: [[image:iciclesimage]]

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== Regressive Completion ==

Though generally less useful than [[progressive completion]], you can
also widen, instead of narrow, the current set of [[completion]]
candidates, by providing an alternative pattern ([[regexp]]) to match.
By analogy, I call this '''[::regressive completion]'''.

The analogy is not exact. By definition, 
your current input is always matched against all candidates in the domain of discourse.
With progressive completion, a recursive [[minibuffer]] is entered for
each new pattern to match.  The candidates that matched the
previous input of the progression become the new domain of discourse
for the current act (recursive level) of completion.

That same technique is not applicable for widening.  Instead, you
enter, using `RET', a new pattern to match as an alternative, and
'''Icicles''' changes the current input to a regexp that matches either
what the previous input matched or the alternative pattern.  In
other words, it is just a short cut for typing a regexp that
matches a choice: '''##\(...\|...\)##'''.    The domain of discourse remains
the same -- in particular, there is no way to widen the domain of
discourse like narrowing narrows it.

You use '''`##M-+##'''' (`icicle-widen-candidates') for regressive completion -- think of the `##+##' as set
union (OR), just as you think of the `##*##' in `##M-*##' as set
intersection (AND).  And, just as for progressive completion,
there is a shortcut, '''`S-backspace'''', for `S-TAB' followed by `##M-+##'.

For example, if you want to see all of the candidates that contain
either `for' or `back', you could type `##\(for\|back\)##' in the
minibuffer, or you could just type `for', then `S-backspace' (or
`S-TAB' followed by `##M-+##'), then `back'.  '''Icicles''' replaces your
input by `##\(for\|back\)##'.  You can continue with additional
alternative match patterns.  And you can combine narrowing with
widening, that is, progressive with regressive completion.

You can of course [[Icicles - Cycling Completions|cycle]] among all matching candidates, regardless
of which alternative they match.  One use of regressive completion
is with '''[[Icicles - Search Commands, Overview|Icicles search]]''' -- it corresponds to the OR searching of
common search engines.

'''Gotcha:''' When completing file names that are not absolute
(e.g. using `C-x C-f', not `C-u C-x C-f'), be sure that the
default directory is not included in your minibuffer input when
you hit `##M-+##'.  You do not want the overall regexp that `##M-+##'
constructs to be something like ##\(/my/default/dir/foo\|bar\)## -- you
want it to be just ##\(foo\|bar\)##.  For absolute file name
completion there is no such problem, because the completion  candidates themselves have a directory component.  So either use a
`nil' value of `insert-default-directory' or use `M-k' to remove the directory component before hitting `##M-+##'.


'''See Also:''' [[Icicles - Candidate Sets#RecompletingUsingTheOriginalDomain|Recompleting Using the Original Domain]] for how to
widen back to the original set of candidates.










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DrewsElispLibraries referenced here: Lisp:icicles.el


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