What happened to this RegEx for email verification?

Question

Here's the regular expression for checking email - \ S+@ \S+\.\S+ , I didn't write it. I am new to regular expressions and don't understand them all well.

I have a couple of questions:

• What is wrong with the above RegEx?
• What is a good RegEx for email confirmation?

Answer 1

"How to validate regular expression emails" is one of the most popular questions that arise when using regular expressions, and the only real good answer is "you don't." This has been discussed on this very website in cases . You must understand that if you really want to follow the specification, your regular expression will look like this . Obviously this is a monster, and it is more an exercise demonstrating how ridiculously difficult it is to adhere to what you have to accept. With this in mind, if you absolutely need to know that the email address is valid, the only real way to check this is to actually send the message to the email address and check whether it bounces or not. Otherwise, this regular expression will correctly validate most cases, and in most cases, in most cases, enough. In addition, this page will discuss issues with trying to validate email using regular expression.

Answer 2

I will only answer your first question in terms of technical regular expression .

What is wrong with the regular expression \ S+@ \S+\.\S+ is that it can do the job too slowly. What happens if someone enters an email line like the one below and you need to check it?

[e-mail]abcdefghijklmnopqrstuvwxyz0123456789

Or even worse (yes, it's 100 @ after the dot):

@. @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

Slowing down is happening. At first, the regular expression would greedily match as many characters as possible for the first \S+ . That way, it will initially match the entire line. Then we need the @ symbol, so it will return until it finds it. At this point, we still have \S+ , so it will again consume everything to the end of the line. Then he must go back until he finds a point. Can you imagine how many return paths occur before the regular expression fails in the second line of the letter?

To kill backtracking, I suggest using possessive character classes in this case, which have the added benefit of not allowing multiple @ on the same line.

[^@\s] ++@ [^@\s.]++\.[^@\s]++

I did a quick test for two regular expressions against "100 @email". Mine is about 95 times faster.

Answer 3

What is wrong with the above RegEx?

It checks only "@" and a. There are many things that are definitely not legitimate email addresses that match this combination.

For example, if the user wrote [e-mail] , it will match, but this is obviously a mistake. Some more difficulties in the regular expression would catch him and help the user.

The same thing if he put [e-mail] - he made a mistake "net". Either he put two @@ (user @@ yahoo.com / user @yahoo @ yahoo.com - which is actually quite common) or two points (user @ yahoo..com). The best regex is to catch them.

[Although the best checks often dwell on other errors, such legitimate multiple dots before and after the "at" that can be dropped or canceled ( [e-mail] )]

If you do not want to be picky, you do not even need a regular expression. indexOf ('@')! = -1 will catch most of the errors. After checking, you should do better.

What is a good RegEx for checking email?

http://www.gooli.org/blog/useful-regular-expressions

http://www.regular-expressions.info/email.html

Answer 4

I see that @liam posted a link to RFC822 . But, in keeping with the idea that stackoverflow is the destination, and incase ex-parrot.com removes the link or whatever you have. There is enthusiasm in him ...

Mail :: RFC822 :: Address: Regular Expression Address Verification

Mail :: RFC822 :: Address - Perl module for checking email addresses according to RFC 822 gramma . It provides the same functions as RFC :: RFC822 :: Address, but uses Perl regular expressions, not the Parse :: RecDescent parser. This means that the module loads much faster, since it does not need to compile the grammar at startup.

Download Mail :: RFC822 :: Address-0.4.tar.gz or read the documentation .

The grammar described in RFC 822 is surprisingly complex. Implementing validation testing with valid expressions somewhat pushes the framework of what is reasonable to do with regular expressions, although Perl does a good job:

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[ \t]))*"(?:(?:\r\n)?[ \t])*)*:(?:(?:\r\n)?[ \t])*(?:(?:(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]| \\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<> @,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|" (?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)?[ \t] )*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\ ".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(? :[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[ \]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*|(?:[^()<>@,;:\\".\[\] \000- \031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|( ?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)*\<(?:(?:\r\n)?[ \t])*(?:@(?:[^()<>@,; :\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([ ^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\" .\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\ ]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*(?:,@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\ [\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\ r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\] |\\.)*\](?:(?:\r\n)?[ \t])*))*)*:(?:(?:\r\n)?[ \t])*)?(?:[^()<>@,;:\\".\[\] \0 00-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\ .|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@, ;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(? :[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)?[ \t])* (?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\". \[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[ ^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\] ]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*\>(?:(?:\r\n)?[ \t])*)(?:,\s*( ?:(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\ ".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)(?:\.(?:( ?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[ \["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t ])*))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t ])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(? :\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+| \Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*|(?: [^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\ ]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)*\<(?:(?:\r\n) ?[ \t])*(?:@(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\[" ()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n) ?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<> @,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*(?:,@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@, ;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t] )*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\ ".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*)*:(?:(?:\r\n)?[ \t])*)? (?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\". \[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)(?:\.(?:(?: \r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\[ "()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t]) *))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t]) +|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\ .(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z |(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*\>(?:( ?:\r\n)?[ \t])*))*)?;\s*) 

This regular expression will only check addresses that have been marked and replaced with spaces (this is done by the module).

Answer 5

What happened to the above RegEx?
The RFC822 standard for email addresses is very loose, so it’s hard to find a short regular text that captures all possible valid emails. The matter is complicated by the fact that not every mail server / client applies this standard, so the actual data may be

Although you can probably guess or enforce a particular format, writing a special expression is pretty much a guarantee that you will either have many unwanted email addresses or deny valid ones.

What is a good RegEx for checking email?
This is a reference regular expression for RFC validation, it is implemented here as a perl module, but it is also the final list in O'Reillys Mastering Regular Expressions

Mail :: RFC822 :: Address