The syntax for the back reference of a named group is as follows:

    k<group1> 

Here, group1 is the name of the named capturing group.

For example, we can write our regular expression of repeating numbers, using a named group and named back reference, as follows:

    ^(?<matchedDigits>d+)s+k<matchedDigits>$ 

Here, we are defining a captured group called num to capture a number using the first, and then, we are using a back reference of the named group using k<num>.

This will match inputs such as 1234 1234 or 989 989.

Since the named capturing groups are numbered automatically as well, we can write the same regular expression as follows:

    ^(?<num>d+)s+1$