Please check this question before. I frequently heard about
"... don't trust the client ! never trust the client ! never trust the user inputs !"
I agree with client-side validation alone is a very bad idea. But if so , what is the advantages of using client-side validation ? Is it necessary or not? When should I use client-side validation ?
Forget about some bad guys (who try to hack), please think about should every request reach to server side ? I don't think so . Normal users can input wrong datas and these should be mostly end at the client side. I believe that it may improve site's performance.
I tried to Disable developer tools in my site to moderate bad guys. I use both client-side and server-side validations for my sites. I believe client-side validations were also needed. Any suggestions for using client-side validation ?
I personaly use like you both of them.
Imagine Email Regex:
Clientside:
You help the User do know about a mistake, if he forget @ or something like that
Serverside:
The best Regex can't help you, to know the Email is availible. You need to send him a Email.
General
Use Clientside Validation to help the User to detect Errors, use Serverside Validation, if you need some Data in a specific format.