Back References or sometimes called groups could be created using () and later be referred by \{digit} depending on the programming languages. Usually, the whole matched pattern will be grouped as group 0.
Vim
In Vim’s regexp, we could refer to the group by using the syntax \[0-9]. The maximum groups in a pattern is 10. There is no mechanism, at least from what I know, to support more than 10 regexp groups.
C++
Back references could be accessed via regex_iterator from regex object. From there, we could iterate over the matched groups from the regex.
In order to not create a back reference, we could use the syntax (?:).
Rust
In order to get the back references from Regex object, we need to obtain a Capture object from Regex method captures. From Capture, we use the method get to get the particular group as a Match object.
Note: Remember to unwrap or map them to desirable type.