Email and Mobile No Validations in asp.net

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeBehind="Default.aspx.cs" Inherits="Test_Website_13_12_2015._Default" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
<script  type="text/javascript" >
    function validateEmail_Ext() {

   
    }

    function validateTxt() {
        var obj = document.getElementById('<%= txtEmail.ClientID %>');
        var regex = /^[a-zA-Z0-9._-]+@([a-zA-Z0-9.-]+\.)+[a-zA-Z0-9.-]{2,4}$/;
        if (regex.test(obj.value)) {
            //You can also assign stylesheet by
            //obj.className='....';
            obj.style.backgroundColor = '';
            obj.style.backgroundColor = '';
            alert("valid Email Address");
        }
        else {
            //Changing Background Color, so that user can understand that its invalid
            //You can also assign stylesheet by
            //obj.className='....';
            alert("please enter valid email");
            obj.style.backgroundColor = '#FD5E53';
            obj.style.borderColor = '#CD4A4A';
            return false;
        }

        var inputtxt = document.getElementById('<%= TxtPhone.ClientID  %>');
        //     var phoneno = /^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{8})$/;
               var phoneno = /^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{8})$/;
  if(inputtxt.value.match(phoneno)) {
      alert("valid Phone Number");
         
     }
   else {
       inputtxt.style.backgroundColor = '#FD5E53';
       inputtxt.style.borderColor = '#CD4A4A';
         alert("Not a valid Phone Number");
       return false;
     }

   return true;

    }
</script>
</asp:Content>

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">

    <h2>
        Welcome to ASP.NET!
    </h2>
    <p>
        To learn more about ASP.NET visit <a href="http://www.asp.net" title="ASP.NET Website">www.asp.net</a>.
    </p>
    <p>
        You can also find <a href="http://go.microsoft.com/fwlink/?LinkID=152368&amp;clcid=0x409"
            title="MSDN ASP.NET Docs">documentation on ASP.NET at MSDN</a>.
    </p>

   <asp:TextBox ID='txtEmail' runat="server" placeholder="demo@demo.com"></asp:TextBox>
   <br />
   <asp:TextBox ID='TxtPhone' runat="server" placeholder="022-252-12345678"></asp:TextBox>
   <br />
<%--<asp:Button id="cmdTest" runat="server" Text="Check Email Address" OnClientClick="validateEmail();" /><br />--%>
<asp:Button id="cmdTest_Ext" runat="server" Text="Save" OnClientClick="return validateTxt();" />

<p>

</p>
</asp:Content>