What is the regular expression that matches a space, but does not match "other whitespace characters" (tab, form feed, and line feed)

I want a regular expression to match a-z, A-Z, 0-9, and space. For this particular situation, I do not want users to be allowed to input a tab character, or line feed, or form feed.

I know the following regular expression will do everything except the space requirement:

var alphaRE = /^[a-zA-Z0-9]+$/;

How do I change that regular expression to include just space characters?

TIA,

Dennis