whitespace-hspace-regexp

whitespace-hspace-regexp is a variable defined in `whitespace.el'.
Its value is

"\\( +\\)"

Documentation:
Specify HARD SPACE characters regexp.

Here are some examples:

"\\(^\xA0+\\)" visualize only leading HARD SPACEs.
"\\(\xA0+$\\)" visualize only trailing HARD SPACEs.
"\\(^\xA0+\\|\xA0+$\\)" visualize leading and/or trailing HARD SPACEs.
"\t\\(\xA0+\\)\t" visualize only HARD SPACEs between TABs.

NOTE: Enclose always by \\( and \\) the elements to highlight.
Use exactly one pair of enclosing \\( and \\).

Used when `whitespace-style' includes `spaces'.

You can customize this variable.