Regex email address validation

Question

Can someone please explain this java Regex to me?

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


        

Answer 1

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

Answer 2

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.

$