In returns true if a specific value is in a list of strings.

Matches returns true if a string value matches a specific regexp pattern.

New is a helper which creates a new Validator instance with an empty errors map.

Unique returns true if all string values in a slice are unique.

Declare a regular expression for sanity checking the format of email addresses (we'll use this later in the book).

Define a new Validator type which contains a map of validation errors.