Argthtjtr

Validating E-mail Ids according to RFC5322 and following

https://en.wikipedia.org/wiki/Email_address

Below is the sample code using java and a regular expression to validate E-mail Ids.

public void checkValid() {

    List<String> emails = new ArrayList();

    //Valid Email Ids

    emails.add("simple@example.com");

    emails.add("very.common@example.com");                   

    emails.add("disposable.style.email.with+symbol@example.com");

    emails.add("other.email-with-hyphen@example.com");

    emails.add("fully-qualified-domain@example.com");

    emails.add("user.name+tag+sorting@example.com");

    emails.add("fully-qualified-domain@example.com");

    emails.add("x@example.com");

    emails.add("carlosd'intino@arnet.com.ar");

    emails.add("example-indeed@strange-example.com");

    emails.add("admin@mailserver1");

    emails.add("example@s.example");

    emails.add("" "@example.org");

    emails.add(""john..doe"@example.org");



    //Invalid emails Ids

    emails.add("Abc.example.com");

    emails.add("A@b@c@example.com");

    emails.add("a"b(c)d,e:f;g<h>i[j\k]l@example.com");

    emails.add("just"not"right@example.com");

    emails.add("this is"not\allowed@example.com");

    emails.add("this\ still"not\allowed@example.com");

                    emails.add("1234567890123456789012345678901234567890123456789012345678901234+x@example.com");

    emails.add("john..doe@example.com");

    emails.add("john.doe@example..com");



    String regex = "^[a-zA-Z0-9_!#$%&'*+/=? \"`{|}~^.-]+@[a-zA-Z0-9.-]+$";



    Pattern pattern = Pattern.compile(regex);

    int i=0;

    for(String email : emails){

        Matcher matcher = pattern.matcher(email);

        System.out.println(++i +"."+email +" : "+ matcher.matches());

    }

}

Actual Output:

   1.simple@example.com : true

   2.very.common@example.com : true

   3.disposable.style.email.with+symbol@example.com : true

   4.other.email-with-hyphen@example.com : true

   5.fully-qualified-domain@example.com : true

   6.user.name+tag+sorting@example.com : true

   7.fully-qualified-domain@example.com : true

   8.x@example.com : true

   9.carlosd'intino@arnet.com.ar : true

   10.example-indeed@strange-example.com : true

   11.admin@mailserver1 : true

   12.example@s.example : true

   13." "@example.org : true

   14."john..doe"@example.org : true

   15.Abc.example.com : false

   16.A@b@c@example.com : false

   17.a"b(c)d,e:f;g<h>i[jk]l@example.com : false

   18.just"not"right@example.com : true

   19.this is"notallowed@example.com : false

   20.this still"notallowed@example.com : false

   21.1234567890123456789012345678901234567890123456789012345678901234+x@example.com    : true

   22.john..doe@example.com : true

   23.john.doe@example..com : true

Expected Ouput:

1.simple@example.com : true

2.very.common@example.com : true

3.disposable.style.email.with+symbol@example.com : true

4.other.email-with-hyphen@example.com : true

5.fully-qualified-domain@example.com : true

6.user.name+tag+sorting@example.com : true

7.fully-qualified-domain@example.com : true

8.x@example.com : true

9.carlosd'intino@arnet.com.ar : true

10.example-indeed@strange-example.com : true

11.admin@mailserver1 : true

12.example@s.example : true

13." "@example.org : true

14."john..doe"@example.org : true

15.Abc.example.com : false

16.A@b@c@example.com : false

17.a"b(c)d,e:f;g<h>i[jk]l@example.com : false

18.just"not"right@example.com : false

19.this is"notallowed@example.com : false

20.this still"notallowed@example.com : false

21.1234567890123456789012345678901234567890123456789012345678901234+x@example.com : false

22.john..doe@example.com : false

23.john.doe@example..com : false

How can I change my regular expression so that it will invalidate the below patterns of email ids.

1234567890123456789012345678901234567890123456789012345678901234+x@example.com

john..doe@example.com

john.doe@example..com 

just"not"right@example.com

Below are the criteria for regular expression:

Local-part

The local-part of the email address may use any of these ASCII characters:

1. uppercase and lowercase Latin letters A to Z and a to z;


2. digits 0 to 9;


3. special characters !#$%&'*+-/=?^_`{|}~


4. dot ., provided that it is not the first or last character unless
   quoted, and provided also that it does not appear consecutively
   unless quoted (e.g. John..Doe@example.com is not allowed but
   "John..Doe"@example.com is allowed);


5. 
   space and "(),:;<>@[] characters are allowed with restrictions
   (they are only allowed inside a quoted string, as described in the
   paragraph below, and in addition, a backslash or double-quote must
   be preceded by a backslash); comments are allowed with parentheses
   at either end of the local-part; e.g.
   john.smith(comment)@example.com and
   (comment)john.smith@example.com are both equivalent to
   john.smith@example.com.

Domain

1. uppercase and lowercase Latin letters A to Z and a to z;


2. digits 0 to 9, provided that top-level domain names are not
   all-numeric;


3. hyphen -, provided that it is not the first or last character.
   Comments are allowed in the domain as well as in the local-part; for
   example, john.smith@(comment)example.com and
   john.smith@example.com(comment) are equivalent to
   john.smith@example.com.

You could RFC5322 like this

( reference regex modified )

"(?im)^(?=.{1,64}@)(?:("[^"\\]*(?:\\.[^"\\]*)*"@)|((?:[0-9a-z](?:\.(?!\.)|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)?[0-9a-z]@))(?=.{1,255}$)(?:(\[(?:\d{1,3}\.){3}\d{1,3}\])|((?:(?=.{1,63}\.)[0-9a-z][-\w]*[0-9a-z]*\.)+[a-z0-9][\-a-z0-9]{0,22}[a-z0-9])|((?=.{1,63}$)[0-9a-z][-\w]*))$"  

https://regex101.com/r/ObS3QZ/1

 # (?im)^(?=.{1,64}@)(?:("[^"\]*(?:\.[^"\]*)*"@)|((?:[0-9a-z](?:.(?!.)|[-!#$%&'*+/=?^`{}|~w])*)?[0-9a-z]@))(?=.{1,255}$)(?:([(?:d{1,3}.){3}d{1,3}])|((?:(?=.{1,63}.)[0-9a-z][-w]*[0-9a-z]*.)+[a-z0-9][-a-z0-9]{0,22}[a-z0-9])|((?=.{1,63}$)[0-9a-z][-w]*))$



 # Note - remove all comments '(comments)' before runninig this regex

 # Find  ([^)]*)  replace with nothing



 (?im)                                     # Case insensitive

 ^                                         # BOS



                                           # Local part

 (?= .{1,64} @ )                           # 64 max chars

 (?:

      (                                         # (1 start), Quoted

           " [^"\]* 

           (?: \ . [^"\]* )*

           "

           @

      )                                         # (1 end)

   |                                          # or, 

      (                                         # (2 start), Non-quoted

           (?:

                [0-9a-z] 

                (?:

                     .

                     (?! . )

                  |                                          # or, 

                     [-!#$%&'*+/=?^`{}|~w] 

                )*

           )?

           [0-9a-z] 

           @

      )                                         # (2 end)

 )

                                           # Domain part

 (?= .{1,255} $ )                          # 255 max chars

 (?:

      (                                         # (3 start), IP

           [

           (?: d{1,3} . ){3}

           d{1,3} ]

      )                                         # (3 end)

   |                                          # or,   

      (                                         # (4 start), Others

           (?:                                       # Labels (63 max chars each)

                (?= .{1,63} . )

                [0-9a-z] [-w]* [0-9a-z]* 

                .

           )+

           [a-z0-9] [-a-z0-9]{0,22} [a-z0-9] 

      )                                         # (4 end)

   |                                          # or,

      (                                         # (5 start), Localdomain

           (?= .{1,63} $ )

           [0-9a-z] [-w]* 

      )                                         # (5 end)

 )

 $                                         # EOS

It's not the question you asked, but why re-invent the wheel?

Apache commons has a class that covers this already.

org.apache.commons.validator.routines.EmailValidator.getInstance().isValid(email)

This way you aren't responsible for keeping up to date with changing email format standards.

A regular expression is the most difficult and error-prone way to validate emails addresses. If you are using an implementation of javax.mail to send the emails, then the simplest way to determine if it will work is by using the provided parser, because whether the email is compliant or not, if the library cannot use it, then it doesn't matter.

public static boolean validateEmail(String address) {

    try {

        // if this fails, the mail library can't send emails to this address

        InternetAddress ia = new InternetAddress(address, true);

        return ia.isGroup() && ia.getAddress().charAt(0) != '@';

    }

    catch (Throwable t) {

        return false;

    }

}

Invoking it with false allows emails without a @domain part when strict parsing. And since the checkAddress function invoked internally is private and we can't just call checkAddress(addr,false,true) since we don't want routing information (a feature practically designed for fraud through server bouncing), we have to check the first letter of the validated address.

Now what you may notice here is that this validation method is actually compliant to RFC 2822, rather than 5822. The reason for this is because unless you are implementing your own SMTP sender library, then you're using one that depends on this one, and if you have an address that is 5822-valid but 2822-invalid, then your 5822-validation is rendered useless. But if you are implementing your own 5822 SMTP library, then you should learn from the existing ones and write a parser function, rather than a regular expression.