replace regexp r by R if cursor is not on r in emacs
Suppose you visit a file F in an emacs buffer B, let r and R be some replacement regular expressions. Now I want to replace all occurrences of strings r_i which matches r (in some region) by the corresponding replacement-string R_i defined by R such that the following conditions are fulfiled:
It would be nice if it would be possible to highlight (eg different color, or underline ...) the replacements and control this highlighting depending on r.
I guess that such a functionality already exists but I don't know how it is called. What I described above is in some way similar to preview mode for editing latex-files.
It looks like you just want some strings displayed differently, not any actual replacement. I think font-lock-mode can handle your requirements. For example, you can have all occurrences of the word "pi" in a buffer displayed as the greek letter π by evaluating this in the buffer:
(font-lock-add-keywords
nil `(("<pi>" (0 `(face default display "π")))))
(push 'display font-lock-extra-managed-props)
Ch v font-lock-keywords will give more details.
I'm not aware of any user-level feature to do this. There's hi-lock-mode which lets you highlight some text but not show a replacement text instead.
The programmer-level feature used by preview mode and other packages that cause a different text to be displayed from what is in the buffer is called overlays. If you want to code this up, it would look something like this: for every occurrence of r, put an overlay on this occurence of r with the original text r_1 as the help-echo property, r_2 as the display property, a modification-hooks property that reacts to any change, plus probably a face or category property.