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