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34.3.3 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 regexp18 that matches a block comment in the C programming language: 

(rx "/*"                          ; Initial /*
    (zero-or-more
     (or (not (any "*"))          ;  Either non-*,
         (seq "*"                 ;  or * followed by
              (not (any "/")))))  ;  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.

• Rx Constructs:  Constructs valid in rx forms.
• Rx Functions:  Functions and macros that use rx forms.
• Extending Rx:  How to define your own 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.