allout-depth-one-regexp is a variable defined in `allout.el'.
Its value is

  • Automatically becomes buffer-local when set.

Regular expression to match a heading line prefix for depth one.

This var is set according to the user configuration vars by
`set-allout-regexp'. It is prepared with format strings for two
decimal numbers, which should each be one less than the depth of the
topic prefix to be matched.