Regular Expression to match string starts with specific substring using C# and VB.Net in ASP.Net

Hey rakibxl,

Please refer below sample.

HTML

<div>
    <asp:TextBox runat="server" ID="txtId" />
    <asp:Button Text="Check" runat="server" OnClick="Check" />
</div>

Namespaces

C#

using System.Text.RegularExpressions;

 VB.Net

Imports System.Text.RegularExpressions

Code

C#

protected void Check(object sender, EventArgs e)
{
    Regex pattern = new Regex(@"^[I-T]");
    if (!pattern.IsMatch(txtId.Text))
    {
        ScriptManager.RegisterStartupScript(this, this.GetType(), "Message", "alert('input must Start with IT.');", true);
    }
}

VB.Net

Protected Sub Check(ByVal sender As Object, ByVal e As EventArgs)
    Dim pattern As Regex = New Regex("^[I-T]")
    If Not pattern.IsMatch(txtId.Text) Then
        ScriptManager.RegisterStartupScript(Me, Me.[GetType](), "Message", "alert('input must Start with IT.');", True)
    End If
End Sub

Screenshot