In this article I will explain with an example, how to perform Indian Mobile Number validation on Server Side using Data Annotation in ASP.Net Core Razor Pages.
The Indian Mobile Number validation will be performed using Regular Expression (Regex) in ASP.Net Core Razor Pages.
The following Model class consists of one property MobileNumber. 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 Mobile Number.
The following conditions must satisfy for a Mobile Number to be termed as valid.
1. It should be 10 digits long.
2. The first digit should be a number. 6 to 9.
3. The rest 9 digits should be any number. 0 to 9.
4. It can have 11 digits including 0 at the starting.
5. It can have 12 digits including 91 at the starting.
Examples: 9876543210, 09876543210, 919876543210
using System.ComponentModel.DataAnnotations;
namespace Indian_MobileNumber_Validation_ServerSide_Razor_Core.Models
    public class PersonModel
        [Display(Name = "Mobile Number:")]
        [Required(ErrorMessage = "Invalid Mobile Number.")]
        [RegularExpression(@"^(0|91)?[6-9][0-9]{9}$", ErrorMessage = "Invalid Mobile Number.")]
        public string MobileNumber { get; set; }
Razor PageModel (Code-Behind)
The PageModel consists of following two Handler methods.
Handler method for handling GET operation
This Handler method handles the GET calls, for this particular example it is not required and hence left empty.
Handler method for handling POST operation
This Handler 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 IndexModel : PageModel
    public PersonModel Person { get; set; }
    public void OnGet()
    public void OnPostSubmit(PersonModel person)
        if (ModelState.IsValid)
            // Validation Success.
Razor Page (HTML)
The Form
The HTML of Razor Page consists of an HTML Form which has been created using following ASP.Net Tag Helpers attribute.
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 has been set with the following Tag Helpers attributes:-
asp-for – The Model property to which validation will be performed. In this case MobileNumber.
asp-validation-for – Displaying the validation message for the Model property.
The Submit Button has been set with the POST Handler method using the asp-page-handler attribute. When the Submit button is clicked, the Form gets submitted and the MobileNumber value is sent to the Handler method.
Note: In the Razor PageModel, the Handler method name is OnPostSubmit but here it will be specified as Submit when calling from the Razor HTML Page.
@model Indian_MobileNumber_Validation_ServerSide_Razor_Core.Pages.IndexModel
@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">
        <label asp-for="Person.MobileNumber"></label>
        <input type="text" asp-for="Person.MobileNumber" />
        <span asp-validation-for="Person.MobileNumber" class="error"></span>
        <input type="submit" value="Submit" asp-page-handler="Submit" />
ASP.Net Core Razor Pages: Server Side Indian Mobile Number validation using Data Annotations