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re-builder: the Interactive regexp builder

Even if you're a wizard at regular expressions, you may occasionally want a little help. Emacs's regexp builder highlights matches and capturing groups in a buffer of your choice, and it works with a range of regular expression flavors.
Updated for Emacs 28
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I doubt it’s a well-kept secret that Emacs has a regexp “helper” called M-x re-builder. But if you haven’t heard about it before, Emacs’s re-builder lets you interactively build a regular expression and see what it matches on the screen. It’ll even uniquely color capturing groups so you can tell them apart.

Syntax

What most people don’t know is re-builder’s support for different syntax; but, sadly, not PCRE – sorry!

There are five different syntax choices (see table below). You can either use customize (M-x customize-variable RET reb-re-syntax RET) or set the variable (reb-re-syntax) directly.

read

Default. Similar to string but requires “double escaping” of backslashes like you would be required to do in elisp. Example: \\\\(foo\\\\\|bar\\\\)

string

Recommended. Similar to read but you don’t have the issue of backslash plague that haunts the default settings. Example: \\(foo\\\|bar\\)

sregex

Deprecated. A symbolic regular expression engine that uses s-expressions instead of strings.

lisp-re

Deprecated. Yet another regular expression engine that uses s-expressions

rx

Recommended. A third, and far more advanced, s-expression regexp engine. Use this and not sregex or lisp-re if you want to use a lisp-style regexp engine.

The backslash madness

I recommend you switch to string right away; there’s little reason to use read, and the extra escaping will drive you insane unless you’re used to writing regexp in elisp.

Add this to your .emacs to switch the default syntax to string:

(require 're-builder)
(setq reb-re-syntax 'string)

Useful Keybinds

If you do write a lot of elisp, you probably use (or should use!) rx to make your regexp experience in Emacs a bit more pleasant. Unfortunately, you can only have one default setting at a time so you have to switch manually with C-c TAB in re-builder.

You can enter the sub-expression mode with C-c C-e to only highlight capturing groups; you can toggle the case sensitivity with C-c C-i; and you can move between matches with C-c C-s and C-c C-r.

The re-builder keybind C-c C-w bears mention as well: it will copy (and convert, where applicable) the expression to a string format suitable for use in elisp.

Further Reading

Have you read my Reading Guide yet? It's a curated guide to most of my articles, and I guarantee you'll learn something whether you're a beginner or an expert. And why not check out my book?

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