code: func (c *Constraint) UnmarshalText(text []byte) error {
  	constraint, err := NewConstraint(string(text))
  	if err != nil {
  		return err
  	}
  	c.Key = constraint.Key
  	c.MustMatch = constraint.MustMatch
  	c.Regex = constraint.Regex
  	return nil
  }
docs: // UnmarshalText defines how unmarshal in TOML parsing