I wanted to find a regular expression to validate e-mail addresses, but when I searched I found dozens of slightly different versions of the same expression. Almost all of them had people leaving comments about how it failed to validate or invalidate some rare but completely valid/invalid address syntax. But dispite all the criticism, I couldn't find a definitive "best" expression. So, I setup arrays of some of the most promising expressions as well as a (hopefully) complete sampling of valid and invalid addresses. It's my philosophy that it's better to accept a few invalid addresses than reject any valid ones, so I'm shooting for 0 false-positives and as few false-negatives as possible.

Conclusion:
James Watts and Francisco Jose Martin Moreno are the first to develop one which passes all of the tests.
/^([\w\!\#$\%\&\'\*\+\-\/\=\?\^\`{\|\}\~]+\.)*[\w\!\#$\%\&\'\*\+\-\/\=\?\^\`{\|\}\~]+@((((([a-z0-9]{1}[a-z0-9\-]{0,62}[a-z0-9]{1})|[a-z])\.)+[a-z]{2,6})|(\d{1,3}\.){3}\d{1,3}(\:\d{1,5})?)$/i

Arluison Guillaume has also improved Warren Gaebel's regex. This one will work in JavaScript:
/^[-a-z0-9~!$%^&*_=+}{\'?]+(\.[-a-z0-9~!$%^&*_=+}{\'?]+)*@([a-z0-9_][-a-z0-9_]*(\.[-a-z0-9_]+)*\.(aero|arpa|biz|com|coop|edu|gov|info|int|mil|museum|name|net|org|pro|travel|mobi|[a-z][a-z])|([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}))(:[0-9]{1,5})?$/i

Notes:

• There's no perfect regular expression to validate e-mail addresses
• I think it's better to allow any TLD, even if it doesn't exist, than to put a static list of currently-existing TLDs into the regex. The problem with a static list is that the regex will be used in production environments, but probably won't be updated when new TLDs come out.
• Table of valid and invalid characters in the local part of an e-mail address
• Some characters (like the ampersand, apostrophe, asterisk etc) are technically valid, but can cause problems due to their significance in operating systems, programming languages and applications and therefore shouldn't really be used. I've just used my judgement on whether they should be treated as valid or not.
• Some of these are dependent on being executed using case-insenstive regex functions.
• This page uses PHP's ereg() and preg_match() functions, but some of these will work with JavaScript and other languages
• These regex's only check if the address is syntactically valid. It's entirely possible that an address which does not exist on any e-mail server would pass this test. Checking to see if an address actually exists isn't always practical or necessary, though.
• If you need something more advanced than a simple regex, Cal Henderson and Dominic Sayers have PHP parsers available. Their test suites are much more comprehensive than mine.

Results: