This is the third article in a small series inspired by my recent cleanup of Prelude and my personal Emacs configuration, following the ones on repeat-mode and read-extended-command-predicate. I’ve been going through the Emacs 28-30 changelogs for features I had ignored so far, and this one from Emacs 30 immediately caught my eye.

Writing Emacs regexps has always been a bit of a dark art. Between the double-escaped backslashes and the various group syntaxes (\(...\), \(?:...\), \(?N:...\)), it’s easy to lose track of what you’re actually matching. You type something into query-replace-regexp, press RET, and hope for the best.

Emacs 30 added minibuffer-regexp-mode, a minor mode that gives you live visual feedback as you compose a regexp in the minibuffer:

(minibuffer-regexp-mode 1)

When active, the mode highlights the structure of your regexp as you type it in the minibuffer. Capture groups, character classes, and other constructs get color-coded so you can see at a glance whether your grouping is right.

I find this particularly useful when building a regexp with multiple groups for query-replace-regexp, where you need to get the group numbering right for the replacement string (e.g., \1, \2). The visual feedback makes it obvious which group is which.

How Does This Compare to re-builder?

You might be wondering how this compares to re-builder (M-x re-builder). They’re complementary, really. re-builder shows matches in the buffer as you type a regexp in a dedicated editing window – great for developing complex patterns against actual text. minibuffer-regexp-mode, on the other hand, highlights the regexp itself in the minibuffer. It kicks in automatically whenever you’re prompted for a regexp (e.g., isearch-forward-regexp, query-replace-regexp, keep-lines, etc.).

One helps you see what your regexp matches; the other helps you see what your regexp says. I’d suggest using both.

That’s all I have for you today. Keep hacking!