whitespace-tab-regexp

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

"\\(	+\\)"

Documentation:
Specify TAB characters regexp.

If you're using `mule' package, there may be other characters
besides "\t" that should be considered TAB.

Here are some examples:

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

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

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

You can customize this variable.