outline-regexp

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

"[*]+"

• This variable is safe as a file local variable if its value
  satisfies the predicate `stringp'.

Documentation:
Regular expression to match the beginning of a heading.
Any line whose beginning matches this regexp is considered to start a heading.
Note that Outline mode only checks this regexp at the start of a line,
so the regexp need not (and usually does not) start with `^'.
The recommended way to set this is with a Local Variables: list
in the file it applies to. See also `outline-heading-end-regexp'.