[svn:perlfaq] r8070 - perlfaq/trunk

Author: rafael
Date: Tue Nov 14 01:59:53 2006
New Revision: 8070

Modified:
   perlfaq/trunk/perlfaq9.pod

Log:
* perlfaq9 : update to "How do I check a valid mail address" by
    Aaron Sherman


Modified: perlfaq/trunk/perlfaq9.pod
==============================================================================
--- perlfaq/trunk/perlfaq9.pod	(original)
+++ perlfaq/trunk/perlfaq9.pod	Tue Nov 14 01:59:53 2006
@@ -407,34 +407,40 @@
 
 =head2 How do I check a valid mail address?
 
-You can't, at least, not in real time.  Bummer, eh?
+This isn't as simple a question as it sounds.  There are two parts:
+
+a) How do I verify that an email address is correctly formatted?
+
+b) How do I verify that an email address targets a valid recipient?
 
 Without sending mail to the address and seeing whether there's a human
-on the other end to answer you, you cannot determine whether a mail
-address is valid.  Even if you apply the mail header standard, you
-can have problems, because there are deliverable addresses that aren't
-RFC-822 (the mail header standard) compliant, and addresses that aren't
-deliverable which are compliant.
-
-You can use the Email::Valid or RFC::RFC822::Address which check
-the format of the address, although they cannot actually tell you
-if it is a deliverable address (i.e. that mail to the address
-will not bounce).  Modules like Mail::CheckUser and Mail::EXPN
-try to interact with the domain name system or particular
-mail servers to learn even more, but their methods do not
-work everywhere--especially for security conscious administrators.
-
-Many are tempted to try to eliminate many frequently-invalid
-mail addresses with a simple regex, such as
-C</^[\w.-]+\@(?:[\w-]+\.)+\w+$/>.  It's a very bad idea.  However,
-this also throws out many valid ones, and says nothing about
-potential deliverability, so it is not suggested.  Instead, see
-http://www.cpan.org/authors/Tom_Christiansen/scripts/ckaddr.gz ,
-which actually checks against the full RFC spec (except for nested
-comments), looks for addresses you may not wish to accept mail to
-(say, Bill Clinton or your postmaster), and then makes sure that the
-hostname given can be looked up in the DNS MX records.  It's not fast,
-but it works for what it tries to do.
+on the other end to answer you, you cannot fully answer part I<b>, but
+either the C<Email::Valid> or the C<RFC::RFC822::Address> module will do
+both part I<a> and part I<b> as far as you can in real-time.
+
+If you want to just check part I<a> to see that the address is valid
+according to the mail header standard with a simple regular expression,
+you can have problems, because there are deliverable addresses that
+aren't RFC-2822 (the latest mail header standard) compliant, and
+addresses that aren't deliverable which, are compliant.  However,  the
+following will match valid RFC-2822 addresses that do not have comments,
+folding whitespace, or any other obsolete or non-essential elements.
+This I<just> matches the address itself:
+
+
+    my $atom = qr{[\w!\#$\%&'*+/=?\^`{}~\-]+};
+    my $dot_atom = qr{$atom(?:\.$atom)*};
+    my $quoted = qr{"(?:\\[^\r\n]|[^\\"])*"};
+    my $local = qr{(?:$dot_atom|$quoted)};
+    my $domain_lit = qr{\[(?:\\\S|[\x21-\x5a\x5e-\x7e])*\]};
+    my $domain = qr{(?:$dot_atom|$domain_lit)};
+    my $addr_spec = qr{$local\@$domain};
+
+Just match an address against C</^${addr_spec}$/> to see if it follows
+the RFC2822 specification.  However, because it is impossible to be sure
+that such a correctly formed address is actually the correct way to
+reach a particular person or even has a mailbox associated with it, you
+must be very careful about how you use this.
 
 Our best advice for verifying a person's mail address is to have them
 enter their address twice, just as you normally do to change a password.