This property returns the text matched by the last sub-expression which was part of the most recent match. It will contain a null or undefined value if there was no previous match.
Since this is a class property (a static property), it belongs to the global built-in RegExp object. This means it is shared by all RegExp object instances and therefore is very transient and will be overwritten as soon as the next regular expression is evaluated. If you want to preserve the value, you will need to copy it immediately after your regular expression has been evaluated and before you call another.