whitespace-space-after-tab-regexp

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

("	+\\(\\( \\{%d\\}\\)+\\)" . "\\(	+\\) +")

Documentation:
Specify regexp for 8 or more SPACEs after TAB.

It is a cons where the cons car is used for SPACEs visualization
and the cons cdr is used for TABs visualization.

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

You can customize this variable.