A Validator question can validate the responses of other questions.
In Design, you can define any number of "comparisons" on other questions, much like you would with Conditions. If these "comparisons" fail, then an error message will be displayed on the form.
Each "comparison" is made up of the following pieces:
Question/Answer - A reference to another question/answer on the form to check
Compare - A "Compare" type comparison will directly compare the value of the referenced question/answer to another value. For example, it could be used to check that an "Age" field was greater than 18.
Regular Expression - Use a "Regular Expression" type comparison to validate that the value of a reference question/answer matches an entered regular expression. For example, it could be used to check that a value is a valid email address.
Field Length - A "Field Length" type comparison checks the actual length of the entered value. For example, it could be used to check that a "Name" field is less than 50 characters long.
Comparison - The options for this property will change based on the "Type" selected. It gives you the options available for your comparison. A "Field Length" type only has Minimum field length and Maximum field length; whereas a "Compare" comparison has a number of options.
In the example above, 'Error on page' will appear as an error if less than two letters have been typed into "New Question 2".
Applies to Infiniti v8.6.5 or later
The 'Message' field supports question references from version 8.6.5:
validation error message