Thank you all for your help.
I found a JS code that do what I was looking for, here is the result:
http://cadtuts.eu/page7.php
Client-side validation only helps the visitor to your site, you MUST have server-side validation since external data can be set to anything in a request and cannot be trusted.
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add “required” to your input fields
<input type="text" id="diametre" name="diametre" placeholder="diametre" required >