It also provides a lot of methods to access certain parts of the address (left-hand side, right-hand side, personal names, comments, etc), to parse/validate mailbox-list headers, to parse/validate the return-path (which is unique among the headers), and so forth. The email address can only be checked for its format conformance. I'd also stay away from so called simple 'non-restrictive' regex; there's no such thing.
Apache Commons is generally known as a solid project. J" = 17 Top-Level-Domain muss 2 Stellen haben null = 1 Eingabestring nicht gesetzt " " = 2 durch TRIM eine Laenge von 0 " A . D " = 12 Leerzeichen innerhalb der e Mail-Adresse "(? ]@.: " = 12 Sonderzeichen sind nicht erlaubt Thank you for your interest in this question.
Les Hazlewood has written a very thorough RFC 2822 compliant email validator class using Java regular expressions. Simple regex won't understand it, even though the email should be valid.
Anything more complex becomes error-prone or even contain hidden performance killers. The only comprehensive RFC compliant regex based validator I'm aware of is email-rfc2822-validator with its 'refined' regex appropriately named
If the data entered by a client was incorrect or was simply missing, the server would have to send all the data back to the client and request that the form be resubmitted with correct information.
This was really a lengthy process which used to put a lot of burden on the server.