Problem

You want to find a pattern that will verify the validity of a supplied mail address.

Solution

There isn't one. You cannot do real-time validation of mail addresses. You must pick from a number of compromises.

Discussion

The common patterns that people try to use for this are all quite incorrect. As an example, the address fred&barney@stonehenge.com is valid and deliverable (as of this writing), but most patterns that allegedly match valid mail addresses fail miserably.

RFC-822 documents have a formal specification for what constitutes a syntactically valid mail address. However, complete processing requires recursive parsing of nested comments, something that one single regular expression cannot do. If you first strip off legal comments:

1 while $addr =~ s/\([^()]*\)//g;

You could then in theory use the 6598-byte pattern given on the last page of Mastering Regular Expressions to test for RFC-conformance, but that's still not good enough, for three reasons.

First, not all RFC-valid address are deliverable. For example, foo@foo.foo.foo.foo is valid in form, but in practice is not deliverable. Some people try to do DNS lookups for MX records, even trying to connect to the host handling that address's mail to check if it's valid at that site. This is a poor approach because most sites can't do a direct connect to any other site, and even if they could, mail receiving sites increasingly either ignore the SMTP VRFY command or fib about its answer.

Second, some RFC-invalid addresses, in practice, are perfectly deliverable. For example, a lone postmaster is almost certainly deliverable but doesn't pass RFC-822 muster. It doesn't have an @ in it.

Thirdly and most important, just because the address happens to be both valid and deliverable doesn't mean that it's the right one. president@whitehouse.gov, for example, is valid by the RFC and deliverable. But it's very unlikely that would really be the mail address of the person submitting information to your CGI script.

The script at http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz makes a valiant (albeit provably imperfect) attempt at doing this incorrectly. It jumps through many hoops, including the RFC-822 regular expression from Mastering Regular Expressions, DNS MX record look-up, and stop lists for naughty words and famous people. But this is still a very weak approach.

Our best advice for verifying a person's mail address is to have them enter their address twice, just as you would when changing a password. This usually weeds out typos. If both versions match, send mail to that address with a personal message such as:

Dear someuser@host.com,

Please confirm the mail address you gave us Wed May  6 09:38:41
MDT 1998 by replying to this message.  Include the string
"Rumpelstiltskin" in that reply, but spelled in reverse; that is,
start with "Nik...".  Once this is done, your confirmed address will
be entered into our records.

If you get the message back and they've followed your directions, you can be reasonably assured that it's real.

A related strategy that's less open to forgery is to give them a PIN (personal identification number). Record the address and PIN (preferably a random one) for later processing. In the mail you send, ask them to include the PIN in their reply. If it bounces, or the message is included via a vacation script, it'll be there anyway. So ask them to mail back the PIN slightly altered, such as with the characters reversed, one added or subtracted to each digit, etc.

See Also

The "Matching an Email Address" section of Chapter 7 of Mastering Regular Expressions; Recipe 18.9