whitespace-space-regexp

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

"\\( +\\)"

Documentation:
Specify SPACE characters regexp.

If you're using `mule' package, there may be other characters
besides " " that should be considered SPACE.

Here are some examples:

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