java - 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.

$