C#.net Extract numbers fron string

C#.net Extract numbers fron string:
1. add textbox ,label and button to form
2.textbox contain string number means it numbers
3.textbox contain only strings means it returns value "zero"
4.min,max amount validation for done here..


using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

static string ExtractNumbers(string expr)
{
return string.Join(null, System.Text.RegularExpressions.Regex.Split(expr, "[^\\d]"));
}




private void button2_Click(object sender, EventArgs e)
{
string numValue = ExtractNumbers(textBox1.Text);

if (numValue != "")
{
string aString = numValue;
int aInt = int.Parse(aString);

if (aInt >= 100 && aInt <= 40000)
{
label1.Text = Convert.ToString(aInt + ".00");

}
else
{

label1.Text = "Not valid";
}
}

else
{
label1.Text = "0";

}

}
}
}