Switch from AllowAlphaNumeric to UserNameRegex

Can we call this UserNameValidationRegex? My motivation is to try to make it as easy as possible to grok, because while UserNameRegex is an order of magnitude more powerful it is also an order of magnitude harder to than AllowOnlyAlphanumericNames for someone that just wants to switch the behavior between one and the other. (I understand from the code below that you can set this to null if you just want to allow everything).