Hello,
I have made a regex (it's a little version not entire version of my regex but it doesn't work too) ^[0-9a-z]*(?:@[0-9a-z]*)?$ (Regexper) it can check when a user typing if the email was correctly typed.
on JS it's work proper (only test with it) but in java you can put an infinity of @ without any problem
tested on java:
java version "1.7.0_51"
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)
sorry for my poor english