ActiveRecord has many awesome validators for common things like presence, numericality, etc. Unfortunately, it is missing one common format - regex, or regular expressions. These are normally saved as string fields and, if not validated, can lead to page errors when your app tries to use a field that contains an invalid regular expression. This is especially the case if you allow the user to input a regular expression. With the addition of one small validation class, you will be able to prevent these sorts of bugs with your models.
The above code adds a validator called RegularExpressionValidator to app/validators/regular_expression_validator.rb. It then adds the validation to the Bear model on the field my_regex_field. With this, if you try and create a Bear with an invalid regular expression, an error will be thrown and the save will not complete.