import ( "github.com/astaxie/beego/validation" "fmt" ) type User struct { Username string `valid:"Required;MinSize(3);MaxSize(20)"` Email string `valid:"Email;MaxSize(50)"` } func main() { u := User{Username: "John", Email: "[email protected]"} valid := validation.Validation{} b, err := valid.Valid(&u) if err != nil { fmt.Println(err) } if !b { // validation does not pass for _, err := range valid.Errors { fmt.Println(err.Key, err.Message) } } }In the above code, the "valid" tag is used to define the validation rules for each field. The "Required", "MinSize", "MaxSize", and "Email" rules are applied to the "Username" and "Email" fields respectively. The "valid.Valid(&u)" method call checks if the data in the "User" struct is valid based on the defined rules. This package is used by the Beego framework to validate user input data and ensure that it meets the required validation rules.