The PatternMatcher interface defines the operations a regular expression matcher must implement. However, the types of the Pattern implementations recognized by a matcher are not restricted. Typically PatternMatcher instances will only recognize a specific type of Pattern. For example, the Perl5Matcher only recognizes Perl5Pattern instances. However, none of the PatternMatcher methods are required to throw an exception in case of the use of an invalid pattern. This is done for efficiency reasons, although usually a CastClassException will be thrown by the Java runtime system if you use the wrong Pattern implementation. It is the responsibility of the programmer to make sure he uses the correct Pattern instance with a given PatternMatcher instance. The current version of this package only contains the Perl5 suite of pattern matching classes, but future ones for other regular expression grammars may be added and users may also create their own implementations of the provided interfaces. Therefore the programmer should be careful not to mismatch classes.