Example in a discussion on help-gnu-emacs

perl6-mode-imenu-chunk-bad.el

;;; perl6-imenu.el --- Imenu support Perl 6 -*- lexical-binding: t; -*-
;;; fragment

;; Imenu functions and variables are defined here.
;; a new definition of the Variables entry regex:
(defconst perl6-vars
  (concat
    "^\\s-*"                     ; leading ws allowed
    "\\(?:my\\|our\\)\\s-+"      ; scope of var, followed by mandatory ws
    "\\("                        ; start capture group 1 for the var name
         "\\(?:\\$\\|@\\|%\\)"   ;   sigil for type of var
         "\\(?:[_[:alnum:]]+\\)" ;   the var name ends with ws
    "\\)"                        ; end of capture group 1
    ))

;; now subsitute the var above in the var below to replace the explicit regex
(defvar perl6-imenu-generic-expression
  `(
    ;; the names are in reverse desired order since they are evaluated here last first
    ;;("Variables" "^\\s-*\\(?:my\\|our\\)\\s-+\\(\\(?:\\$\\|@\\|%\\)\\(?:[_[:alnum:]]+\\)\\)" 1)
    ("Variables" ,perl6-vars 1)
    ("Subs/Methods" "^\\s-*\\(?:my\\s-+\\|our\\s-+\\)?\\(?:multi\\s-+sub\\|multi\\s-+method\\|sub\\|method\\|multi\\)\\s-+\\(.+)\\)" 1)
    ))

;; Add imenu support for perl6-mode.  Note that imenu-generic-expression
;; is buffer-local, so we don't need a local-variable for it.
(setq imenu-generic-expression perl6-imenu-generic-expression
      imenu-case-fold-search nil)

;; Imenu support
(setq imenu-auto-rescan t)
(add-hook 'perl6-mode-hook 'imenu-add-menubar-index)
(add-hook 'perl6-mode-hook
	  (lambda ()
	    (setq imenu-generic-expression perl6-imenu-generic-expression)))

;;===========================
(provide 'perl6-imenu)