import java.util.*;
import java.util.regex.*;
class KW
{
    public static void main(String args[])
    {
        String s;
        Scanner sc=new Scanner(System.in);
        System.out.println("____________________________________");
        System.out.println("Implementation of Regular Expression");
        System.out.println("____________________________________");
        System.out.print("Enter a number ");
        s=sc.nextLine();
        if(Pattern.matches("\\d*",s))
        {
            System.out.println("\nNumber accepted");
        }
        else
        {
            System.out.println("\nError: number unacceptable");
        }
        System.out.println("____________________________________");
    }       
}

kodingwindow@kw:~$ javac KW.java
kodingwindow@kw:~$ java KW
____________________________________
Implementation of Regular Expression
____________________________________
Enter a number KW

Error: number unacceptable
____________________________________

kodingwindow@kw:~$ java KW
____________________________________
Implementation of Regular Expression
____________________________________
Enter a number 12

Number accepted
____________________________________
kodingwindow@kw:~$ 

import java.util.*;
import java.util.regex.*;
class KW
{
    public static void main(String args[])
    {
        String s;
        Scanner sc=new Scanner(System.in);
        System.out.println("____________________________________");
        System.out.println("Implementation of Regular Expression");
        System.out.println("____________________________________");
        System.out.print("Enter your 6-digit PIN code ");
        s=sc.nextLine();
        if(Pattern.matches("\\d{6}",s))
        {
            System.out.println("\nCongratulations! your PIN code accepted successfully.");
        }
        else
        {
            System.out.println("\nPlease enter digits only");
        }
        System.out.println("____________________________________");
    }       
}

kodingwindow@kw:~$ javac KW.java
kodingwindow@kw:~$ java KW
____________________________________
Implementation of Regular Expression
____________________________________
Enter your 6-digit PIN code -23566

Please enter digits only
____________________________________

kodingwindow@kw:~$ java KW
____________________________________
Implementation of Regular Expression
____________________________________
Enter your 6-digit PIN code 400099

Congratulations! your PIN code accepted successfully.
____________________________________
kodingwindow@kw:~$