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.
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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.