Build a Custom Validator

See Validation in the Formie developer documentation for PHP rules, browser definitions, and JavaScript registration. It also links to a worked guide that builds a custom field and tests both validation paths.

If you need extra browser logic on an ordinary field, start with Add a Check to an Existing Field. A custom field type is useful when the requirement should travel with a reusable field. Use submission hooks for workflow coordination; abort(message) produces a form-level message, without adding a validation rule for an input.

For server-rendered forms, use the DOM validator registration. React, Vue, Web Components and custom clients using the shared value validator use core rule registration.

The same page covers validator context, displaying a field error and remote checks before submit. For existing rule names and payload options, use Built-in Rules.