Jul 12, 2007 09:56 PM|dvallone|LINK
This will give you the info you need
The Official Standard: RFC 2822
Maybe you're wondering why there's no "official" full-proof and fool-proof regex to match email addresses. Well, there is an official definition, but it's hardly full-proof.
The official standard is known as
RFC 2822. It describes the syntax that valid email addresses must adhere to. You can (but you shouldn't--read on) implement it with this regular expression:
This regex has two parts: the part before the @, and the part after the @. There are two alternatives for the part before the @: it can either consist of a series of letters, digits and certain symbols, including one or more dots. However, dots may not appear
consecutively or at the start or end of the email address. The other alternative requires the part before the @ to be enclosed in double quotes, allowing any string of ASCII characters between the quotes. Whitespace characters, double quotes and backslashes
must be escaped with backslashes.
The part after the @ also has two alternatives. It can either be a fully qualified domain name (e.g. regular-expressions.info), or it can be a literal Internet address between square brackets. The literal Internet address can either be an IP address, or
a domain-specific routing address.
The reason you shouldn't use this regex is that it only checks the basic syntax of email addresses.
email@example.com would be considered a valid email address according to RFC 2822. Obviously, this email address won't work, since there's no "nospam" top-level domain. It also doesn't guarantee your email software will be able to handle it. Not
all applications support the syntax using double quotes or square brackets. In fact, RFC 2822 itself marks the notation using square brackets as obsolete.
We get a more practical implementation of RFC 2822 if we omit the syntax using double quotes and square brackets. It will still match 99.99% of all email addresses in actual use today.
A further change you could make is to allow any two-letter country code top level domain, and only specific generic top level domains. This regex filters dummy email addresses like firstname.lastname@example.org. You will need to update it as new top-level domains are added.
So even when following official standards, there are still trade-offs to be made. Don't blindly copy regular expressions from online libraries or discussion forums. Always test them on your own data and with your own applications.