Form validation is a simple thing: it is the process of ensuring that user input is correct, clean and useful. W3 Schools are providing examples of typical validation tasks: has the user filled in all required fields; has the user entered a valid date; has the user entered text in a numeric field.
In a nutshell: the main purpose of data validation is to ensure correct user input. We can isolate two sides of the data validation: server and client side. Server side validation is performed by the server, after input has been sent to the server, client side validation is performed by the browser, before input is sent to the web server.
If you undervalue the form validation, you might be wrong. It is absolutely required in order to prevent a web site abuse by malicious users or manipulative provided information. Validation of form data is the major reason for security vulnerabilities: automated input, SQL injections, hidden field manipulation, header injections and buffer overflows.
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