using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        Regex regex = new Regex(@"^[-+]?[0-9]\d*(\.\d+)?$");

        Console.WriteLine("Allowed  = " + regex.IsMatch("123"));

        Console.WriteLine("Allowed  = " + regex.IsMatch("12.3"));

        Console.WriteLine("Allowed  = " + regex.IsMatch(".123"));

        Console.WriteLine("Allowed  = " + regex.IsMatch("+.123"));

        Console.WriteLine("Allowed  = " + regex.IsMatch("00123"));

        Console.WriteLine("Allowed  = " + regex.IsMatch("1.2.3"));

        Console.WriteLine("Allowed  = " + regex.IsMatch("12..3"));

        Console.WriteLine("Allowed  = " + regex.IsMatch("+123"));

        Console.WriteLine("Allowed  = " + regex.IsMatch("+-1123"));

    }

}