code: func ValidateRegexp(ch *Changeset, field string, exp *regexp.Regexp, opts ...Option) {
  	val, exist := ch.changes[field]
  	if !exist {
  		return
  	}
  
  	options := Options{
  		message: ValidateRegexpErrorMessage,
  	}
  	options.apply(opts)
  
  	if str, ok := val.(string); ok {
  		match := exp.MatchString(str)
  		if !match {
  			msg := strings.Replace(options.message, "{field}", field, 1)
  			AddError(ch, field, msg)
  		}
  		return
  	}
  }
docs: // ValidateRegexp validates the value of given field to match given regexp.