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23.5 Imenu

Imenu is a feature that lets users select a definition or section in the buffer, from a menu which lists all of them, to go directly to that location in the buffer. Imenu works by constructing a buffer index which lists the names and buffer positions of the definitions, or other named portions of the buffer; then the user can choose one of them and move point to it. Major modes can add a menu bar item to use Imenu using imenu-add-to-menubar.

command imenu-add-to-menubar name​

This function defines a local menu bar item named name to run Imenu.

The user-level commands for using Imenu are described in the Emacs Manual (see Imenu in the Emacs Manual). This section explains how to customize Imenu’s method of finding definitions or buffer portions for a particular major mode.

The usual and simplest way is to set the variable imenu-generic-expression:

variable imenu-generic-expression​

This variable, if non-nil, is a list that specifies regular expressions for finding definitions for Imenu. Simple elements of imenu-generic-expression look like this:

(menu-title regexp index)

Here, if menu-title is non-nil, it says that the matches for this element should go in a submenu of the buffer index; menu-title itself specifies the name for the submenu. If menu-title is nil, the matches for this element go directly in the top level of the buffer index.

The second item in the list, regexp, is a regular expression (see Regular Expressions); anything in the buffer that it matches is considered a definition, something to mention in the buffer index. The third item, index, is a non-negative integer that indicates which subexpression in regexp matches the definition’s name.

An element can also look like this:

(menu-title regexp index function arguments…)

Each match for this element creates an index item, and when the index item is selected by the user, it calls function with arguments consisting of the item name, the buffer position, and arguments.

For Emacs Lisp mode, imenu-generic-expression could look like this:

((nil "^\\s-*(def\\(un\\|subst\\|macro\\|advice\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2)
 ("*Vars*" "^\\s-*(def\\(var\\|const\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2)
 ("*Types*"
  "^\\s-*\
(def\\(type\\|struct\\|class\\|ine-condition\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2))

Setting this variable makes it buffer-local in the current buffer.

This variable controls whether matching against the regular expressions in the value of imenu-generic-expression is case-sensitive: t, the default, means matching should ignore case.

Setting this variable makes it buffer-local in the current buffer.

variable imenu-syntax-alist​

This variable is an alist of syntax table modifiers to use while processing imenu-generic-expression, to override the syntax table of the current buffer. Each element should have this form:

(characters . syntax-description)

The CAR, characters, can be either a character or a string. The element says to give that character or characters the syntax specified by syntax-description, which is passed to modify-syntax-entry (see Syntax Table Functions).

This feature is typically used to give word syntax to characters which normally have symbol syntax, and thus to simplify imenu-generic-expression and speed up matching. For example, Fortran mode uses it this way:

(setq imenu-syntax-alist '(("_$" . "w")))

The imenu-generic-expression regular expressions can then use ‘\\sw+’ instead of ‘\\(\\sw\\|\\s_\\)+’. Note that this technique may be inconvenient when the mode needs to limit the initial character of a name to a smaller set of characters than are allowed in the rest of a name.

Setting this variable makes it buffer-local in the current buffer.

Another way to customize Imenu for a major mode is to set the variables imenu-prev-index-position-function and imenu-extract-index-name-function:

variable imenu-prev-index-position-function​

If this variable is non-nil, its value should be a function that finds the next definition to put in the buffer index, scanning backward in the buffer from point. It should return nil if it doesn’t find another definition before point. Otherwise it should leave point at the place it finds a definition and return any non-nil value.

Setting this variable makes it buffer-local in the current buffer.

variable imenu-extract-index-name-function​

If this variable is non-nil, its value should be a function to return the name for a definition, assuming point is in that definition as the imenu-prev-index-position-function function would leave it.

Setting this variable makes it buffer-local in the current buffer.

The last way to customize Imenu for a major mode is to set the variable imenu-create-index-function:

variable imenu-create-index-function​

This variable specifies the function to use for creating a buffer index. The function should take no arguments, and return an index alist for the current buffer. It is called within save-excursion, so where it leaves point makes no difference.

The index alist can have three types of elements. Simple elements look like this:

(index-name . index-position)

Selecting a simple element has the effect of moving to position index-position in the buffer. Special elements look like this:

(index-name index-position function arguments…)

Selecting a special element performs:

(funcall function
         index-name index-position arguments…)

A nested sub-alist element looks like this:

(menu-title . sub-alist)

It creates the submenu menu-title specified by sub-alist.

The default value of imenu-create-index-function is imenu-default-create-index-function. This function calls the value of imenu-prev-index-position-function and the value of imenu-extract-index-name-function to produce the index alist. However, if either of these two variables is nil, the default function uses imenu-generic-expression instead.

Setting this variable makes it buffer-local in the current buffer.