RE Builder “query replace this”

I use M-x re-builder a lot. Its an interactive regular expression builder that really helps build proper regexps and is an excellent learning tool. However, in using it, I found myself repeating a particular workflow:

• build regexp in re-builder
• copy it
• run query-replace-regexp
• paste regexp as the search
• type in the replacement
• press return and off you go

This was a lot of repeated work, so I wrote a function, which I bind to a key in reb-mode-map (C-c M-%, since plain old M-% is query-replace) which can be run in the re-builder buffer to automatically search the target buffer (the one that re-builder matches as you build) for the regexp you built, and replace it with a string, which is the only prompted argument of the function.

That may all sound complication, but its not. The workflow becomes:

• build regexp in re-builder
• run reb-query-this-regexp (C-c M-%)
• type in the replacement
• press return and off you go

Here is the code:

(defun reb-query-replace-this-regxp (replace)
  "Uses the regexp built with re-builder to query the target buffer.
This function must be run from within the re-builder buffer, not the target
buffer.

Argument REPLACE String used to replace the matched strings in the buffer.
 Sub-expression references can be used (\1, \2, etc)."
  (interactive "sReplace with: ")
  (if (eq major-mode 'reb-mode)
      (let ((reg (reb-read-regexp)))
        (select-window reb-target-window)
        (save-excursion
          (beginning-of-buffer)
          (query-replace-regexp reg replace)))
    (message "Not in a re-builder buffer!")))

(define-key reb-mode-map "\C-c\M-%" 'reb-query-replace-this-regxp)