Outline mode assumes that the lines in the buffer are of two types: heading lines and body lines. A heading line represents a topic in the outline. Heading lines start with one or more asterisk (‘*’) characters; the number of asterisks determines the depth of the heading in the outline structure. Thus, a heading line with one ‘*’ is a major topic; all the heading lines with two ‘*’s between it and the next one-‘*’ heading are its subtopics; and so on. Any line that is not a heading line is a body line. Body lines belong with the preceding heading line. Here is an example:
* Food This is the body, which says something about the topic of food. ** Delicious Food This is the body of the second-level header. ** Distasteful Food This could have a body too, with several lines. *** Dormitory Food * Shelter Another first-level topic with its header line.
A heading line together with all following body lines is called collectively an entry. A heading line together with all following deeper heading lines and their body lines is called a subtree.
You can customize the criterion for distinguishing heading lines by setting the variable outline-regexp. (The recommended ways to do this are in a major mode function or with a file local variable.) Any line whose beginning has a match for this regexp is considered a heading line. Matches that start within a line (not at the left margin) do not count.
The length of the matching text determines the level of the heading; longer matches make a more deeply nested level. Thus, for example, if a text formatter has commands ‘@chapter’, ‘@section’ and ‘@subsection’ to divide the document into chapters and sections, you could make those lines count as heading lines by setting outline-regexp to ‘'@chap\\|@\\(sub\\)*section'’. Note the trick: the two words ‘chapter’ and ‘section’ are equally long, but by defining the regexp to match only ‘chap’ we ensure that the length of the text matched on a chapter heading is shorter, so that Outline mode will know that sections are contained in chapters. This works as long as no other command starts with ‘@chap’.
You can explicitly specify a rule for calculating the level of a
heading line by setting the variable outline-level. The value
outline-level should be a function that takes no arguments
and returns the level of the current heading. The recommended ways to
set this variable are in a major mode command or with a file local