After upgrading from Liferay Portal version 5.2 EE SP4 to 6.2 EE, backslashes in the screen name are not allowed when using the liberal screen name validator. In Portal 5.2, backslashes were allowed in the screen name.
Resolution
The ability to add backslashes in the screen name was never an intended behavior, as only alphanumeric characters, dashes, periods, and underscores should be allowed. The unintended functionality of allowing backslashes in the screen name was fixed on LPS-24601.