speedbar-directory-unshown-regexp

speedbar-directory-unshown-regexp is a variable defined in `speedbar.el'.
Its value is

"^\\(\\..*\\)\\'"

Documentation:
Regular expression matching directories not to show in speedbar.
They should include commonly existing directories which are not
useful. It is no longer necessary to include version-control
directories here; see `vc-directory-exclusion-list'.

You can customize this variable.