+performs the inverse transformation. To include a @samp{\} in the
+text to replace with, you must enter @samp{\\}.
+
+ If you want to enter part of the replacement string by hand each
+time, use @samp{\?} in the replacement string. Each replacement will
+ask you to edit the replacement string in the minibuffer, putting
+point where the @samp{\?} was.
+
+ The remainder of this subsection is intended for specialized tasks
+and requires knowledge of Lisp. Most readers can skip it.
+
+ You can use Lisp expressions to calculate parts of the
+replacement string. To do this, write @samp{\,} followed by the
+expression in the replacement string. Each replacement calculates the
+value of the expression and converts it to text without quoting (if
+it's a string, this means using the string's contents), and uses it in
+the replacement string in place of the expression itself. If the
+expression is a symbol, one space in the replacement string after the
+symbol name goes with the symbol name, so the value replaces them
+both.
+
+ Inside such an expression, you can use some special sequences.
+@samp{\&} and @samp{\@var{n}} refer here, as usual, to the entire
+match as a string, and to a submatch as a string. @var{n} may be
+multiple digits, and the value of @samp{\@var{n}} is @code{nil} if
+subexpression @var{n} did not match. You can also use @samp{\#&} and
+@samp{\#@var{n}} to refer to those matches as numbers (this is valid
+when the match or submatch has the form of a numeral). @samp{\#} here
+too stands for the number of already-completed replacements.
+
+ Repeating our example to exchange @samp{x} and @samp{y}, we can thus
+do it also this way:
+
+@example
+M-x replace-regexp @key{RET} \(x\)\|y @key{RET}
+\,(if \1 "y" "x") @key{RET}
+@end example
+
+ For computing replacement strings for @samp{\,}, the @code{format}
+function is often useful (@pxref{Formatting Strings,,, elisp, The Emacs
+Lisp Reference Manual}). For example, to add consecutively numbered
+strings like @samp{ABC00042} to columns 73 @w{to 80} (unless they are
+already occupied), you can use
+
+@example
+M-x replace-regexp @key{RET} ^.\@{0,72\@}$ @key{RET}
+\,(format "%-72sABC%05d" \& \#) @key{RET}
+@end example