Regular expression for mobile number with fix country code

=>Web API
using System.ComponentModel.DataAnnotations;

[RegularExpression(@"^(91)?\d{10}$", ErrorMessage = "invalid country code or mobile no.")]   

public string mobileno { get; set; }

=>C Sharp

using System.Text.RegularExpressions;

string regex = @"^(91)?\d{10}$";

string inputString = "919876543210";

Match match = Regex.Match(inputString, regex);

=>Java Script

<script>

    function Validate() {

        var text = document.getElementById("TextBox1").value;

        var pattern = "^(91)?\d{10}$";

        var result = text.match(pattern);

        if (!result)

            alert("invalid country code or mobile no.");

    }

</script>

<asp:TextBox ID="TextBox1" runat="server" ClientIDMode="Static"></asp:TextBox>

<asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="return Validate();" />

Note:- Input string only valid when mobile no passed with Indian country code i.e. 91.