recentf-arrange-rules

recentf-arrange-rules is a variable defined in `recentf.el'.
Its value is

(("Elisp files (%d)" ".\\.el\\'")
 ("Java files (%d)" ".\\.java\\'")
 ("C/C++ files (%d)" "c\\(pp\\)?\\'"))

Documentation:
List of rules used by `recentf-arrange-by-rule' to build sub-menus.
A rule is a pair (SUB-MENU-TITLE . MATCHER). SUB-MENU-TITLE is the
displayed title of the sub-menu where a '%d' `format' pattern is
replaced by the number of items in the sub-menu. MATCHER is a regexp
or a list of regexps. Items matching one of the regular expressions in
MATCHER are added to the corresponding sub-menu.
SUB-MENU-TITLE can be a function. It is passed every items that
matched the corresponding MATCHER, and it must return a
pair (SUB-MENU-TITLE . ITEM). SUB-MENU-TITLE is a computed sub-menu
title that can be another function. ITEM is the received item which
may have been modified to match another rule.

You can customize this variable.