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(emacs)Replacement and Case

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Replace Commands and Case
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   If the first argument of a replace command is all lower case, the
command ignores case while searching for occurrences to
replace--provided `case-fold-search' is non-`nil'.  If
`case-fold-search' is set to `nil', case is always significant in all
searches.

   In addition, when the NEWSTRING argument is all or partly lower
case, replacement commands try to preserve the case pattern of each
occurrence.  Thus, the command

     M-x replace-string <RET> foo <RET> bar <RET>

replaces a lower case `foo' with a lower case `bar', an all-caps `FOO'
with `BAR', and a capitalized `Foo' with `Bar'.  (These three
alternatives--lower case, all caps, and capitalized, are the only ones
that `replace-string' can distinguish.)

   If upper-case letters are used in the replacement string, they remain
upper case every time that text is inserted.  If upper-case letters are
used in the first argument, the second argument is always substituted
exactly as given, with no case conversion.  Likewise, if either
`case-replace' or `case-fold-search' is set to `nil', replacement is
done without case conversion.