Gravity Forms
+ Cross-Field Validation Rules

Gravity Forms checks each field on its own: is it filled in, is it a valid date, is the number inside its range. What it cannot check out of the box is whether two answers make sense together. A visitor can pick a return date before the departure, an end time before the start time, a maximum budget below the minimum, percentages that add up to 94, or a deposit larger than the total. Each value is valid on its own, so the entry goes through and you find out later.

The Cross-Field Validation Rules add-on adds a Validation Rules tab to every form. A rule compares one field with another field, with a fixed value or with today or now, optionally with an offset such as plus 3 days or plus 30 minutes. It works with dates in every Gravity Forms date format, 12 and 24 hour times, a date plus a time from two fields, numbers, currency fields and the order total, and text. Group rules check that a set of fields adds up to a value, or that at least one, exactly one, or all or none of them are filled in.

Every rule has its own error message, shown on the field you choose, and can be limited by a condition such as only when Trip type is Return. Rules are checked live in the browser as visitors fill in the form, and always again on the server when the form is submitted, so a visitor who switches off scripts or posts the form directly is still stopped. On multi-page forms a rule runs on the page that shows its error, fields hidden by conditional logic are skipped, and empty fields are only an error when you say so. Everything runs on your own server, with no outside connection. For Gravity Forms 2.9 and newer.

Rules are stored with the form. When the form is shown, the add-on gives the browser the rules and a few facts about the fields they use (the date format, the time format, the decimal separator), and a small script checks each rule when a visitor changes or leaves one of its fields, and again before the visitor moves to the next page or submits. Every value is turned into something that can be compared: a date becomes a day number whatever its format, a time becomes minutes after midnight, a number is read with the decimal separator of the field or the Gravity Forms currency, and text can ignore upper and lower case. The same steps run on the server in the Gravity Forms gform_validation filter for every submission, so the result is the same in the browser and on the server, and the server decides. A rule is skipped when one of its fields is hidden by conditional logic, when its condition does not match, or when a compared field is empty and the rule does not say that empty is an error. On multi-page forms the server runs a rule on the page that shows its error and sends the visitor back to that page on the final submit. Everything happens on your own server — the add-on makes no outside HTTP request beyond the licence and update checks.