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35.3.3 Therx 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.
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