You can easily make a restiction on what characters are allowed in the field value
1) Open the field properties and navigate to the "Data" section
2) Click on "Edit" under the Validation
3) Check the "Content constraint" field and use the "Allowed values" or the "Regular expression" options. You can choose "Use an external source of allowed values" if you want to check the value with the data in the Database or File.
4) Check "Treat validation error as warning" if you want to get the warning instead of the error when the validation rule is broken.