Replace Commands and Case
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.
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