This subsection describes the behavior of replace commands with respect to lax matches (see Lax Search) and how to customize it. In general, replace commands mostly default to stricter matching than their search counterparts.
Unlike incremental search, the replacement commands do not use lax
space matching (see lax space matching) by default.
To enable lax space matching for replacement, change the variable
replace-lax-whitespace to non-
nil. (This only affects
how Emacs finds the text to replace, not the replacement text.)
A companion variable replace-regexp-lax-whitespace controls
query-replace-regexp uses lax whitespace matching when
searching for patterns.
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-
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
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
replacement is done without case conversion.
The replacement commands by default do not use character folding
(see character folding) when looking for the text to
replace. To enable character folding for matching in
replace-string, set the variable
replace-char-fold to a non-
nil value. (This
setting does not affect the replacement text, only how Emacs finds the
text to replace. It also doesn’t affect