Here is an example of how to use the RegexpClassNameMatcher class from the Apache Commons IO library in Java:

import org.apache.commons.io.filefilter.RegexpClassNameMatcher;
import java.io.File;
import java.io.FileFilter;

public class Example {
    public static void main(String[] args) {
        FileFilter filter = new RegexpClassNameMatcher("^java.util.*");
        File dir = new File("src/main/java");
        File[] files = dir.listFiles(filter);
        for (File file : files) {
            System.out.println(file.getName());
        }
    }
}

This example creates an instance of RegexpClassNameMatcher with a regular expression "^java.util.*" and applies it to a directory containing java files to filter the class name that match the regular expression.

The RegexpClassNameMatcher is a implementation of FileFilter that filters files by matching their class name against a regular expression. It returns all files whose class name matches the regular expression.

Please note that this class is not thread-safe.