Need some Regex help

[3lou 55]

Hello there,

So we were getting annoyed on our forums with stupid registration names.
So we wanna setup a regex now so that they can't do this anymore.
I know that this feature is available in the admin panel and we will also use that.
I just need some help with the regex.

Basicly what we want is we wanna allow these chars a-z A-Z 1234567890 and one space inbetween the name.
The space inbetween the name is not a MUST, but if there are spaces in the name it should only be inside the name, and it should only be one.

If anyone could help me with this one it would be greatly appreciated.