def add_blank_option(regexp, blank)
      case blank
        when String then Regexp.union(regexp, Regexp.new(Regexp.quote(blank)))
        when true then   Regexp.union(regexp, //)
        else regexp
      end
    end
def add_blank_option(regexp, blank)
      case blank
        when String then Regexp.union(regexp, Regexp.new(Regexp.quote(blank)))
        when true then   Regexp.union(regexp, //)
        else regexp
      end
    end
Allow the field to be blank if this option is given. This can be true to
 allow an empty string or a string alternative for the nil value.