In Java, regular expressions are supported through the java.util.regex package. The java.util.regex package contains two main classes: Pattern and Matcher.

Here's an example of how to use regular expressions in Java:

import java.util.regex.*;

public class Example {
    public static void main(String[] args) {
        String input = "Hello World!";
        String pattern = "l+";

        Pattern p = Pattern.compile(pattern);
        Matcher m = p.matcher(input);

        while (m.find()) {
            System.out.println("Match found at index " + m.start() + " - " + m.group());
        }
    }
}

In this example, we create a regular expression pattern that matches one or more "l" characters. We then use the Pattern class to compile the pattern and create a Matcher object to search for the pattern in a given input string. We use the find() method of the Matcher class to search for all occurrences of the pattern in the input string. Finally, we print out the start index and the matching substring for each match.