Evaluate the replacement text.
This method will be called for every single "visited" place
in the text under work.
The content of this visited place will be referred to by (max.)
- opening brace (pattern),
- the found text (itself),
- closing brace (pattern).
These can all be null, but (according to contract) at least one,
, should be non null.
The return value of this method is the new text, that shall replace all
(!) three Strings constituting the visited place in the text under
That means, if the original text shall be kept unchanged at the visited
or just return null.
The contract says, that returning null will also induce the caller to
leave the place untouched. The visit then shall not be considered or
counted as a replacement. If you want the visited spot emptied