List of definition matchers for creating an Imenu index.
Each element of this list should have the form
(MENU-TITLE REGEXP INDEX [FUNCTION] [ARGUMENTS...])
MENU-TITLE should be nil (in which case the matches for this
element are put in the top level of the buffer index) or a
string (which specifies the title of a submenu into which the
matches are put).
REGEXP is a regular expression matching a definition construct
which is to be displayed in the menu. REGEXP may also be a
function, called without arguments. It is expected to search
backwards. It must return true and set `match-data' if it finds
another element.
INDEX is an integer specifying which subexpression of REGEXP
matches the definition's name; this subexpression is displayed as
the menu item.
FUNCTION, if present, specifies a function to call when the index
item is selected by the user. This function is called with
arguments consisting of the item name, the buffer position, and
the ARGUMENTS.
The variable `imenu-case-fold-search' determines whether or not
the regexp matches are case sensitive, and `imenu-syntax-alist'
can be used to alter the syntax table for the search.
If non-nil this pattern is passed to `imenu--generic-function' to
create a buffer index.
For example, see the value of `fortran-imenu-generic-expression'
used by `fortran-mode' with `imenu-syntax-alist' set locally to
give the characters which normally have "symbol" syntax
"word" syntax during matching.