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+ <h4 class="subsection">The rx Structured Regexp Notation</h4>   <p>As an alternative to the string-based syntax, Emacs provides the structured <code>rx</code> 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. </p> <p>Here is an <code>rx</code> regexp<a id="DOCF22" href="#FOOT22"><sup>22</sup></a> that matches a block comment in the C programming language: </p> <div class="example"> <pre class="example">(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 /
+</pre>
+</div> <p>or, using shorter synonyms and written more compactly, </p> <div class="example"> <pre class="example">(rx "/*"
+    (* (| (not "*")
+          (: "*" (not "/"))))
+    (+ "*") "/")
+</pre>
+</div> <p>In conventional string syntax, it would be written </p> <div class="example"> <pre class="example">"/\\*\\(?:[^*]\\|\\*[^/]\\)*\\*+/"
+</pre>
+</div> <p>The <code>rx</code> notation is mainly useful in Lisp code; it cannot be used in most interactive situations where a regexp is requested, such as when running <code>query-replace-regexp</code> or in variable customization. </p> <table class="menu" border="0" cellspacing="0"> <tr>
+<td align="left" valign="top">• <a href="rx-constructs" accesskey="1">Rx Constructs</a>
+</td>
+<td> </td>
+<td align="left" valign="top">Constructs valid in rx forms. </td>
+</tr> <tr>
+<td align="left" valign="top">• <a href="rx-functions" accesskey="2">Rx Functions</a>
+</td>
+<td> </td>
+<td align="left" valign="top">Functions and macros that use rx forms. </td>
+</tr> <tr>
+<td align="left" valign="top">• <a href="extending-rx" accesskey="3">Extending Rx</a>
+</td>
+<td> </td>
+<td align="left" valign="top">How to define your own rx forms. </td>
+</tr> </table><div class="_attribution">
+  <p class="_attribution-p">
+    Copyright &copy; 1990-1996, 1998-2022 Free Software Foundation, Inc. <br>Licensed under the GNU GPL license.<br>
+    <a href="https://www.gnu.org/software/emacs/manual/html_node/elisp/Rx-Notation.html" class="_attribution-link">https://www.gnu.org/software/emacs/manual/html_node/elisp/Rx-Notation.html</a>
+  </p>
+</div>