Multiple Replacements with query-replace

Contents

• Motivation
• The solution
  • The code

As its name suggests, Emacs’s query-replace function, bound to M-% by default, can be used to replace occurences of one string with another—and it’s quite good at what it does. However, there is one crucial feature missing from its default functionality: the ability to create multiple from → to pairs. But this is Emacs, after all, which means that I can write that query-replace-many function I’ve always wanted, and even share it with others! The code is packaged as query-replace-many, available on GitLab and GitHub.

I quite like the workflow that query-replace offers. In comparison to other tools that are used for similar purposes—keyboard macros and multiple-cursors—the whole process after entering the from and to strings is interactive all the way through: it’s quite fast to step through the individual matches and decide whether one would like to replace them or not. In true Emacs fashion, the function also takes way too many arguments: among other things, it can operate on the current region, backwards, or only on things surrounded by words boundaries.

Motivation§

Originally, my motivation came through work, where I write a lot of LaTeX. When polishing papers, it sometimes happens that I would like to change or unify the notation of certain objects in the current equation/environment/file.

When an alteration like this only encompasses a single action, like switching T to H, a simple query-replace after narrowing to the current region of interest is totally sufficient. For others, like changing T to H and S to G, this solution, along with multiple-cursors and other tools people usually go for, would already be unsatisfactory—the whole region would need to be traversed twice. Now imagine that you want to change T to U and U to T: chaos! Save having to give some sort of temporary name to one of the objects, which would be even slower, query-replace is quite useless in this situation. It’s possible to cook up a manual solution using the alternative query-replace-regexp function and capture groups, but I’m unsure how many people know their elisp regular expressions well enough for that to be time efficient. I don’t, and almost certainly never will, so it seemed much easier to automate this instead!

The solution§

Thankfully, since replace.el sports a decent api, writing a version of query-replace that accepts multiple arguments turns out to be easy enough. The high-level overview is this: we read in multiple queries until an empty input is given, build up a regular expression of the form "\\(?:query-1\\|query-2\\|…\\)", and—when it comes to replacing—test the current thing to be replaced against all of the queries to select the correct one.

The beauty of this is that, since it’s really just a thin wrapper over two functions from replace.el that do the heavy lifting, all of the modules regular functionality, like the keybindings and history, just work.

For example, in the following I replace T with U and, at the same time, U with T. The first few matches are stepped through and the rest is just accepted wholesale. At the bottom, you can see the default query-replace interface when interacting with the query.

The only cosmetic imperfection of this is that, while the replacement candidate itself is correctly updated, we see the whole regular expression \(?U:\|T\) as the thing to be replaced instead of the bit that’s actually matching currently. However, since this would seem to require some work and one of course sees what’s to be replaced by looking at the thing at point, I can live with this for the moment.

The code§

As one might imagine, the code is actually quite straightforward—it only consists of two functions! The first one is a little helper, querying the user for multiple pairs.

The actual query-replace-many function now just reads some pairs from the user by virtue of the above function, and then calls perform-replace with an appropriately generated regular expression.

And that’s it! As an aside, calling query-replace-many also works from lisp; (query-replace-many '(("1" . "2") ("2" . "1"))) has exactly the effect one would imagine it to have. As I said, everything is conveniently packaged up on GitLab and GitHub—get it while it’s hot!