Back reference is the term used when a capturing group is referenced after we capture it. Likewise, a forward reference is the term for referencing a group before capturing the text using a captured group. In other words, the forward reference refers to a capturing group that appears later in the regular expression pattern. Forward references are only useful if we use them inside a repeated group, since the regex engine will be able to resolve captured groups and populate the forward reference while repeating a match using quantifiers.
As with most modern languages, Java allows us to use the forward reference of the captured group. Forward reference is a fairly complex concept to understand in regex.
Let's consider an example ...
