Java program to check whether a given mail id is valid or invalid.

import java.util.regex.*;

class RegexDemo7

{

   public static void main(String[] args) 

   {

      Pattern p=Pattern.compile("[a-zA-Z0-9][a-zA-Z0-9_.]*@[a-zA-Z0-9]+([.][a-zA-Z]+)+");

      Matcher m=p.matcher(args[0]);

      if(m.find() && m.group().equals(args[0]))

      {

        System.out.println("valid mail id");

       }

       else

       {

         System.out.println("invalid mail id");

        }

    }

}

Output

1st run : 

java RegexDemo7  trilochan4u@gmail.com

valid mail id

2nd run : 

java RegexDemo7  trilochan4u

nvalid mail id

   

Java program to check whether a given number is a valid mobile number or not.

import java.util.regex.*;
class RegexDemo6
{
   public static void main(String[] args)
    {
        Pattern p=Pattern.compile("(0/91)?[7-9][0-9]{9}");
        Matcher m=p.matcher(args[0]);
        if(m.find() && m.group().equals(args[0]))
        {
System.out.println("valid mobile number");
         }
        else
        {
System.out.println("invalid number");
         }
      }
   }

Output 

                         1st run : java RegexDemo6  9439202111
                         valid mobile number
                         2nd run : java RegexDemo6  929291929394
                         invalid number

Java program to check whether a given string is palindrome or not without taking reverse.

import java.util.*;
public class StringPalindrome {

public static void main(String[] args)
{
String str;
int l,c=0;
Scanner s= new Scanner(System.in);
System.out.println("enter a string");
str= s.nextLine();
l=str.length();
for(int i=0;i<(l-1)/2; i++)
{
if(str.charAt(i)==str.charAt(l-1-i))
{
continue;
}
else
{
c++;
}
}
if(c==0)
System.out.println("the given string is palindrome");
else
System.out.println("the given string is not palindrome");

}

}

Output
1st Run:
enter a string
katak
the given string is palindrome

2nd Run:
enter a string
Silan
the given string is not palindrome