<p>It is documented in <a href="https://www.geany.org/manual/current/index.html#regular-expressions" rel="nofollow">the manual</a>:</p>
<table>
<thead>
<tr>
<th>\n</th>
<th>Where n is 1 through 9 refers to the first through ninth tagged region when searching or replacing. Searching for (Wiki)\1 matches WikiWiki. If the search string was Fred([1-9])XXX and the replace string was Sam\1YYY, when applied to Fred2XXX this would generate Sam2YYY.</th>
</tr>
</thead>
</table>
<p>Maybe it needs to be called-out more, since the GLib docs don't do a good job either.</p>

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