DocumentFilter.insertString method documentation says:

Invoked prior to insertion of text into the specified Document. Subclasses that want to conditionally allow insertion should override this and only call supers implementation as necessary, or call directly into the FilterBypass

This is misleading, as the expected behavior is, replace method will be called prior to insertion of text into the Document. Conditional insertion can be done by over-riding only the replace method and not the insertString method.