Allow Spaces but not other random characters?

[eluser]Corey Freeman[/eluser]
For my blogging simulator (fun fun), people want to be able to use spaces in their blog's names. I personally want the game to retain some kind of realism by not letting people name their blogs random strings of special characters (e..x "U$%9*$)&%(#*$)*^)( Blog) but I don't see any reason not to allow spaces or something.

How do I keep the alphanumeric rule AND allow spaces? And I do know that a space isn't an alphanumeric character.