whitespace-trailing-regexp

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

"\\([	  ]+\\)$"

Documentation:
Specify trailing characters regexp.

There may be other characters besides:

" " "\t" "\u00A0"

that should be considered blank.

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

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

You can customize this variable.