with the specification:

function checkMailSubject($subject){
    $regex = '/^[^<>()[]，;:@“]+$/'; // 验证规则
    if(preg_match($regex, $subject)){
        return true;
    }else{
        return false;
    }
}

The preg_match() function is used in the above code to perform regular expression matching, and the verification rules use the rules mentioned above. If the function returns true, it means that the email subject complies with Specification, otherwise it will not comply with the specification.

4. Example of PHP regular expression verification of email subject
The following is a complete PHP program to demonstrate how to use regular expressions to verify email subject:

";

// 测试包含特殊符号的情况
$subject2 = '这是一个包含特殊符号的邮件主题：';
if(checkMailSubject($subject2)){
    echo '邮件主题验证通过';
}else{
    echo '邮件主题验证不通过';
}
?>

In the above code, the first test case will pass the verification normally because the email subject complies with the specification and does not contain any special symbols. The second test case contains special symbols such as the email address, so it cannot pass Verification.

5. Summary
Through the introduction of this article, we can understand the basic knowledge of PHP regular expressions and the rules of email subjects, as well as how to write PHP code to verify the legitimacy of email subjects. In In actual development, we can adjust the verification rules of the email subject according to specific needs to ensure the robustness and user experience of the program.

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