Rx Notation (GNU Emacs Lisp Reference Manual)

35.3.3 The rx Structured Regexp Notation

As an alternative to the string-based syntax, Emacs provides the structured rx notation based on Lisp S-expressions. This notation is usually easier to read, write and maintain than regexp strings, and can be indented and commented freely. It requires a conversion into string form since that is what regexp functions expect, but that conversion typically takes place during byte-compilation rather than when the Lisp code using the regexp is run.

Here is an rx regexp²⁷ that matches a block comment in the C programming language:

(rx "/*" ; Initial /*
 (zero-or-more
 (or (not "*") ; Either non-*,
 (seq "*" ; or * followed by
 (not "/")))) ; non-/
 (one-or-more "*") ; At least one star,
 "/") ; and the final /

or, using shorter synonyms and written more compactly,

(rx "/*"
 (* (| (not "*")
 (: "*" (not "/"))))
 (+ "*") "/")

In conventional string syntax, it would be written

"/\\*\\(?:[^*]\\|\\*[^/]\\)*\\*+/"

The rx notation is mainly useful in Lisp code; it cannot be used in most interactive situations where a regexp is requested, such as when running query-replace-regexp or in variable customization.

• Constructs in rx regexps
• Functions and macros using rx regexps
• Defining new rx forms

Footnotes

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It could be written much simpler with non-greedy operators (how?), but that would make the example less interesting.