  We consider the a posteriori error estimation for
convection-diffusion-reaction equations in both diffusion-dominated and
convection/reaction-dominated regimes. We present an explicit hybrid estimator,
which, in each regime, is proved to be reliable and efficient with constants
independent of the parameters in the underlying problem. For
convection-dominated problems, the norm introduced by Verf{\"u}rth
\cite{verf2005confusion} is used to measure the approximation error. Various
numerical experiments are performed to (1) demonstrate the robustness of the
hybrid estimator; (2) show that the hybrid estimator is more accurate than the
explicit residual estimator and is less sensitive to the size of reaction, even
though both of them are robust.
