  Regular languages are closed under a wealth of formal language operators.
Incorporating such operators in regular expressions leads to concise language
specifications, but the transformation of such enhanced regular expressions to
finite automata becomes more involved.
  We present an approach that enables the direct construction of finite
automata from regular expressions enhanced with further operators that preserve
regularity. Our construction is based on an extension of the theory of
derivatives for regular expressions. To retain the standard results about
derivatives, we develop a derivability criterion for the compatibility of the
extra operators with derivatives.
  Some derivable operators do not preserve regularity. Derivatives provide a
decision procedure for the word problem of regular expressions enhanced with
such operators.
