Add a function: `wp_spaces_regexp()`. It is not used yet.

This function is required by a bunch of patches by miqrogroove. Needs filter docs.

Props miqrogroove.
See #27588.


git-svn-id: https://develop.svn.wordpress.org/trunk@28708 602fd350-edb4-49c9-b593-d223f7449a82