You can enter a single word, or a single word followed by a space, but you can't enter two words separated by a space.
So this prohibits entering a name for example.
Of course I could enter it using _, but a space is the normal convention for separating words.
The only other option is to set the field value to none, but that then allows for too much leeway.
Any chance <space> and , (comma) can be added to this field value as I suspect that's how most people will want to use it.
Edit: I suppose I can always use the custom regex field...