In this article I will explain with an example, how to perform GST Number validation on Server Side using Data Annotation in ASP.Net Core MVC.
The GST Number validation will be performed using Regular Expression (Regex) in ASP.Net Core MVC.
The following Model class consists of one property GSTNumber. The property is decorated with the following Data Annotation attributes for performing validations.
1. Required Data Annotation attribute.
2. RegularExpression Data Annotation attribute.
Note: The Data Annotations attributes can be used with the Entity Data Model (EDM), LINQ to SQL, and other data models.
The Required Data Annotation and the RegularExpression Data Annotation attributes have been specified with a property ErrorMessage with a string value. As the name suggests, this string value will be displayed to the user when the respective validation fails.
The RegularExpression Data Annotation attribute has been set with a Regular Expression for validating GST Number.
The following conditions must satisfy for a GST Number to be termed as valid.
1. It should be 15 characters long.
2. The first 2 characters should be a digit representing State Code.
3. The next 10 characters should be PAN number of the taxpayer.
Note: For more details on PAN Card Number validation, please refer my article ASP.Net Core MVC: Server Side PAN Card Number validation using Data Annotations and Regular Expressions.
4. The 13th character should be any digit (1-9) or an alphabet.
5. The 14th character should be Z by default.
6. The 15th character should be an alphabet or any digit. 0-9.
Example: 06BZAHM6385P6Z2
using System.ComponentModel.DataAnnotations;
namespace GSTNumber_Validation_ServerSide_MVC_Core.Models
    public class PersonModel
        [Display(Name = "GST Number:")]
        [Required(ErrorMessage = "Invalid GST Number.")]
        [RegularExpression(@"^[0-9]{2}[A-Z]{5}[0-9]{4}[A-Z]{1}[1-9A-Z]{1}Z[0-9A-Z]{1}$", ErrorMessage = "Invalid GST Number.")]
        public string GSTNumber { get; set; }
The Controller consists of following two Action methods.
Action method for handling GET operation
Inside this Action method, simply the View is returned.
Action method for handling POST operation
This Action method handles the POST operation and when the form is submitted, the object of the PersonModel class is sent to this method.
The state of the submitted Model is checked using ModelState.IsValid property.
Note: ModelState.IsValid property is an inbuilt property which verifies two things:
1. Whether the Form values are bound to the Model.
2. All the validations specified inside Model class using Data annotations have been passed.
public class HomeController : Controller
    public IActionResult Index()
        return View();
    public IActionResult Index(PersonModel person)
        if (ModelState.IsValid)
            // Validation success.
        return View();
Inside the View, in the very first line the PersonModel class is declared as Model for the View.
The Form
The View consists of an HTML Form which has been created using following ASP.Net Tag Helpers attributes.
asp-action – Name of the Action. In this case the name is Index.
asp-controller – Name of the Controller. In this case the name is Home.
method – It specifies the Form Method i.e. GET or POST. In this case it will be set to POST.
Implementing Validation
The Form consists of a Label, a HTML INPUT TextBox, a SPAN element and a Submit Button.
The TextBox have been set with the following Tag Helpers attributes:-
asp-for –The Model property to which validation will be performed. In this case GSTNumber.
asp-validation-for - Displaying the validation message for the Model property.
When the Submit button is clicked, the Form gets submitted and the GSTNumber value is sent to the Controller.
@model GSTNumber_Validation_ServerSide_MVC_Core.Models.PersonModel
@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
    Layout = null;
<!DOCTYPE html>
    <meta name="viewport" content="width=device-width" />
    <style type="text/css">
        body { font-family: Arial; font-size: 10pt; }
        .error { color: Red; }
        .gst { text-transform: uppercase; }
    <form method="post" asp-controller="Home" asp-action="Index">
        <label asp-for="GSTNumber"></label>
        <input type="text" asp-for="GSTNumber" class="gst" />
        <span asp-validation-for="GSTNumber" class="error"></span>
        <hr />
        <input type="submit" value="Submit" />
ASP.Net Core MVC: Server Side GST (Goods and Services Tax) Number validation using Data Annotations