In this article I will explain with an example, how to perform Aadhaar Number validation on Server Side using Data Annotation in ASP.Net Core MVC.
The Aadhaar Number validation will be performed using Regular Expression (Regex) in ASP.Net Core MVC.
The following Model class consists of one property AadhaarNumber. 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 Error Message 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 Aadhaar Number.
The following Aadhaar Number formats will be termed as valid.
1. 12 digits without space. Ex: 123456789012
2. White space after every 4th digit. Ex: 1234 5678 9012
3. Hyphen (-) after every 4th digit. Ex: 1234-5678-9012
using System.ComponentModel.DataAnnotations;
namespace Aadhaar_Number_Validation_ServerSide_MVC_Core.Models
    public class PersonModel
        [Display(Name = "Aadhaar Number:")]
        [Required(ErrorMessage = "Aadhaar Number is required.")]
        [RegularExpression(@"^([0-9]{4}[0-9]{4}[0-9]{4}$)|([0-9]{4}\s[0-9]{4}\s[0-9]{4}$)|([0-9]{4}-[0-9]{4}-[0-9]{4}$)", ErrorMessage = "Invalid Aadhaar Number.")]
        public string AadhaarNumber { 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 AadhaarNumber.
asp-validation-for – Displaying the validation message for the Model property.
When the Submit button is clicked, the Form gets submitted and the AadhaarNumber value is sent to the Controller.
@model Aadhaar_Number_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; }
    <form method="post" asp-controller="Home" asp-action="Index">
        <label asp-for="AadhaarNumber"></label>
        <input type="text" asp-for="AadhaarNumber" />
        <span asp-validation-for="AadhaarNumber" class="error"></span>
        <hr />
        <input type="submit" value="Submit" />
ASP.Net Core MVC: Server Side Aadhaar Number validation using Data Annotations