whitespace-indentation-regexp

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

("^	*\\(\\( \\{%d\\}\\)+\\)[^
	]" . "^ *\\(	+\\)[^
]")

Documentation:
Specify regexp for 8 or more SPACEs at beginning of line.

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 `indentation',
`indentation::tab' or `indentation::space'.

You can customize this variable.