Another useful validation feature is the pattern attribute, which expects a Regular Expression as its value. Server-side validation requires more time first occurring on the server, which requires the user's input to be submitted and sent to the server before validation occurs. The difficult part is to make it generic enough to use both cross-platform and on any form you might create. If you want to report an error, or if you want to make a suggestion, do not hesitate to send us an e-mail: W3Schools is optimized for learning and training. If isn't supported, we hide the native picker and show the fallback (