You can use a validation hook to verify that the information
a user has entered in a field is valid. If the information is not
valid, the user is prompted for valid information.
About this task
When you define a new BASIC or Perl
hook for a field or action, the calling syntax for that hook is added
to the script editor window. The calling syntax cannot be edited.
Because
hooks can affect the behavior of a field, design and test hooks carefully
before making them available to users. For example, this user input
hook example effectively makes the user_number field a mandatory field,
regardless of the setting in the Behaviors grid.