How Do I Limit Values In A Text Box

// Javascript function to move the text to next line in textbox if it exceeded the limit of maximum charcaters in one line i.e. maxCharInLine 

 <script type="text/javascript">
          function lineLimit(maxCharInLine) {
              var txtDemo = document.getElementById("<%=txtDemo.ClientID%>");
              if (txtDemo.value.lastIndexOf('\n') != -1 || txtDemo.value.length >= maxCharInLine)
              {
                  var trailingText = txtDemo.value.substr(txtDemo.value.lastIndexOf('\n') + 1);
                  if (trailingText.length >= maxCharInLine)
                  {
                      txtDemo.value = txtDemo.value + '\n';
                  }
              }
        }
    </script>

// In declaration of textbox, you have to call above function "lineLimit" on key press and also define the maximum limit of character for one line. Here I have defined 5 as maximum character limit. 

  <asp:TextBox ID="txtDemo" runat="server" TextMode="MultiLine" onkeypress="javascript:lineLimit(5);"></asp:TextBox>

<asp:Label ID="lblMessage" runat="server" ForeColor="Red" EnableViewState="false" />
    <p>
        <asp:TextBox ID="txtDetails" runat="server" TextMode="MultiLine" Columns="50" Rows="5" /></p>
    <asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="SubmitData" />

protected void SubmitData(object sender, EventArgs e)
    {
        // allow only 500 characters
        var maxLength = 50;
        if (txtDetails.Text.Trim().Length > maxLength)
        {
            lblMessage.Text = string.Format("Sorry, only {0} characters are allowed.",
            maxLength);
            return;
        }
        // go ahead and write code to save the data
    }