How would I implement a bare bones function that 're-evaluates' with every key-press? I mean key-press loosely; I'm looking to emulate the behavior of isearch and friends.
I would be using this to interactively test XPath expressions against an open nxml-mode buffer using something along the lines of buffer narrowing. For the purposes of this question though, just a function that calls message on whatever is in the minibuffer would be absolutely grand.
Depending of what you want, you could cheat and use isearch-mode: its third argument (OP-FUN) is a function called after each input is processed, and its 5th argument (WORD) can be any function that will transform the string typed in isearch into a regexp to look for in the buffer.
For exemble:
(defun my-op-fun (string &optional lax)
(let ((words (split-string string ":")))
(mapconcat '(lambda (word)
(concat "\\<" (regexp-quote word)))
words "\\>.*")))
(defun my-isearch-beggining-of-line ()
(interactive)
(isearch-mode t t nil t 'my-op-fun))
if you type "foo" it will look for a word beginning with "foo", it you type "foo:bar" it will look for the "foo" word followed by some other char then "bar" as the beggining of a word.
Some regexp mastery might be needed to make this work for you.
You could also just look at the source of isearch to see how it is done.
