whitespace-space-before-tab-regexp

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

"\\( +\\)\\(	+\\)"

Documentation:
Specify SPACEs before TAB regexp.

Used when `whitespace-style' includes `space-before-tab',
`space-before-tab::tab' or `space-before-tab::space'.

You can customize this variable.