Here's an example of how to use the RegexFileFilter class from the Apache Commons IO library to filter files based on a regular expression:

import java.io.File;
import org.apache.commons.io.filefilter.RegexFileFilter;

public class Main {
    public static void main(String[] args) {
        // The regular expression to match
        String regex = "file[1-9]\\.txt";

        File dir = new File("/path/to/directory");
        File[] matchingFiles = dir.listFiles((FileFilter) new RegexFileFilter(regex));

        for (File file : matchingFiles) {
            System.out.println(file.getName());
        }
    }
}

In this example, /path/to/directory should be replaced with the path to the directory you want to search through. The listFiles method is called on the dir object, and it takes an instance of RegexFileFilter as an argument. The RegexFileFilter constructor takes a string as an argument, which is the regular expression to match. The filtered matching files are stored in the matchingFiles array, and their names are printed out using a for loop.

You can use any regular expression to match file names, for example you can use the regular expression to select all files that have numbers in the middle of their name

File[] matchingFiles = dir.listFiles((FileFilter) new RegexFileFilter("^.*[0-9].*\\.txt$"));

This will return all files that have numbers in the middle of the name and have the .txt extension.

Note that the regular expression is a powerful tool but you must be careful when using it, it can cause performance issues if the pattern is too complex or the directory has too many files.