Validating drop down lists in javascript

If the input field comes from a fixed set of options, like a drop down list or radio buttons, then the input needs to match exactly one of the values offered to the user in the first place.Free-form text, especially with Unicode characters, is perceived as difficult to validate due to a relatively large space of characters that need to be whitelisted.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.

If it's well structured data, like dates, social security numbers, zip codes, e-mail addresses, etc.

then the developer should be able to define a very strong validation pattern, usually based on regular expressions, for validating such input.

This does not mean that other users cannot access this mailbox, for example when the user makes use of a service that generates a throw away email address.

As the local-part of email addresses are, in fact - case sensitive, it is important to store and compare email addresses correctly.

Input validation should be applied on both syntactical and semantic level.