The expression is taken from: https://github.com/balexand/email_validator/blob/master/lib/email_validator.rb
Just did not want to include whole gem and keep the possibility to change the validations on demand.
It does allow ```name+surname@domain.tld```, the only symbol it does not allow in email prefix is spaces.
http://www.w3schools.com/jsref/jsref_regexp_whitespace.asp