Form Validation

Definition: Form validation is the process of checking whether information entered into a form meets defined requirements before the form is accepted. Rules may verify that required fields are completed, email addresses use a valid structure, numbers fall within an allowed range, or passwords meet specified criteria. Client-side validation happens in the visitor’s browser and can provide immediate feedback, while server-side validation checks the data again after submission and protects against invalid or unsafe input. Effective validation explains the problem near the relevant field, preserves the visitor’s existing entries, and uses clear instructions instead of vague messages such as “Invalid input.” Validation should prevent genuine errors without creating unnecessary barriers. For example, a phone number field should accept reasonable formatting variations rather than reject every format that differs from one preferred pattern. Properly implemented validation improves data quality, reduces manual cleanup, helps integrations run reliably, and can lower form abandonment by resolving problems before submission. It should support accessibility by associating messages with their fields and making errors understandable to people using assistive technology.

Example: A webinar registration form requires a work email and company size. If someone leaves the email blank or enters an incomplete address, an inline message identifies the specific issue while keeping the other fields filled in. The form accepts phone numbers with spaces or parentheses, then performs a second server-side check before adding the registrant to the campaign.