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In emacs I'm using M-% to replace occurrences of b_abcd (of which there are only a few) with B_abcd (of which there are very many). Currently Emacs asks if it's OK to replace at each B_abcd, which takes a lot of time to wade through all B_abcds it finds.
How can I make Emacs ask only about replacing b_abcd occurrences?
Bind or set case-fold-search to nil.
C-h k M-% tells us:
M-%runs the commandquery-replace(found inglobal-map), which is an interactive compiled Lisp function inreplace.el.It is bound to
M-%,<menu-bar> <edit> <replace> <query-replace>.
(query-replace FROM-STRING TO-STRING &optional DELIMITED START END BACKWARD REGION-NONCONTIGUOUS-P)Probably introduced at or before Emacs version 1.3.
Replace some occurrences of
FROM-STRINGwithTO-STRING.As each match is found, the user must type a character saying what to do with it. For directions, type
C-hat that time.
...
Matching is independent of case if both
case-fold-searchandsearch-upper-caseare non-nilandFROM-STRINGhas no uppercase letters; ifsearch-upper-caseisnil, then whether matching ignores case depends oncase-fold-searchregardless of whether there are uppercase letters inFROM-STRING. Replacement transfers the case pattern of the old text to the new text, if bothcase-fold-searchandcase-replaceare non-nilandFROM-STRINGhas no uppercase letters. (Transferring the case pattern means that if the old text matched is all caps, or capitalized, then its replacement is respectively upcased or capitalized.)
If your string to replace had uppercase letters and you wanted to always respect case then you would set case-fold-search to non-nil and search-upper-case to nil. In your case the string to replace has no uppercase letters, so it's sufficient to set case-fold-search to non-nil.
You can customize either or both of these variables (options), to make them have the values you want most often by default. That way, you will only need to set or bind them occasionally, for your less frequent uses. To customize them, use M-x customize-option.
Use isearch (C-s), turn on case sensitivity (M-c) then launch replace (M-%) from there.
M-c runs the command isearch-toggle-case-foldM-% runs the command isearch-query-replace