[CentOS] E-mail advice sought

[Ned Slider]

On 01/05/16 13:23, Alice Wonder wrote:
> On 05/01/2016 05:10 AM, Alice Wonder wrote:
>>
>> I think this is my autism coming in to play, I think what is very clear
>> to me I just am not able to adequately communicate because clearly
>> people are not even remotely grasping what I am trying to convey.
>>
>
> Basically whether it is a white list or a black list doesn't matter.
>
> One is just the complement set of the other.
>
> It's the set I'm after. Or its complement. I don't care. Not whether my
> code should use set A or its complement.
>

See Gordon's previous answer above:

That is, [A-Za-z0-9._][A-Za-z0-9._-]

In other words, allowable characters are A-Za-z (upper and lower case), 
0-9 (numerics), . and _ (period and underscore).

Hyphens are allowed, but NOT as the first character, so maybe easier to 
just not allow hyphens in your account usernames.