Regex email address validation

Question

Can someone please explain this java Regex to me?

^[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+(?:[A-Z]{2}|com|org|net|gov|mil|biz|info|mobi|name|in|aero|jobs|museum)$

This regex is used to validate an email address .

Answer 1

Validating email addresses is now considered bad practice ( stop validating email addresses with regex ), especially with such expression as in your question. For example here's a more complete expression .

As for this expression let's break it in parts:

Beginning of the matched string

^

Matches at least one character from the list

[a-z0-9!#$%&'*+/=?^_`{|}~-]+

Non-capturing ( see backreference ) group which can be repeated 0..n times, that matches a . and then at least one character from the list.

(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*

Just this character

@

Non-capturing group matching one character in this list [a-z0-9] and then possibly more characters from the following lists. Matched string must start and end with [a-z0-9] and inside it can have [a-z0-9-].

(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+

Non-capturing group that matches 2 uppercase letters or one of the words.

(?:[A-Z]{2}|com|org|net|gov|mil|biz|info|mobi|name|in|aero|jobs|museum)

End of the string.

$

Answer 2

^                                       # Beginning of the line
[a-z0-9!#$%&'*+/=?^_`{|}~-]+            # One or more (+) characters from the 
                                        bracket expression, i.e., letters [a-z],
                                        numbers [0-9], !, $, %, et cetera
(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*    # Zero or more (*) of the above
                                        expression, preceded by a dot \\.
@                                       # Literal @
(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+ # A digit or a letter, followed by 
                                        optional digits, letters, or dashes, 
                                        followed by a a dot
(?:[A-Z]{2}|com|org|net...)             # Country code ([A-Z]{2}), or a top level
                                        domain, such as com, org, net.
$                                       # End of the line

Using a concrete example, john@foo.com . The first part of the e-mail, john , will be matched by ^[a-z0-9!#$%&'*+/=?^_{|}~-]+ . The @ will be matched by, well, @ . The domain foo , as well as the dot, is matched by (?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\\\.)+ . Finally, the TLD com is matched by the alternation (?:[AZ]{2}|com|org|net|gov|mil|biz|info|mobi|name|in|aero|jobs|museum) .