Named groups for query-replace-regexp

Question

I'd like to define a function to do a query-replace-regexp with a pattern I often use. When not using capture groups, it works:

(defun my-replace ()
    (interactive)
    (query-replace-regexp "[[:nonascii:][:ascii:]]*xxx" "replacedText" nil (point-min) (point-max)))

But I couldn't figure out how to use capture groups -- neither '\(\)' nor '()' works:

(defun my-replace ()
  (interactive)
  (query-replace-regexp "([[:nonascii:][:ascii:]]*)xxx" "\1replacedText" nil (point-min) (point-max)))


(defun my-replace ()
    (interactive)
    (query-replace-regexp "\([[:nonascii:][:ascii:]]*\)xxx" "\1replacedText" nil (point-min) (point-max)))

But when invoked with M-x query-replace-regexp, \([[:nonascii:][:ascii:]]*\)xxx would work.

Answer 1

In Emacs Lisp string literals, backslash is an escape character. To express a single literal backslash character, as required by many regular expression constructs, you need two backslashes, eg:

"\\([[:nonascii:][:ascii:]]*\\)xxx"

You don't need the extra backslashes when you run query-replace-regexp interactively because you're not entering a string literal, ie, an Emacs Lisp language construct; you're entering the characters of the regexp directly.

Try hitting M-: (eval-expression) and entering this expression:

"\(foo\)"

You'll see:

"(foo)"

...indicating that an escaped parenthesis in a string literal is the same as an unescaped parenthesis. So the regular expression engine was seeing parentheses without a preceding backslash, which match only themselves; they weren't starting or ending a capturing group as you expected.

Answer 2

Use "\" to quote "\":

(defun my-replace ()
  (interactive)
  (query-replace-regexp "\\([[:nonascii:][:ascii:]]*\\)xxx" "\\1replacedText"
                        nil (point-min) (point-max)))