SSN, date, currency symbol) while semantic validation should enforce correctness of their values in the specific business context (e.g.start date is before end date, price is within expected range).In a web application, validation is usually implemented twice: on the client and server sides.Client-side validation is implemented mostly for user experience.Input validation can be implemented using any programming technique that allows effective enforcement of syntactic and semantic correctness, for example: , where the ' character is fully legitimate.For more information on XSS filter evasion please see the XSS Filter Evasion Cheat Sheet.
Input validation should happen as early as possible in the data flow, preferably as soon as the data is received from the external party.
In an application, the inputs should be validated first.
The input can be sent by a user or another application.
It's also free-form text input that highlights the importance of proper context-aware output encoding and quite clearly demonstrates that input validation is not the primary safeguards against Cross-Site Scripting — if your users want to type apostrophe (') or less-than sign ( References: Input validation of free-form Unicode text in Python Developing regular expressions can be complicated, and is well beyond the scope of this cheat sheet.
There are lots of resources on the internet about how to write regular expressions, including: and the OWASP Validation Regex Repository.