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 regexp[^27] 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.