code: func ValidateRegexp(v interface{}, k string) (ws []string, errors []error) {
  	if _, err := regexp.Compile(v.(string)); err != nil {
  		errors = append(errors, fmt.Errorf("%q: %s", k, err))
  	}
  	return
  }
docs: // ValidateRegexp returns a SchemaValidateFunc which tests to make sure the
  // supplied string is a valid regular expression.