The Regular Expression is an API to define pattern for searching or manipulating strings.

Java Regex API provides 1 interface and 3 classes in java.util.regex package i.e.

1.    MatchResult interface

2.    Matcher class

3.    Pattern class

4.    PatternSyntaxException class

The Matcher and Pattern classes are widely used in java regular expression.

Matcher Class:

It implements MatchResult interface. It is a regex engine i.e. used to perform

match operations on a character sequence.

Pattern Class:

It is the compiled version of a regular expression. It is used to define a pattern for the regex engine.

For Example, we see the implementation of java regex with the help of following example 

import java.util.regex.*;  
public class RegexEx{  
public static void main(String
args[]){  
//First way 
Pattern p =
Pattern.compile(".s");//compiles the given regex and return the
instance of pattern.
Pattern q =
Pattern.compile("a.");
//. represents single character in Regular
Expression 
 
Matcher m = p.matcher("as"); //creates a matcher that matches the given input with pattern. 
boolean b = m.matches();/*It compiles the regular expression and matches the given input with the pattern.  */ 
  
//Second  way  
 boolean b2=Pattern.matches("[amn]", "a");//true (among a or m or n)    
//Third way  boolean b3 = Pattern.matches("[amn]", "a");  
boolean b4 = Pattern.matches("a.", "as");
System.out.println(b+" "+b2+" "+b3+" "+b4);
// another operations 
System.out.println("metacharacters d...."); //d means digit  
System.out.println(Pattern.matches("\\d", "abc"));//false (non-digit)  
System.out.println(Pattern.matches("\\d", "1")); //true (digit and comes once)    
System.out.println("metacharacters D....");//D means non-digit    
System.out.println(Pattern.matches("\\D", "abc"));//false (non-digit but comes more than once) 
System.out.println(Pattern.matches("\\D", "m"));//true (non-digit and comes once)
 
System.out.println("metacharacters D with quantifier....");  
System.out.println(Pattern.matches("\\D*", "mak"));//true (non-digit and may come 0 or more times)  
 }
}  

The output of the above program is shown below: