using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var mobileNumber="09207311668";

        if(Validation.ValidateMobileNumber(mobileNumber))

        {

            Console.WriteLine("success");

        }

        else{

            Console.WriteLine("failed");

        }

    }

     public static class Validation

    {

        public static bool ValidateMobileNumber(string mobileNumber)

        {

            string MatchPhoneNumberPattern = @"[789]\d{9}$";

            var isValidNumber = false;

            mobileNumber = mobileNumber.Trim();

​

            // if length is less than 10 return false

            if (mobileNumber.Length < 10)

            {

                return false;

            }

​

            // check length if greater than 10 try to remove +91 or 0

            else if (mobileNumber.Length > 10)

            {

                mobileNumber = mobileNumber.Substring(mobileNumber.Length - 10, 10);

            }

​

            isValidNumber = Regex.IsMatch(mobileNumber, MatchPhoneNumberPattern);

            return isValidNumber;

        }

    }

}