In Java, regular expressions are implemented through various classes and interfaces provided in the java.util.regex package. These classes and interfaces enable developers to work with regular expressions for pattern matching, searching, and manipulation. Here are the key classes and interfaces:

1. Pattern Class:

• The Pattern class represents a compiled regular expression pattern.
  It provides static methods for compiling regular expressions into Pattern objects.
• Allows for pattern matching, searching, splitting, and replacing strings based on the pattern.
• Methods include compile(), matcher(), matches(), split(), and replaceAll().

2. Matcher Class:

• The Matcher class represents the engine that performs match operations on a character sequence by interpreting the compiled pattern.
• It provides methods for matching the pattern against input strings, finding occurrences, extracting groups, etc.
• Methods include find(), group(), start(), end(), matches(), and replaceAll().

3. MatchResult Interface:

• The MatchResult interface represents the result of a match operation performed by a Matcher object.
• It provides methods for retrieving information about the match, such as the start and end indices of matched groups.
• It is Implemented by the Matcher class.

4. PatternSyntaxException Class:

• The PatternSyntaxException class represents an unchecked exception that indicates syntax errors in a regular expression pattern.
• It is thrown by the Pattern.compile() method if the specified regular expression pattern contains syntax errors.
• These classes and interfaces form the core of Java’s regular expression API and provide the necessary functionality for working with regular expressions. They allow developers to define complex patterns, search for matches within strings, extract matched substrings, and perform various string manipulation tasks based on the specified patterns.

Here’s a brief example demonstrating the use of these classes and interfaces:

import java.util.regex.*;

public class RegexExample {
public static void main(String[] args) {
String text = "The quick brown fox jumps over the lazy dog";
String pattern = "\\b\\w{4}\\b"; // Match words with 4 characters

// Compile the regular expression pattern
Pattern regex = Pattern.compile(pattern);

// Create a matcher object
Matcher matcher = regex.matcher(text);

// Find and print all matches
while (matcher.find()) {
System.out.println("Match found: " + matcher.group());
}
}
}

In this example, we compile a regular expression pattern to match words with exactly four characters. We then create a Matcher object to search for occurrences of this pattern within the input text. The find() method of the Matcher class is used to find all matches, and the group() method is used to retrieve the matched substrings.