Parentheses in regular expression (RegEx) can be used for grouping expression and can be named. To name it, after the opening parenthesis follows it with the following:

 ?<name>

Within the same expression you can backreference the group using the following:

\k<name>

On the replace field, based on the group position it will be numbered starting from 1 and increasing from the left of the expression and must be preceded with dollar sign (i.e. $). Use this number to access the captured match.

Note: if your group exceeds a single digit use ${<nn>} (e.g. ${10}) notation.

Example

<?xml version="1.0"?>
<fruits>
 <a>apple</a>
 <b></b>
 <c>cashew</c>
 <d></d>
</fruits>

From the XML above find all the empty elements and add an attribute empty that is set to true.

Find Replace Comment
<(?<tag>\w*[^>])></\k<tag>> <$1 empty="true"></$1> The named group (i.e. blue text) is assigned to $1.

The \k<tag> (i.e. green text) is the backreference.