This client-side validation should never be used as the only validation technique to validate data on a Web page.
Server Side Validation When the validation code is placed the server side, the process of validation might be slow because a form might involve multiple roundtrips to the Web Server before all the data is validated.
You can always turn the validation on the client side off using the enable client side property to false.
Lets see the difference between the client side validation and the server side validation.
In order to set the Required validator on the Text Box control just drag and drop the validator on the webform and set its Controlto Validate property to the Text Box id you want to validate. Is Valid property is true if the page does not have any error messages to display.
The Required field validator performs the validation first on the client side and than on the server side.
Since the user must enter the username in the Text Box in order to access the website.This tutorial covers Validating User Input with C# covers Overview of ASP.NET Validation Controls , Using the Simple Validators , Using the Complex Validators and Summarizing Results with the Validation Summary Control.Regular Expression Validator is used to check the user input against some built in regular expressions.The control provides the Regular Expression collection property that can be used to select the desired Regular Expression.On the other hand, because the Web Server is performing all the validation, you can trust the validated data.