Replace two strings with each other

Question

I would like to change all occurrences of a string (say string1) with another string (say string2) and vice-versa. To be precise I have a long document and these strings occur many times. What I currently do, is to replace string1-->string3, then string2-->string1 and finally string3-->string2.

I am wondering if there is already a function in emacs which does this. Other solutions are welcomed.

Edit: This is actually a duplicate of the question linked in the comment. But the problem is that the accepted answer of that question does not always work (see the comment below). I think it is better to wait until the author of that accepted answer corrects his/her answer or some other solution comes out. Secondly before posting my question, I googled the question and that question did not show up. I think having two duplicate questions is sometimes useful since they use different wording and if a user find the duplicate one, he/she can find the right one.

Edit 2: The accepted answer of Search/replace-like feature for swapping text seems to be fine now.

Answer 1

For the interactive case query-replace-regexp (C-M-%) can do this, using the relatively unknown \, for the replacement.

C-M-% \(string1\)\|\(string2\)
\,(if (equal \& "string1") "string2" "string1")

If the replacement text contains \, followed by a lisp expression, it uses the value of that expression as the replacement. In this case the expression is an "if" which takes a condition and the return value for the true and false cases.

In the comments it is suggested to use

\,(if \1 "string2" "string1")

This works because \1 will be "string1" if the pattern matches string1, and will be nil otherwise. Lisp takes any non-nil value as true. Whilst this works and is shorter, I think my answer is more useful. I think the code is more readable to someone whose lisp knowledge is limited. It has a second advantage as it still works if the "from" pattern is given as

\(string1\|string2\)

or even

string1\|string2

Answer 2

Well, maybe with a macro this would be easy:

F3C-saRETC-bM-tF4. Using this macro on your question yells this result:

I would like to change all occurrences of string a (say string1) with another string (say string2) and vice-versa. To be precise I have long a document and these strings occur many times. What I currently do, is to replace string1-->string3, then string2-->string1 and finally string3-->string2.

I am wondering if there is already function a in emacs which does this. Other solutions are welcomed.

I'm transposing positions between whatever word is after an a and that word. You can later edit such macro to transpose other words as well without needing to redefine the entire macro, you can edit the macro with C-xC-kRET.