“Character masks” is one of the patterns that you can select on the Match panel. With this pattern you can easily make a field match a certain combination of letters, digits, and punctuation. This pattern uses text masks similar to the masks used by masked edit controls in various development tools to force the user to enter a certain combination of letters, digits, and/or punctuation.
Type in one or more character masks for the strings that the field is allowed to match. You can enter one mask per line. The characters listed below have a special meaning in the masks. All other characters simply match themselves. Masks are always applied case insensitively.
In the "none" field validation mode, the field will allow any of the characters allowed by any of the masks anywhere in the field. The shortest and longest mask determine the minimum and maximum length of the field.
In the "loose", "average", and "strict" field validation modes, the field will use the masks exactly as you specified them.