EmacsWiki: String Search And Replace

String search and replace in EmacsLisp can be done in a variety of ways and variations, each can be multiplied again through the use of RegularExpressions.

The EmacsLispManual has an example of search and replace in the buffer using ‘re-search-forward’ and ‘replace-match’ that is an easily recognizable idiom:

     (while (re-search-forward "foo[ \t]+bar" nil t)
       (replace-match "foobar"))

Sometimes, search and replace needs to be done on a string rather than in a buffer.

Note: Whether workin in a buffer is better or worse compared to working with strings is something that depends on the exact circumstances. If in doubt, use the EmacsLispProfiler (‘elp’) to figure out which approach is more efficient. As a rule of thumb, use a buffer (‘with-temp-buffer’) if you’re working on large strings. The reason is that strings are immutable so every operation taking a string and returning a new string will in fact return a copy of the string.

(defun string-search-and-replace (search replace string)
  "Replace all instances of SEARCH with REPLACE in STRING."
  (replace-regexp-in-string (regexp-quote search) replace string t t))

Before Emacs 21, the canonical code to search and replace all instances found in a string was less recognizable.

(defun string-search-and-replace (search replace string)
  "Replace all instances of SEARCH with REPLACE in STRING."
  (let ((quoted-search (regexp-quote search))
	(start 0))
    (while (string-match quoted-search string start)
      (setq string (replace-match replace nil t string))
      (setq start (+ (match-end 0) (- (length replace)
				      (length search)))))
    string))

The above idiom still appears in various places in EmacsLisp libraries with a variety of mutations.